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Classification of Elements - The Periodic Table

Improve Your Learning
 

I. Conceptual Understanding
 

1. Newlands proposed the law of octaves. Mendeleev suggested eight groups for elements in his table. How do you explain these observations in terms of modern periodic classification?   (AS - 1) (4 Marks)

A: * According to Newland's law of octaves every eighth element starting from a given element resembles in its properties to that of the starting element.

* If we start at Lithium, then the eighth element is sodium and next coming eighth is potassium and so on. These elements show similar physical and chemical properties.

* According to modern periodic table elements lithium, sodium, potassium .... etc. are all placed in the same group starting that they have similar physical and chemical properties.

* Mendeleev's periodic table has eight vertical columns. These are groups.

There are seven horizontal rows. These are periods. According to Mendeleev's table elements present in a group have similar properties.

* What is proposed by Mendeleev was supported in modern periodic table?

* For example Mendeleev gave the general formula for the first group of elements as R2O and for second group of elements as RO, the same thing was followed in modern periodic table.

2. What are the limitations of Mendeleev's periodic table? How could the modern periodic table over come the limitations of Mendeleev's table?   (AS - 1) (4 Marks)

A: Limitations of Mendeleev's periodic table

1. Anomalous pair of elements: Certain elements of highest atomic weights precede those with lower atomic weights.

For example, tellurium (atomic weight 127.6) precedes iodine (atomic weight 126.9).

2. Dissimilar elements placed together: Elements with dissimilar properties were placed in same group as sub-group A and sub-group B. For example, alkali metal like Li, Na, K etc., of I A group have little resemblance with coinage metals like Cu, Ag, Au of I B group. Cl is of VII A group and 'Mn' is of VII B, but chlorine is a non metal, where as manganese is a metal.

Modern periodic table: Over coming the above limitations:

* The periodic law is changed from atomic weight concept to atomic number concept in the modern periodic law.

* a) In Mendeleev's periodic table tellurium (Te) (atomic weight 127.6) precedes iodine (atomic weight 126.9).

b) In modern periodic table this placement is made justified because the atomic number of tellurium is 52 and that of iodine is 53.

c) So in the long form of periodic table these dissimilar elements changed their place in groups and periods.

* a) In the long form of periodic table 'Cl' falls in 3rd period and VII A group and 'Mn' falls in 4th period and VII B group.

b) Metals and non-metals are separated here.

3. Define the modern periodic law. Discuss the construction of the long form of the periodic table.   (AS - 1) (4 Marks)

A: Modern periodic law: ''The physical and chemical properties of elements are periodic function of electronic configuration of their atoms".

Construction of the long form of the periodic table

* The modern periodic table has 18 vertical columns known as groups and 7 horizontal rows known as periods.

* The groups are represented by using Roman numeral I through VIII with letters A and B in traditional notation.

* According to latest recommendation of the IUPAC, these groups are represented by Arabic numerals 1 through 18 with no A and B designations.

* There are seven periods in the modern periodic table and the first period starts with K - shell. It contains only one sub - shell, the 1s. It accommodates 2 electrons. They are 1s1(H) and 1s2 (He).

* Therefore the first period contains only two elements.

* Second period consists of two s - block elements and six p-block elements. So the total number of elements in second period is 8.

* Third period starts with third main shell (M). It has 3 sub-shells, namely 3s, 3p and 3d, but while electrons are being filled into the shell '3d' gets electrons only after 4s is filled. So 3rd period has 8 elements only.

* 4th main shell (N) has four sub-shells, 4s, 4p, 4d and 4f. But electrons enter the atoms in the order 4s, 3d and 3p. Due to this the 4th period contains 18 elements.

* On the same line, we can explain why there are 18 elements in the 5th period.

* There are 32 elements in the sixth period.

* Seventh period is incomplete. It contains 2 elements from s -block (7s), 14 elements from f - block (5f), 10 elements from d block (6d) and some elements from p-block (7p).

* The f- block elements known as lanthanoids and actinoids are shown separately at the bottom of the periodic table.

4. Explain how the elements are classified into s, p, d and f-block elements in the periodic table and give the advantage of this type of classification. (4 Marks)

A: * Depending on to which sub - shell, the differentiating electron enters the atom of the given element, the elements are classified as s, p, d and f block elements.

s - block elements:

* The differentiating electron enters into s - sub shell.

* I A and II A groups of elements belong to s -block.

* Except hydrogen, all s - block elements are metals.

* The general electronic configuration of these elements is ns1 to ns2.

p - block elements:

* The differentiating electron enters into p - sub shell.

* III A to VIII A or zero groups of elements belong to p - block.

* He is the only element whose differentiating electron does not enter into p-sub shell.

* The general electronic configuration of these elements is ns2 np1 to ns2 np6.

d-block elements:

* The differentiating electron enters into d -sub shell.

* I B to VIII B groups of elements belong to d - block.

* All d - block elements are metals.

* The general electronic configuration of these elements is ns2np6 (n - 1)d1 to ns2np6 (n - 1)d10.

f - block elements:

* The elements in which f - orbitals are being filled in their atoms are called f-block elements.

* The lanthanoids and actinoids below the periodic table are all f-block elements.

Note:

* s and p - block elements together are called representative elements.

* d - block elements are called transition elements.

* f - block elements are called inner transition elements.

Advantages:

* Division of elements into different block elements is useful to identify their physical and chemical properties easily.

* It is easy to choose substitute elements in the study of chemical reactions.
 

5. Given below is the electronic configuration of elements A, B, C, D.  (AS - 1) (4 Marks)

A. 1s2 2s2             B. 1s2 2s2 2p6 3s2

C. 1s2 2s2 2p6 3s2 3p3             D. 1s2 2s2 2p6

a. Which are elements coming with in the same period?

A: * A and D elements belong to the same period as their outer most shell is L (n = 2).

* The atomic numbers of these elements are Be (Z = 4) and Ne (Z = 10).

b. Which are the ones coming within the same group?

A: * A and B elements are coming within the same group.

* The atomic numbers of these elements are

Be (Z = 4) and Mg (Z = 12).

c. Which are the noble gas elements?

A: * D element is the noble gas element.

* Its atomic number is Ne (Z = 10).

d. To which group and period does the element 'C' belong?

A: * Element with electronic configuration in C is P (Z = 15).

* It belongs to the 3rd period and V A group.
 

6. Write down the characteristics of the elements having atomic number 17. (4 Marks)

a) Electronic configuration .... b) Period number .....

c) Group number ..... d) Element family .....

e) No. of valence electrons ..... f) Valency .....

g) Metal or non-metal .....

A: a) Electronic configuration: 1s2 2s2 2p6 3s2 3p5 (Z = 17)

b) Period number: 3rd period

c) Group number: VII A group (17th)

d) Element family: Halogen

e) No. of valence electrons: 7 (2 + 5)

f) Valency: 1 (8 - 7)

g) Metal or non-metal: Non-metal
 

7. a) State the number of valence electrons, the group number and the period number of each element given in the following table.  (AS - 1) (4 Marks)


 

7. b) State whether following elements belong to a group (G), period (P) or neither group nor period (N).  (AS - 1) (4 Marks)


 

8. Elements in a group generally possess similar properties, but elements along a period have different properties. How do you explain this statement? (AS - 1) (2 Marks)

A: * Elements in a group have similar electronic configuration. So all the elements in a group possess similar properties.

* Elements in a period do not have similar electronic configuration. So the elements in a period have different properties and do not have similar properties.
 

9. s-block and p-block elements except 18th group elements are some times called as 'Representative elements' based on their abundant availability in nature. Is it justified? Why?  (AS - 1) (4 Marks)

A: The statement given is justified. Because

* Elements in groups 1, 2, 13, 14, 15, 16, 17 of the periodic table are representative elements.

* These group elements atoms have outer most orbit filled partly or nearly completely with electrons in s and p orbitals.

* The 18th group elements atoms are completely filled with electrons in their s and p orbitals and so these do not come under representative elements.

* The metals are present in the left group of the representative elements and non - metals are present in the right and semi - metals are present in the middle.

* 18th group elements are noble gases.

* The representative elements are all having incomplete outer most orbits. So they are chemically reactive to obtain stable electronic configuration of noble gases.

* These elements are therefore abundant in nature in the form of compounds.
 

10. Complete the following table using the periodic table.    (AS - 1 ) (4 Marks)

11. Complete the following table using the periodic table. (AS - 1) (4 Marks)

12. The electronic configuration of the elements X, Y, Z are given below. (2 Marks)

   a) X = 2     b) Y = 2, 6   c) Z = 2, 8, 2

i. Which element belongs to second period?

A: Y belongs to 2nd period.

ii. Which element belongs to second group?

A: Z belongs to 2nd group.

iii. Which element belongs to 18th group?

A: X belongs to 18th group.

13. Identify the element that has larger atomic radius in each pair of the following and mark it with a symbol .    (AS - 1) (1 Mark)

   I) Mg or Ca  II) Li or Cs    III) N or P    IV) B or Al

A: I) Mg or Ca                                     II) Li or Cs 

III) N or P                                   IV) B or Al 

14. Identify the element that has lower ionization energy in each pair of the following and mark it with a symbol  (AS - 1) (1 Mark)

I) Mg or Na        II) Li  or O          III) Br or F        IV) K or Br

A: I) Mg or Na                                    II) Li  or O

    III) Br  or F                          IV) K  or Br

15. In the period 2, element X is to the right of element Y. Then find which of the elements have

I) Low nuclear charge                                  II) Low atomic size

III) High ionization energy                       IV) High electronegativity

V) More metallic character   (AS - 1) (4 Marks)

A: I) Low nuclear charge: Element Y has low nuclear charge.

II) Low atomic size: Element Y has low atomic size.

III) High ionization energy: Element Y has high ionization energy.

IV) High electronegativity: Element Y has high electronegativity.

V) Metallic character: X has more metallic character.
 

16. How does metallic character change when we move

I) Down a group

II) Across a period.  (AS - 1) (2 Marks)

A: I) Down a group: Metallic character of elements increases as we move down a group.

II) Across a period: Metallic character of elements decreases as we move across a period.

17. Why was the basis of classification of elements changed from the atomic mass to atomic number?  (AS - 1) (4 Marks)

A: * At the time (1829) when Dobereiner established the law of triads electrons were not discovered. With the available information of atomic weights he established the law of triads.

* Even at the time of John Newlands (1865) with the same available data of atomic weights of elements he established the law of octaves.

* The same process continued with Mendeleev while developing the periodic table.

* Mendeleev's periodic table faced limitations and the anomalous pair of elements, and dissimilar elements placed together paved way to put the minds of the scientists on this issue.

* Incidentally in 1913 Moseley's X-ray analysis introduced the concept that the atomic number is more fundamental characteristic of an element than its atomic weight.

* As a result the modern periodic law is stated as 'The properties of the elements are periodic function of their atomic number'.
 

18. What is a periodic property? How do the following properties change in a group and period? Explain.

a) Atomic radius   b) Ionization energy

c) electron affinity       d) electronegativity    (AS - 1) (4 Marks)

A: Periodic properties: When elements are arranged in the order of their electronic configuration, the repetition of physical and chemical properties of the elements at regular intervals is called periodic properties.

Explanation:

a) Atomic radius: The distance between the centre of the nucleus to the outermost shell of an atom is called atomic radius. (or) Atomic radius is half of the distance between the radii of two bonding atoms.

* In a group the atomic radius increases as we go from top to bottom. This is due to the addition of shells.

* In a period the atomic radius decreases as we move from left to right. This is because more electrons enter the shell as move from left to right in the period. This increases the nuclear attraction on the outer shell and the size of the atom decreases.

b) Ionizatin energy: Ionization energy is the energy required to remove an electron from the outer shell of a neutral gaseous atom.

* In a group as the atomic radius increases from top to bottom, it becomes comparatively easy to remove the electron from the outer shell. So ionization energy decreases in a group as we go down a group.

* In a period as we move from left to right more electrons join the outer shell and it becomes difficult to remove the electron from the outer shell. So the ionization energy generally increases in a period as we move from left to right.

c) Electron affinity: Electron affinity is defined as the energy released when an electron is added to a gaseous neutral atom.

* As the atomic radius decreases when we go down a group the electron affinity decreases.

* In a period as more electrons join the same shell when we move from left to right the atomic radius decreases and the electron affinity increases.

d) Electronegativity: Electronegativity is the relative tendency of its atom to attract electrons towards itself when it is bounded to the atom of another element.

* Electronegativity decreases as we go down a group.

* Electronegativity increases as we move from left to right in a period.
 

18. b) Explain the ionization energy order in the following sets of elements.

a) Na, Al, Cl                  b) Li, Be, B                  c) C, N, O

d) F, Ne, Na                  e) Be, Mg, Ca   (Each 1 Mark)

A: a) Na, Al, Cl: All these elements belong to the same period. The order of their atomic size is Na > Al > Cl. As we move from left to right in a period the ionization energy increases.

The order of ionization energy of these elements is Cl > Al > Na.

b) Li, Be, B: These three elements belong to the same period. Their electronic configurations are

Li (Z = 3): 1s2 2s1, Be (Z = 4): 1s2 2s2 and B (Z = 5): 1s2 2s2 2p1.

It is easy to remove an electron from 2p rather than from 2s.

So the order of ionization energy is Be > Li > B.

c) C, N, O: These three elements belong to the same period. Their electronic configurations are:

C (Z = 6): 1s2 2s2 2p2, N (Z = 7): 1s2 2s2 2p3, O (Z = 8): 1s2 2s2p4.

As N has half filled degenerated orbitals it is more stable relative to C and O. So its ionization energy is more.

The order of ionization energy is N > C > O.

d) F, Ne, Na: Electronic configuration of

F (Z = 9): 1s2 2s2 2p5, Ne (Z = 10): 1s2 2s2 2p6

Na (Z = 11): 1s2 2s2 2p6 3s1.

Na is an inert gas so has highest ionization energy. Na atom is comparatively larger in size than F atom.

So the order of ionization energy is Ne > F > Na.

e) Be, Mg, Ca: These elements belong to the same group. Their atomic size is in the order Ca > Mg > Be.

As atomic size increases the ionization energy decreases. The order of ionization energy is Be > Mg > Ca.
 

II. Asking questions and making hypothesis

19. Name two elements that you would expect to have chemical properties similar to Mg. What is the basis for your choice?       (AS - 2) (4 Marks)

A: * The two elements I expect to have the chemical properties similar to Mg are Be and Ca.

* The basis for my choice is that these three elements Be, Mg and Ca belong to the same group i.e. group 2.

* All these elements which are in the same group have the same type of electronic configuration for their outer shell and same chemical properties.

* The outer shell electronic configuration for these three elements is as 2s2 (Be), 3s2 (Mg) and 4s2 (Ca).
 

20. On the basis of atomic numbers predict to which block the elements with atomic number 9, 37, 46 and 64 belong to?    (AS - 2) (4 Marks)

A: * Electronic configuration of element with atomic number 9 is 1s2 2s2 2p5 - So this belongs to p -block.

* Electronic configuration of element with atomic number 37 is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1 - So this belongs to s - block.

* Electronic configuration of element with atomic number 46 is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 5p6 4d2 - So this belongs to d -block.

* Electronic configuration of element with atomic number 64 is 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 5p6 4d10 6s2 6p6 4f2 - So this belongs to f - block.
 

21. Using periodic table, predict the formula of compound formed between an element X of group 13 and another element Y of group 16.   (AS - 2) (1 Mark)

A: * Element X of group 13 has a valency 3.

* Element Y of group 16 has a valency = 18 - 16 = 2

* So formula of the compound is X2Y3 .
 

22. An element X belongs to 3rd period and group 2 of the periodic table. State 

a) The no. of valence electrons    b) The valency

c) Whether it is a metal or a non-metal?  (AS - 2) (2 Marks)

A: The element X that belongs to 3rd period and group 2 is Mg.

a) The no. of valence electrons = 2

b) The valency = 2

c) Mg is a metal.
 

23. An element has atomic number 19. Where would you expect this element in the periodic table? And why?  (AS - 2) ( 2 Marks)

A: * An element with atomic number 19 will be in the 4th period and 1st group of the periodic table.

* The reason for this is that the electronic configuration of this element is

1s2 2s2 2p6 3s2 3p6 4s1.

* It is clear that the differentiating electron enters the 4th shell. So it belongs to 4th period.

* The differentiating electron is in the 's' orbital. It means it belongs to s -block.

* The outer most orbital has only one electron. So it belongs to first group.
 

III. Experimentation and Field Investigation

24. Aluminium does not react with water at room temperature but it reacts with both dil. HCl and NaOH solutions. Verify these statements experimentally. Write your observations with chemical equations. From these observations, can we conclude that Al is a metalloid? (AS - 3) (4 Marks)

A: * Aluminium is a metal. It is covered with an oxide layer which protects the metal from the air attack.

 * If the oxide layer on aluminium is damaged then it is easily attacked by water also.

* Aluminium reacts with dilute HCl and H2 gas is liberated.

    2 Al + 6 HCl   2 AlCl3 + 3 H2

* Similarly Aluminium reacts with NaOH and H2 gas is liberated.

2 Al + 6 H2O + 2 NaOH  2 Na[Al(OH)4] + 3 H2

* Aluminium reacts both with acids and bases so it is amphoteric.

* Aluminium is available in abundance on the earth's crust.

* Aluminium is not available free in nature but occurs as Bauxite, Corundum etc.
 

IV. Information Skills and Projects

25. Collect the information about reactivity of VIII A group elements (noble gases) from internet or from your school library and prepare a report on their special character when compared to other elements of periodic table.  (AS - 4) (4 Marks)

A: * VIII A group elements are noble gases or inert gases. As the name implies that they do not react with any other element under ordinary conditions.

* This is because except He, other inert gas elements have stable 'octet' configuration.

* Even for He, the two available orbitals in 1s is filled with electrons and so it is also very inactive.

* These elements can neither loose electrons nor gain electrons as they have very high ionization energy and zero electron affinity.

* In recent times experiments have established that under suitable conditions heavier elements of this group can form compounds with F2, O2.

* Xenon (Xe) shows a tendency to loose an electron and exist as a positive ion. So it reacts with highly electronegative elements like F2 and O2 only.

e.g.: XeO3, XeO4, XeF2, XeF4, .........

* Under favourable conditions Ar is able to form coordination compounds with BF3. The composition of this new compound is Ar.n.BF3.

* N.Barltel prepared xenon fluoroplatinate Xe (PlF6). This is the first compound prepared with an inert gas.

* Incidentally, it is identified that these compounds have varied applications in different fields.
 

26. Collect information regarding metallic character of elements of I A group and prepare a report to support the idea of metallic character increases in a group as we move from top to bottom.  (AS - 4) (4 Marks)

A: * Li, Na, K, Rb, Cs and Fr are the elements of I A group.

* These are metals and have metallic character.

* The tendency of an element to loose electrons and form positive ions is called metallic character.

* As we go down in this group, the atomic size increases and electrons in the outer shell experiences less nuclear attraction and can loose electrons easily. This is increased metallic character.

* These elements have less electronegative character.

* Reaction with non-metals:

2 Na + O2 Na2O2

Here Na+ is cation in the chemical characteristic of metals, they form cations in ionic compounds with non-metals.

* These metals form ionic halides.
    

This MH is an ionic halide and the order of ionic nature of halides is

LiH < NaH < KH < RbH < CsH.

* Metals form basic oxides.

2 M + O2 M2O2 (except Li)

4 Li + 2 O2 2 Li2O2

All metals form peroxides.

* From the above discussion we can conclude that group I A elements are metals and their metallic character increases as we go down the group.

VI. Appreciation and Aesthetic Sense, Values

27. How do you appreciate the role of electronic configuration of the atoms of elements in periodic classification.  (AS - 6) (4 Marks)

A: * H.J. Moseley (1913) analysed the X - ray pattern obtained when elements are subjected to bombardment by high energy electrons.
* Moseley realised that the atomic number is more fundamental characteristic of an element than its atomic weight.

* This atomic number concept forced the periodic law to be changed.

* So the modern periodic law is 'The physical and chemical properties of elements are the periodic function of electronic configurations of their atoms'.

* According to the modern periodic law, the elements having same outer shell electronic configuration are kept in the same group. These elements have similar chemical properties.

* This classification rectified all the short comings of Mendeleev's classification and also enabled the study of physical and chemical properties of elements easily.

* I appreciate the usefulness of electronic configuration in arranging elements in periodic table so that the place of an element in this table can be easily identified.
 

28. Without knowing the electronic configurations of the atoms of elements Mendeleev still could arrange the elements nearly close to the arrangements in the modern periodic table - How can you appreciate this?   (AS - 6) (4 Marks)

A: * At the time when Mendeleev introduced his periodic table even electrons were not discovered. Even then the periodic table was able to provide a base for the study of chemistry of elements. I very much appreciate this.

* Based on the arrangement of the elements in the table he predicted that some elements were missing and left blank places at the appropriate places in the table. This speaks of his excellent vision in the arrangement of elements.

* Mendeleev believed that some new elements would be discovered definitely. He predicted the properties of these new additional elements in advance purely depending on his table.

* His predicted properties were almost the same as the observed properties of those elements after their discovery. This confirms his confidence on the table of elements arranged which should be appreciated.

* Another great victory of Mendeleev's table is helped in correcting atomic masses of some elements like beryllium, indium and gold.

* This indicates his extraordinary intelligence because with the help of his theoretical table he could correct the atomic masses of some elements found with experimental calculations.

* I always remember that in his honour the 101th element was named Mendelevium. What more recognition a scientist gets more than this for his work.

VII. Application to Daily Life, Concern to Biodiversity

29. Comment on the position of Hydrogen in the periodic table.  (AS - 7) (4 Marks)

A: * The position of hydrogen in the periodic table is unique. It is placed at the top of both alkali metals (group I A) and halogens (VII A).

* This is because it can loose one electron like alkali metals as well as gain one electron like halogens.

* In fact hydrogen has the simplest atomic structure of all the elements (1s1).

* Hydrogen has high ionization enthalpy, so it does not easily give up its electron. Where as alkali metals have low ionization enthalpy and they readily can form M+ (g) ions and also M+ (aq) ions. So they form ionic bonds.

* Hydrogen has a greater tendency to pair up its electron and form covalent bonds like halogens. So it can be placed in that group.

* However the position of hydrogen is not certain. So in some of the periodic tables hydrogen is not placed in any of the groups.

* Hydrogen is treated as a very special element and placed alone at the head of the periodic table.
 

30. How the positions of elements in the periodic table help you to predict its chemical properties? Explain with an example.  (AS - 7) (4 Marks)

A: * The position of elements in the periodic table helps us to predict the chemical properties.

* For example the elements which are in the left of the periodic table are metals which are highly reactive.

* Li, Na, K, Mg, Ca are all metals present on th left side of the periodic table.

* Non-metals and gases are present on the right side of the periodic table.

* O, F, Cl, S, ...... etc., are all on the right side of the periodic table. These are all non metals.

* Noble gases are in the 18th group. These are inert chemically. Positions of elements in the periodic table can easily reveal their chemical properties by knowing the chemical properties of any element in its group or in its period.

* For example consider the third period elements namely Na, Mg, Al, Si, P, S and Cl. In a period on moving from left to right the chemical reactivity of elements first slowly decreases and then increases.

* In the elements mentioned Na is very reactive Mg is less reactive, Al is still less reactive and Si is chemically least reactive.

* When we consider P it is quite reactive, S is more reactive and Cl is much more reactive.

* Thus in a period the chemical reactivity first decreases and then increases.
 

THINK AND DISCUSS


1. What relation about elements did Dobereiner want to establish? (AS - 2) (2 Marks)

A: * Dobereiner wanted to establish a relation between the atomic masses and the physical, chemical properties of elements.

* This led to the formation of periodic table of Mendeleev putting the elements in groups and periods keeping in view their atomic masses, and their physical, chemical properties.

* It so happened finally, the modern periodic table came in to use.

2. The densities of calcium (Ca) and Barium (Ba) are 1.55 and 3.51 gm cm-3 respectively. Based on Dobereiner's law of triads can you give the approximate density of strontium (Sr). (AS - 1) (2 Marks)

A: Given: Density of Calcium = 1.55 gm cm-3

Density of Barium = 3.51 gm cm-3

Density of Strontium = ?

                                      = 2.53 gm cm-3

* Actually density of Strontium = 2.64 gm cm-3

* The average density of Calcium and Barium is approximately equal to the density Strontium based on Dobereiner's law.
 

3. Do you know why Newlands proposed the law of octaves? Explain your answer in terms of the modern structure of the atom?  (AS - 1) (2 Marks)

A: * According to the law of octaves every eighth element starting from a given element resembles in its properties to that of the starting element.

* In fact Newlands arranged the elements in the increasing order of their atomic weights.

* According to the structure of the atom, after completion of octet (ns2 np6) configuration, the properties of the elements are repeated.
 

4. Do you think that Newlands' law of octaves is correct? Justify.  (AS - 1) (2 Marks)

A: * Newlands law of octaves is correct only upto calcium with atomic mass 40. It is not suitable for elements which had atomic masses more than 40.

* In order to fit elements into his table, he adjusted two elements in the same slot.

e.g.: Co and Ni are grouped with F, Cl and Br which have different properties.

* In the same way iron (Fe) which resembles Co and Ni was placed far away.

5. Why Mendeleev had to leave certain blank spaces in his periodic table? What is your explanation for this? (2 Marks)

A: * Predicting the properties of missing elements.

* Based on the arrangement of the elements in the table, he predicted that some elements were missing and left blank spaces at the appropriate places in the table.

* Mendeleev believed that some new elements would be discovered definitely.

* He predicted the properties of these new additional elements in advance purely depending on his table.

* His predicted properties were almost the same as the observed properties of those elements after their discovery.

6. What is your understanding about Ea2O3 , EsO2? (AS - 1) (2 Marks)

A: * Ea2O2 is the oxide of Eka-Aluminium predicted by Mendeleev.

* EsO2 is the oxide of Eka-Silicon predicted by Mendeleev.

* In 1875 Eka-Aluminium was found to be Ga2O3 Gallium oxide.

* In 1886 Eka-Silicon was found to be GeO2 Germanium oxide

7. All alkali metals are solids but hydrogen is a gas with diatomic molecules. Do you justify the inclusion of hydrogen in first group with alkali metals? (AS - 1) (2 Marks)

A: * Hydrogen element has been placed at the top of group I (Alkali metals). This is because the electronic configuration of hydrogen is similar to the electronic configuration of alkali metals.

* But hydrogen differs from alkali metals in many of its properties.

* So hydrogen is not included in alkali metals when we discuss about alkali metals.
 

8. Why lanthanoids and actinoids placed separately at the bottom of the periodic table?  (AS - 1) (2 Marks)

A: * The 'f' block elements are called as Lanthanoids and Actinoids.

* So they are shown separately at the bottom of the periodic table as these cannot be fitted in the general periodic table.
 

9. If they are inserted within the table imagine how the table would be?  (AS - 1) (2 Marks)

A: * The periodicity of elements would not have been maintained of lanthanoids and actinoids are inserted within the periodic table.

* It would not have been possible to put the elements of similar properties under the same group.
 

10. Second ionization energy of an element is higher than its first ionization energy. Why?  (AS - 1) (2 Marks)

A: * The energy required to remove an electron (first electron) from the outer most orbit of a neutral gaseous atom is called first ionization energy.

* The energy required to remove an electron (second electron) from unipositive ion is called second ionization energy.

* The nuclear attractive force on the outermost electron of unipositive ion is more than the nuclear attractive force on the outer most electron of a neutral gaseous atom.

* So second ionization energy is higher than the first ionization energy.
 

11. The calculated electron gain enthalpy values for alkaline earth metals and noble gases are positive. How can you explain this?  (AS - 1) (2 Marks)

A: * For alkaline earth metals, the electronic configuration for the outer shell will be ns2.

* s - orbital already accommodated the maximum number of electrons in it. So these are all stable.

* It is difficult to add an electron to the outermost orbit of alkali earth metal atoms.

* Energy is required to add an electron to it. Hence their electron gain enthalpy is positive.

* For the atoms of noble gases, the general electronic configuration is ns2 np6 for their outer shell. These have stable octet electronic configuration. Energy is needed to add an electron to the atoms of these inert gases.

* As such for intert gases electron gain enthalpy values are positive.
 

12. The second period element for example 'F' has less electron gain enthalpy than the third period element of the same group for example 'Cl'. Why? (AS - 1) (2 Marks)

A: * An atom can gain an electron when the electron outside the atom is attracted by the nucleus of the atom.

* During this process energy is liberated. This energy is called electron gain enthalpy.

* Fluorine 'F' is more electronegative than chlorine. So F attracts an electron by releasing less energy compared to chlorine which is less electronegative than fluorine.

* So electron gain enthalpy of 'F' is less than that of 'Cl'.
 

Questions and Answers given in the Lesson

1. How does the valency vary in a period on going from left to right? (AS - 1) (1 Mark)

A: * The variation of valency does not follow a regular trend as we move from left to right in a period.

* It first increases and then decreases and finally becomes 'O' for inert gases.

2. How does the valency vary on going down a group?              (AS - 1) (1 Mark)

A: As the number of valence electrons is the same for all elements in a group, the valency remains the same on going down a group.
 

3. Do the atom of an element and its ion have same size?       (AS - 1) (1 Mark)

A: * No. The positive ion is smaller in size than the atom.

* The negative ion is larger in size than the atom.
 

4. Which one between Cl and Cl - would have more size? Why? (AS - 1) (2 Mark)

A: * Chlorine atom has the electronic configuration.

Cl (Z = 17): 1s2 2s2 2p6 3s2 3p5

* Chlorine - ion has the electronic configuration 1s2 2s2 2p6 3s2 3p6.

* The nuclear attraction in Cl atom is more than that in Cl- ion. So Cl- ion is bigger in size than Cl.

5. Which one between Na and Na+ would have more size? Why? (AS - 1) (2 Marks)

A: * Atomic number of sodium (Na) is 11. Therefore sodium (Na) atom contains 11 protons and 11 electrons with outer electron as 3s1.

* On the other hand Na+ ion has 11 protons but only 10 electrons. The 3s shell of Na+ level has no electron in it.

* Hence its outer shell configuration is 2s2 2p6. As proton number is more than electrons in it, the nucleus of Na+ ion attracts outer shell electrons with strong nuclear force.

* As a result the Na+ ion shrinks in size.

* Therefore, the size of Na+ ion is less than 'Na' atom. In general the positive ion (cation) of an element has less size than its neutral atom.
 

6. Which one in each of the following pairs is larger in size?

a) Na, Al                    b) Na, Mg+2             c) S2-, Cl- d)

d) Fe2+, Fe3+           e) C4-, F-   (AS - 1) (1 Mark each)

A: a) Na is larger in size in the pair Na, Al.

b) Na is larger in size in the pair Na, Mg+2.

c) S2- is larger in size in the pair S2-, Cl-.

d) Fe2+ is larger in size in the pair Fe2+, Fe3+.

e) C4- is larger in size in the pair C4-, F-.

Activities

1. Observe the following table and fill the gaps.  (AS - 4) (4 Marks)

Elements in each row represent a triad.

A:

1. Can you establish the same relationship with the set of elements given in the remaining rows?

A: Except in the first row, we cannot establish the relationship as stated by the law of triads.

2. Find average atomic weights of first and third elements in each row and compare it with the atomic weight of the middle element.

A: Except in group A, in the remaining groups the average atomic weights of the first and third elements is approximately equal to the atomic weight of the middle element.

3. What do you observe?

A: The law of triads gave a clue that the atomic weights can be corrected by knowing the properties of elements.
 

Activity: 2

2. Some main group elements of s - block and p - block have family names as given in the following table. Observe the long form of the periodic table and complete the table with proper information.  (AS - 4) (4 Marks)

A:


Activity: 3

3. Find out the valencies of first 20 elements. (AS-1) (4 Marks)

A:

1. How does the valency vary in a period on going from left to right?

A: While moving from left to right in a period the valency increases from 1 to 4 and then decreases to get 0.

2. How does the valency vary on going down a group?

A: The valency does not change on going down a group.

Additional Questions and Answers

I. Conceptual Understanding

1. How did Robert Boyle define an element? (1 Mark)

A: Robert Boyle (1661) defined an element as any substance the cannot be decomposed into a further simple substance by a physical or chemical chance.

2. How many elements are now known to us? (1 Mark)

A: By now, including synthetic elements, there are more than 115 elements.

3. State and explain Dobereiner's law of triads. Statement. (2 Marks)

A: Dobereiner stated that when elements with similar properties are taken three at a time and arranged in the ascending order of their atomic weights, the atomic weight of the middle element is the average of the atomic weights of the first and third elements. This statement is called the Dobereiner's law of triads.

Explanation:

* Lithium (Li) (At. Wt. : 7.0), Sodium (Na) (At. Wt. = 23) and Potassium (K) (At. Wt. : 39) is a Dobereiner triad.

4. In what way Dobereiner's attempt helped the scientists?    (2 Marks)

A: * Dobereiner's attempts gave a clue that atomic weights could be correlated with properties of elements.

* It made chemists look at elements in terms of groups of elements with similar chemical and physical properties.

* This eventually led to rigorous classification of elements and the modern periodic table of elements.
 

5. What are the limitations of Dobereiner's law of triads?  (2 Marks)

A: Limitations

i) All the known elements at that time could not be arranged in the form of triads.

ii) The law failed for very low mass or for very high mass elements. In case of F, Cl, Br, the atomic weight of Cl is not an arithmetic mean of atomic weights of F and Br.

iii) As the techniques improved for measuring atomic masses accurately, the law was unable to remain strictly valid.
 

6. State Newlands law of octaves?

A: The law of octaves states that when elements are arranged in the ascending order of their atomic weights they fall into a pattern in which their properties repeat at regular intervals. Every eighth element starting from a given element resembles in its properties to that of the starting element.
 

7. What are the merits and demerits of Newland's table of elements? (4 Marks)

A: * Newlands' table is not without problems.

* There are instance of two elements fitted into the same slot.

e.g. Cobalt and nickel.

* Certain elements, totally dissimilar in their properties, were fitted into the same group.

* For example he arranged Co, Ni, Pd, Pt and Ir which have different properties compared with halogens in the same row (F, Cl, Br, I). (See Newlands' first horizontal row)

* It was found that the law of octaves holds good only for the elements upto calcium.

* The law was not valid for elements that had atomic masses higher than calcium.

* Newlands periodic table was restricted to only 56 elements and did not leave any room for new elements.

* Elements that were discovered later could not be fitted into Newlands table in accordance with their properties.

* Newlands attempted to link the periodicity of the chemical properties of elements with the periodicity found in the music.

* In musical scale any note in a key is separated from its octave by an interval of seven notes.

* This must have made him to force all the elements into this active pattern sometimes without caring the similarities.
 

8. Describe the arrangement of elements in Mendeleev's periodic table? (4 Marks)

A: Mendeleev's periodic table

* Menedeleev arranged the elements known at that time in a chart in a systematic order in the increasing order of their atomic weights.

* He divided the chart into 8 vertical columns known as group.

* Each group is divided into A, B sub groups. Each column contained elements of similar chemical properties.

* The elements in the first column, for example, react with oxygen to form compounds with the general formula R2O.

* For example, Li, Na and K when react with oxygen and form compounds Li2O, Na2O and K2O respectively.

* Elements of the second column react with oxygen to form compounds with the general formula RO.

* For example, Be, Mg, and Ca when react with oxygen form BeO, MgO and CaO.

* Mendeleev tried to explain the similarities of elements in the same group in terms of their common valency.
 

9. What is Mendeleev's periodic law?  (2 Marks)

A: The periodic law

* Based on Mendeleev's observations regarding the properties of elements in the periodic table, a law known as the periodic law of the properties of elements was proposed as follows.

* The law states that the physical and chemical properties of the elements are a periodic function of their atomic weights.
 

10. Give an account of the salient features and achievements of the Mendeleev's Periodic table. (4 Marks)

A: Salient features and achievements of the Mendeleev's periodic table

a) Group and Sub - groups:

* There are eight vertical columns in Mendeleev's periodic table called as groups. They are represented by Roman numerals I to VIII.

* Elements present in a given vertical column (group) have similar properties. Each group is divided into two sub-groups 'A' and 'B'.

* The elements within any sub-group resemble one another to great a extent. For example, sub-group I A elements called 'alkali metals' (Li, Na, K, Rb, Cs, Fr) resemble each other very much in their properties.

b) Periods:

* The horizontal rows in Mendeleev's periodic table are called periods.

* There are seven periods in the table, which are denoted by Arabic numerals 1 to 7.

* Elements in a period differ in their properties from one another.

* A period comprises the entire range of elements after which properties repeat themselves.

c) Predicting the properties of missing elements:

* Based on the arrangement of elements in the table he predicted that some new elements would be discovered definitely.

* He predicted the properties of these new additional elements in advance purely depending on his table.

d) Correction of atomic weights:

* The placement of elements Mendeleev's periodic table helped in correcting the atomic masses of some elements like, beryllium, indium and gold.

* For example, at the time of Mendeleev, beryllium (Be) was given atomic weight 13.5

    Atomic weight = Equivalent weight × Valency

* The equivalent weight of Be was found experimentally as 4.5 and its valency was thought as 3. Therefore, the atomic weight of beryllium was given as 4.5 × 3 = 13.5. With this atomic weight it had to be placed in a wrong group in the table.

* He said that its valency should be only 2. Then its atomic weight would be 4.5 × 2 = 9. If atomic weight of 'Be' is 9 it would be fit in the second group and its properties practically are similar to Mg, Ca etc. of the second group elements.

* He also helped in the calculation of the correct atomic weights of 'Indium' and 'Gold' in this manner.

e) Anomalous series:

* Some anomalous series of elements like 'Te' and 'I' were observed in the table. The anomalous series contained elements with more atomic weights like 'Te' (127.6 U) placed before the elements with less atomic weights like 'I' (126.9 U).

* Mendeleev accepted minor inversions in the order of increasing atomic weights as these inversions resulted in elements being placed in the correct groups.

* It was the extraordinary thinking of Mendeleev that made the chemists to accept the periodic table and recognise Mendeleev more than anyone else as the originator of the periodic law.
 

11. What is the outcome of the analysis of Moseley's characteristic X-ray spectra?  (2 Marks)

A: * H.J. Moseley (1913) found that each element emits a characteristic pattern of X - rays when subjected to bombardment by high energy electrons.

* By analyzing the X-ray patterns, Moseley was able to calculate the number of positive charges in the atoms of respective elements.

* The number of positive charges (protons) in the atom of an element is called the atomic number of the element.

* With this analysis Moseley realised that the atomic number is more fundamental characteristic of an element than its atomic weight.
 

12. What changes took place in the periodic table after the identification of atomic number by Moseley?  (4 Marks)

A: * After knowing the atomic numbers of elements, it was recognised that a better way of arranging the elements in the periodic table is according to the increasing atomic number.

* This arrangement eliminated the problem of anomalous series. For example, though tellurium (Te) has more atomic weight than iodine (I), it has atomic number less by one unit compared to iodine.

* This atomic number concept forced the periodic law to be changed.

* The periodic law is changed from atomic weight concept to atomic number concept and now it is called the modern periodic law.

* We know that Mendeleev's periodic law is stated as "The properties of elements are the periodic functions of their atomic weights". Now, let us try to understand modern periodic law.

* The modern periodic law may stated as "the properties of the elements are periodic function of their atomic numbers."
 

13. How is the modern periodic law refined after the introduction of atomic number concept?  (4 Marks)

A: Based on the modern periodic law, the modern periodic table is proposed. It is the extension of the original Mendeleev's periodic table - known as short form of the table and this modern table is called the long form of the periodic table.

* Atomic number of an element (Z) indicates not only the positive charges i.e. the protons in the nucleus of the atom of the element but also the number of electrons in the neutral atom of that element.

* The physical and chemical properties of atoms of the elements depend not on the number of protons but on the number of electrons and their arrangements (electronic configurations) in atoms. Therefore, the modern periodic law may be stated as "The physical and chemical properties of elements are the periodic function of the electronic configurations of their atoms."
 

14. Describe the structure of the modern periodic table and the positions of elements in it?   (4 Marks)

A: ! The modern periodic table has eighteen vertical columns known as groups and seven horizontal rows known as periods.

* Let us see what decides the position of an element in modern periodic table.

* We can explain the classification of the elements in the modern periodic table in terms of certain electron arrangements which are periodically repeated.

* The elements with similar outer shell (valence shell) electronic configurations in their atoms are in the same column called Group.

* Elements listed in a group down to it are in the order of their increasing principal quantum number.

* We have learnt that 's' sub-shell with one orbital contains a maximum of two electrons. Each 'p' sub-shell contains 3 orbitals and accommodates a maximum of six electrons. The 'd' sub-shell contains 5 orbitals and accommodates a maximum of 10 electrons and 'f' sub - shell contains 7 orbitals with 14 electrons maximum.

* Depending upon to which sub - shell the differentiating electron, i.e. the last coming electron enters in the atom of the given element, the elements are classified as 's', 'p, 'd' and 'f' block elements.
 

15. Explain with examples how elements are named as 's', 'p', 'd', 'f' block elements.  (4 Marks)

A: * For example, Sodium (Na) gets its new coming electron (differentiating electron) into 3s level. Therefore 'Na' is an s-block element.

* Aluminium (Al) gets its differentiating electron into 'p' sub-shell and it is a p-block element.

* Scandium (Sc) gets its differentiating electron into 'd' sub-shell.

* Therefore it is a d - block element.

* Cerium (Ce) gets its new coming electron into 'f' sub-shell, hence, it is an f-block element.

* Let us observe the electronic configurations of the following elements. The last coming electron is underlined.


 

16. How are groups represented in the long form of periodic table?

A: * The vertical columns in the periodic table are known as groups. There are eighteen groups in long form of periodic table.

* They are represented by using Roman numeral I through VIII with letters A and B in traditional notation.

* According to latest recommendation of the IUPAC, these groups are represented by Arabic numericals 1 through 18 with no A and B designations.

* We use the latest system with the traditional heading following in parenthesis.

e.g.: Group 2 (II A); Group 16 (VI A).

* Group of elements is also called element family or chemical family.

* For example group 1 (I A) has from Li to Fr with outer shell electronic configuration ns1 and is called Alkali metal family.

17. Describe the distribution of elements in the periods of the long form periodic table.   (4 Marks)

A: * The horizontal rows in the periodic table are called periods. There are seven periods in the modern periodic table. These periods are represented by Arabic numerals 1 through 7.

* The number of main shells present in the atom of particular element decides to which period it belongs.

* For example, hydrogen (H) and helium (He) atoms contain only one main shell (K). Therefore they belongs to period - 1.

* Similarly, the elements Li, Be, B, C, N, O, F and Ne contain two main shells (K and L) in their atoms. Therefore they belong to period - 2.

* The number of elements in period depend on how electrons are filled into various shells. Each period starts with a new main shell 's' subshell and ends when the main shell in filled with respect to the 's' and 'p' subshells (except the first period).

* The first period starts with K - shell. The first main shell (K) contains only one sub- shell, the 1s. For this subshell only two types of electronic configurations are possible and they are 1s1 (H) and 1s2 (He).

* Therefore, the first period contains only two elements.

* Second period starts with the 2nd main shell (L). L - shell has two sub - shells, namely, 2s and 2p. Eight types of configurations are possible in this shell (L) like 2s1 and 2s2 and 2p1 to 2p6.

* Hence the second period contains 8 elements Li, Be, B, C, N, O, F and Ne in the order given.

* Thus, the 2nd period consists two s-block elements (Li, Be) and six p-block elements (B to Ne).

* Third period starts with third main shell (M). This shell (M) has 3 subshells, namely, 3s, 3p and 3d, but while electrons are being filled into the shell '3d' gets electrons only after '4s' is filled.

* Therefore, the 3rd period contains again 8 elements, which includes two s -block elements (Na, Mg) and six p-block elements (Al to Ar)

* Fourth main shell (N). This shell (N) has four sub - shells namely 4s, 4p, 4d and 4f, but while electrons are being filled into the shell, electrons enter the atoms in the order 4s, 3d and 4p.

* Due to this, the fourth period contains 18 elements which includes two s - block (K, Ca), 10 elements from d block (Sc to Zn) and six elements from p - block (31Ga to 36Kr). There are altogether eighteen elements in the fourth period.

On the same lines, we can explain why there are 18 elements in the fifth period ( 37Rb to 54Xe).

* There are thirty two elements in the Sixth period from 55Cs to 86Rn which includes 2 elements from s-block (6s) and 14 elements from f - block (4f).

* 10 elements from d- block (5d) and 6 elements from p - block (6p).

* '4f' elements are called Lanthanoids or lanthanides. Elements from 58Ce to 71Lu possess almost the same properties as 57La. So, the name lanthanoids is the most appropriate one for these elements.

* 7th period is incomplete and contains 2 elements from s-block (7s) and 14 elements from f-block (5f), 10 elements from d-block (6d) and some elements from p -block (7p).

* The 5f elements are called Actinoids or as Actinides. They are from 90Th to 103Lr.

* The f-block elements known as lanthanoids and actinoids are shown separately at the bottom of the periodic table.
 

18. Write a note on metals and non-metals arranged in the periodic table. (4 Marks)

A: * The elements with three or less electrons in the outer shell are considered to be metals and those with five or more electrons in the outer shell are considered to be non metals.

* We may find some exceptions to this.

* 'd' block elements (3rd group to 12th group) are metals and they are also known as transition metals and the metallic character of d- block elements decreases gradually from left to right in periodic table.

* Lanthanoids and actinoids actually belong to 3rd group (III B) which is within the transition elements: hence they are called the inner transition elements.

* Metalloids or semi-metals are elements which have properties that are intermediate between the properties of metals and non metals.

* They possess properties like metals but brittle like non metals. They are generally semi conductors.

     e.g.: B, Si, Ge.

* All elements in s-block are metals, whereas in p-block (except 18th group) there are metals, non metals and metalloids.

* In periodic table you will notice a staircase like demarcation.

* The elements to the left of this demarcation are metals and to the right are non-metals.

* The elements on staircase (or) very near to it like B, Si, As, Ge etc. are metalloids.

19. What do you understand about the periodic properties of the elements in the modern periodic table?

A: Periodic properties of the elements in the modern table

* The modern periodic table is organized on the basis of the electronic configuration of the atoms of elements.

* Physical and chemical properties of elements are related to their electronic configurations particularly the outer shell configurations.

* The atoms of the elements in a group possess similar electronic configurations.

* Therefore, we expect all the elements in a group should have similar chemical properties and there should be a regular gradation in their physical properties from top to bottom.

* Similarly, across the table, i.e. from left to right in any period elements get an increase in the atomic number by one unit between any two successive elements.

* Therefore, the electronic configuration of valence shell of any two elements in a given period is not same.

* Due to this reason elements along a period possess different chemical properties with regular gradation in their physical properties from left to right.
 

20. How does the valence of elements change in groups and periods of the modern periodic table? (4 Marks)

A: * Valence (or) valency of an element was defined as the combining power of an element with respect to hydrogen, oxygen or indirectly any other element through hydrogen and oxygen.

* Valence of an element with respect to hydrogen is the number of hydrogen atoms with which one atom of that element chemically combines.

* Valence of an element with respect to oxygen is twice the number of oxygen atoms with which one atom of that element combines.

* For example one atom of 'Na' chemically combines with one atom of 'H' to give NaH. Therefore, the valence of Na is 1. One atom of 'Ca' combines with one atom of 'O' to give CaO. So the valency of Ca is 2.

* In general, the valence of an element with respect to hydrogen is its traditional group number. If the element is in the group V or above, its valence is 8 - group number. For example, chlorine valence is 8 _ 7 = 1.

* In general, each period starts with valency 1 for 1st group elements, increases upto 4 with respect to the group number and then decreases from 4 to 3 to 2 to 1 to zero in the following groups (this is applicable only for main group elements i.e. 's' and 'p' block elements.)

* Now a days the valence of an element is generally taken as the number of valence shell (outer most shell) electrons in its atom.

* Oxidation number concept almost is the latest substitute to the valence concept in the modern literature.
 

21. What do you understand by the term 'atomic radius' of an element?    (4 Marks)

A: * Atomic radius of an element may be defined as the distance from the centre of the nucleus of the atom to its outer most shell.

* Atomic radius of an element is not possible to measure in its isolated state.

This is because it is not possible to determine the location of the electron that surrounds the nucleus.

* However, we can measure the distance between the nuclei of adjacent atoms in a solid. From this we can estimate the size of the atom by assigning half of this distance to the radius of each atom.

* This method is best suited to elements such as the metals that exist in the solid state.

* More than 75 per cent of the elements are metals and atomic radii of metals are called metallic radii.

* Another way of estimating the size of an atom is to measure the distance between the two atoms in covalent molecules.

* The size of a chlorine atom is estimated by measuring the length of the covalent bond between two chlorine atoms in a Cl2 molecule.

* Half of this distance is taken as atomic radius which is called as the covalent radius of chlorine atom.

* Atomic radius is measured in 'pm' (pico meter) units.

             1 pm = 10-12 m

22. Explain how the atomic radii vary in a group of the periodic table?      (2 Marks)

A: * Atomic radii increase from top to bottom in a group (column) of the periodic table. As we go down in a group, the atomic number of the element increases.

* Therefore to accommodate more number of electrons, more shells are required.

* As a result the distance between the nucleus and the outer shell of the atom increases as we go down the group in spite of increase in nuclear change.

23. Describe the variation of atomic radii in a period of the Periodic table.  (2 Marks)

A: * Atomic radii of elements decrease across a period from left to right. As we go to right, electrons enter into the same main shell or even inner shell in case of 'd' block and 'f' block elements.

* Therefore, there should be no change in distance between nucleus and outer shell but nuclear charge increases because of the increase in the atomic number of elements in period.

* Hence, the nuclear attraction on the outer shell electrons increases.

* As a result the size of the atom decreases.

24. Explain the terms first ionization energy and second ionization energy? (2 Marks)

A: * The energy required to remove an electron from the outer most orbit or shell of a neutral gaseous atom is called ionization energy.

* The energy required to remove the first electron from the outer most orbit or shell of a neutral gaseous atom of the element is called its first ionization energy.

* The energy required to remove the an electron from uni-positive ion of the element is called the 2nd ionization energy of that element and so on.

* M(g) + IE1  M+(g) + e- (IE1 = first ionization energy)

    M+(g) + IE2 M+2(g) + e- (IE2 = second ionization energy)


25. On what Factors does the ionization energy of an element depend?    (2 Marks)

A: Ionization energy of an element depends on

       1) Its nuclear charge.

       2) Screening effect or shielding effect.

       3) Penetration power of the orbitals.

       4) Stable configuration.

       5) Atomic radius.
 

26. How does nuclear charge affect the ఇonization energy of an element?    (1 Mark)

A: More the nuclear charge more the lonization energy.

e.g.: Between 11Na and 17Cl, chlorine atom has more ionizatoni energy.

27. How does the screening effect or shielding effect influence the ionization energy of an element?  (2 Marks)

A: Screening effect or shielding effect.

* More the shells with electrons between the nucleus and the valence shell they act as screens and decrease nuclear attraction over valence electron.

* This is called the screening effect.

* More the screening effect, less is the ionization energy.

* Between 3Li and 55Cs, the element 55Cs with more inner shells has less ionization energy.

28. In what way the penetration power of the orbitals change the value of ionization energy of the elements?  (1 Mark)

A: Penetration power of the orbitals:

* Orbitals belonging to the same main shell have different piercing power towards the nucleus, for example 4s > 4p > 4d > 4f in the penetration.

* Therefore, it is easier to remove 4f electron than 4s.

* Between 4Be 1s2 2s2 and B 1s2 2s2 2p1 , the element B has less ionization energy due to less penetration power of '2p' compared to '2s'.

29. Explain the affect of stable configuration and atomic radius on the ionization energy of an element.   (2 Marks)

A: Stable configuration: It is easier to remove one electron from 8O (1s2 2s2 2p4 ) than

7N (1s2 2s2 2p2). This is because, 7N has stable half filled configuration.

Atomic radius: More the atomic radius, less is the ionization energy. Therefore, ionization energy of 'F' is greater than that of 'I' and the ionization energy of 'Na' is more than that of 'Cs'.

* Ionization energy decreases as we go down in a group and generally increases from left to right in a period.

* Ionization energy is expressed in kJmol -1.

30. What is electron affinity? How do you account for to the 2nd electron affinity of an element?  (2 Marks)

A: * The electron affinity of an element is defined as the energy liberated when an electron is added to its neutral gaseous atom.

* Electron affinity of an element is also called electron gain enthalpy of that element.

            M(g) + e-   M-(g) + EA (EA = Electron affinity)

* The energy liberated when an electron is added to a uni-negative ion of the element is called the 2nd electron affinity of that element.

* How ever, practically no element shows liberation of energy when the 2nd electron is added to its uni-negative ion.

* It does not mean that di-negative or tri-negative ions do not form.

* They do form, but for adding 2nd electron, energy in another way like bond formation must be given.

31. How do the values of electron gain enthalpy change in a group, in a period and in metals?  (2 Marks)

A: * Electron gain enthalpy values decrease as we go down in a group, but increase along a period from left to right.

* Metals have very low electron gain enthalpy values and alkaline earth metals have even positive values.

* Note that the negative sign for energy value in table indicates that energy is liberated or lost, and the positive sign tells that the energy is gained or absorbed.

* All the factors which influence the ionization energy would also influence the electron gain enthalpy.

32. Explain the term 'electronegativity'. What factors influence it?

A: * The electronegativity of an element is defined as the relative tendency of its atom to attract electrons towards it self when it is bonded to the atom of another element.

* All the factors that influence the ionization energy and the electron affinity of elements influence the electronegativity values of these elements.

* Because of this, Millikan proposed that the electronegativity of an element is the average value of its ionization energy and electron affinity.

33. How does the value of electronegativity vary as we go down a group or across a period?   (2 Marks)

A: * Electronegativity values of elements decrease as we go down in a group and increase along a period from left to right.

* The most electronegative element is 'F' and the least electronegative stable element is 'Cs'.

34. Write a note on the properties of 'metals' and 'non - metals'.          (4 Marks)

A: * Metals generally show less electronegative character. In compounds, they generally show a tendency to remain as positive ions.

* This property is often termed as electropositive character. Metals are electropositive elements.

* Non metals are generally more electronegative due to their smaller atomic radii.

* Let us examine the elements of 3rd period.

   3rd period: Na Mg Al Si P S Cl.

* We know that Na and Mg are metals; Al and Si are semi metals (metalloids); P, S and Cl are non metals. So we find metals on left side and non metals on right side of the periodic table.

* This means metallic character decreases while non metallic character increases as we move along a period (from left to right).

* Let us take group 14 (IV A) elements.

    IV A group: C Si Ge Sn Pb

* Here also we know that carbon is non metal, Si and Ge are metalloids, Sn and Pb are metals.

* So we find non metals particularly at the right hand side top and metals at the left and right hand side bottom of the periodic table.

* This means metallic character increases while non metallic character decreases in a group as we move from top to bottom.

IV. Information Skills and Projects

1. Give Newlands table of elements. Write a brief note about the arrangement of elements in it.

A:

* Elements with similar chemical properties are to be present along a horizontal row.

* Newlands was the first to assign atomic numbers to the elements. Unfortunately his work was neither accepted by his seniors nor by the journal of the chemical society, which rejected its publication.

* In Newlands table of elements, if we start with hydrogen and move down and then start at the top the eighth element is fluorine and next eighth element is chlorine and so on.

* The properties hydrogen, fluorine and chlorine are similar.

* Similarly, if you start at Lithium, then eighth element is sodium and next coming eighth is potassium and so on. These elements show similar physical and chemical properties.

2. Write in a tabular form the 1871 version of Mendeleev's periodic table.

A:

3. Collect information about the properties of the predicted elements by Mendeleev and the observed properties of these elements discovered later. (2 Marks)

A:

4. Write in a tabular form the atomic radius of the elements in group 1 and group 17.   (2 Marks)

A:

5. Give an account of the variation of atomic radius of elements present in the 2nd and 3rd periods of periodic table.  (2 Marks)

A:

Period Element (electronegativity with respect hydrogen)
2nd period: Li (152), Be (111), B (88), C (77), N (74), O (66), F (64)
3rd period: Na (186), Mg (160), Al (143), Si (117), P (110), S (104),Cl (99)

6. Note the values of electron affinity for halogens from any source available to you.

A: Electron affinity values of halogens are (in kJ mol -1); F (-328); Cl (-349); Br (-325); I(-295) kJ mol -1. Similarly for group 16 elements, the electron gain enthalpies are O (-141); S (-200); Ge (-195) and Te (-190) kJ mol -1.

7. Collect information about the values of electronegativity of halogens and 2nd period.

A: Pauling assigned the electronegativity values for elements on the basis of bond energies. He assumed that the electronegativity of hydrogen is 2.20 and calculated the values of other elements with respect to hydrogen. Observe the values in the following example.

Period Element (electronegativity with respect hydrogen)
Halogens: F(4.0), Cl (3.0), Br (2.8), I (2.5)
2nd period: Li (1.0), Be (1.47), B(2.0), C(2.5), N(3.0), O(3.5), F(4.0), Ne(-)

V. Communication through Drawing, Model Making

Do You Know?

1. Are you familiar with musical notes?

A: In the Indian system of music, there are seven musical notes in a scale - sa, re, ga, ma, pa, da, ni. In the west, they use the notations - do, re, mi, fa, so, la, ti. Naturally, there must be some repetition on notes. Every eighth note is similar to the first one and it is the first note of the next scale.
 

2. Do you know what Mendeleev said about the melting point of eka Al?

A: 'If I hold it in my hand, it will melt'. The melting point of Ga is 30.2°C and our body temperature is 37°C.

3. How is Mendeleev honoured?

A: At the time when Mendeleev introduced his periodic table even electrons were not discovered. Even then the periodic table was able to provide a scientific base for the study of chemistry of elements. In his honour the 101th element was named Mendelevium.
 

4. Do you know how are the names of certain families of periodic table derived?

A: Alkali metal family: Aliquili = Plant ashes, Na, K etc.. were obtained from plant ash, Group I A elements are called Alkali metals family.

Chalcogen family: Chalcogenous = Ore product, as the elements in group 16 (VI A) form ores with metals. They are called as chalcogenous family.

Halogen family: Halos = Sea salt, Genus = Produced. As most of the elements in Group 17 (VII A) are obtained from nature as sea salt. They are called as halogen family.

Noble gases: As the elements of group 18 (VIII A) are chemically least active.

They are called as noble gases. There outer shell electronic configuration are basis for octet rule.
 

5. What are the arguments about lanthanoids and actinoids?

A: * 'Ide' means 'heir' and it is used generally for a change like Cl to Cl -. Cl is chlorine atom and Cl - is chloride ion. 'Oid' means 'the same'.

* Some scientists suggest lanthanoids as 57La to 70Yb, some suggest them as 58Ce to 71Lu and some take 57La to 71Lu (15 elements).

* There is another argument that even 21Sc and 39Y should be included in lanthanoids.

* All these suggestions have substance because 21Sc, 39Y and 57La to 71Lu all have the similar outer shells configurations.

* Also in the case of actinoids, there are different arguments like actinoids are from 90Th to 103Lr or 89Ac to 102No or 89Ac to 103Lr.


Writer : C.V.Sarveswara Sarma

Posted Date : 26-01-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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