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I. Conceptual Understanding
1. List the factors that determine the type of bond that will be formed between two atoms. (2 Marks)
A: * There are number of factors that determine the type of bond that will be formed between two atoms. They are
* The force of attraction or repulsion between the atoms.
* Number of valence electrons present in the valence shell of the atom.
* Electronegativity difference between the atoms.
a) If electronegativity difference between the two atoms is > 1.9, then ionic bond is formed.
b) If electronegativity difference between the two atoms is < 1.9 covalent bond is formed.
* Atomic size
* Ionisation potential and
* Electron affinity
2. Explain the difference between the valence electrons and the covalency of an element.
3. A chemical compound has the following Lewis notation. (4 Marks)
a. How many Valence electrons does element Y have?
A: Six valence electrons.
b. What is the Valency of element Y?
A: Valency of element Y is two.
c. What is the Valency of element X?
A: The valency of element X is one.
d. How many covalent bonds are there in the molecule?
A: There are two covalent bonds in the molecule
e. Suggest a name for the elements X and Y.
A: Element X = 11Na (Sodium), Element Y = 8O (Oxygen)
4. Why do only Valence electrons involve in bond formation? Why not electron of inner shells? Explain. (4 Marks)
A: * The electrons present in the outermost orbital of an atom are valence electrons. These are very active.
* The valence electrons are weakly attracted by the nucleus of the atom. So these electrons easily involve in the formation of chemical bond.
* The electrons in the inner shells do not involve in bond formation because
a) These electrons are very stable
b) These are strongly attracted by the nucleus.
5. Explain the formation of Sodium Chloride and Calcium Oxide on the basis of the concept of electron transfer from one atom to another atom. (4 Marks)
A: a) Formtion of Sodium Chloride (NaCl): This ionic compound is from the elements Sodium (Na) and Chlorine (Cl).
Formation of Cation: Sodium (Na) atom loses one electron to get octet electron configuration. Thus it becomes a cation (Na+) and gets the electronic configuration of Ne atom.
Na → Na+ + e-
Electronic Configuration: 2, 8, 1 2, 8
Formation of anion: Chlorine has shortage of one electron to get octet in its valence shell. So it gains the electron from Na atom to form anion and gets electron configuration as that of
Cl + e- → Cl-
Electronic Configuration: 2, 8, 7 (2, 8, 8)
Formation of NaCl: Due to transfer of electron from 'Na' atom to 'Cl' atom, they became
Na+ and Cl- ions.
The oppositely charged ions get attracted towards each other due to electrostatic force of attraction and the compound NaCl is formed.
Na+(g) + Cl-(g) → NaCl(s)
b) Formation of Calcium Oxide (CaO): Calcium oxide is formed from the elements Calcium (Ca) and Oxygen (O).
Formtion of Cation: Calcium atom loses two electrons to get octet electronic configuration. It becomes Ca+2 and gets the electronic configuration of Argon (Ar).
Ca - 2 e-→ Ca+2
Electronic Configuration: 2, 8, 8, 2 2, 8, 8
Formation of anion: Oxygen atom has a shortage of two electrons to get octet in its valence shell. So Oxygen will gain the electrons lost by Calcium (Ca) to form O-2 anion. Oxygen anion gets the electronic configuration of Neon (Ne).
O + 2 e- → O-2
Electronic Configuration: 2, 6 2, 8
Formation of CaO: Due to transfer of electrons from 'Ca' to 'O' atoms, they become Ca+2 and O-2 ions.
These oppositely charged ions get attracted towards each other due to electro static forces and form the ionic compound Calcium Oxide.
Ca+2(g) + O-2(g) → CaO(s)
6. A, B and C are three elements with atomic numbers 6, 11 and 17 respectively
a) Which of these cannot form ionic bond why?
b) Which of these cannot form covalent bond why?
c) Which of these can form ionic as well as covalent bonds? (4 marks)
* Element with atomic number 6 cannot form ionic bond.
* This element is Carbon. Its electronic configuration is 1s2 2s2 2p2.
* Carbon has four valence electrons. So for its stability it requires 4 more electrons.
* Hence carbon can form only covalent bonds.
* Element with atomic number 11 cannot form covalent bond.
* Sodium (Na) is the element with atomic number 11. Its Electronic Configuration is 1s2 2s2 2p6 3s1.
* Sodium atom has only one valence electron (3s1).
* Sodium atom loses one electron and gets stable inert gas electronic configuration (Ne).
* So always sodium forms only ionic bond.
* Element with atomic number 17 can form both ionic bonds as well as covalent bonds.
* This element is Chlorine (Cl). Its Electronic Configuration is 1s2 2s2 2p6 3s2 3p5.
* Chlorine atom requires one electron to get octet configuration of Argon in its valence shell.
* So Chlorine can form ionic bonds with metals.
* Chlorine can also form covalent bonds with non metals.
7. How bond energies and bond lengths of molecule helps us in predicting their Chemical Properties? Explain with examples. (4 Marks)
A: * Bond energies and Bond lengths of molecule can give a clue in predicting their chemical properties.
* If the given molecule has low bond energy and high bond length value, it means the molecule is active and is having Polar nature.
* These molecules actively participate in chemical reactions.
* The bond length of iodine molecule is 2.68 Aº and bond energy is 151 kJ/ Mole.
* So Iodine actively participate in chemical reaction.
* If a molecule has low bond length and high bond energy then that molecule is more stable and does not participate in any chemical reaction actively.
* Take HF molecule as an example. Its bond length is (H - F) 0.918 Aº and its bond energy is 570 kJ/ mole.
* So HF cannot actively participate in chemical reactions.
II. ASKING QUESTIONS AND MAKING HYPOTHESIS
8. Predict reasons for low melting point for covalent compounds when compared with ionic compound. (4 Marks)
A: * In covalent compounds, the force of attraction among covalent molecules are weak. These are van der Waals forces which are weak forces.
* As such covalent compounds have low melting point. Only small amount of heat energy is sufficient to break the covalent bonds.
* In ionic compounds strong electrostatic forces exist between the oppositely charged ions of the compound. So these compounds have high melting point.
IV: INFORMATION SKILLS AND PROJECTS
9. Collect the information about properties and uses of covalent compounds and prepare a report. (4 Marks)
A: Properties of covalent compounds:
* At room temperature these are generally gases or liquids.
* Covalent compounds have low melting and boiling points.
* These dissolve in non-polar solvents like Benzene, Carbon tetrachloride... etc. But insoluble in polar solvents like water.
* Moderately or slowly the reactions take place among covalent compounds.
* Covalent compounds are bad conductors of heat and electricity.
* The covalent bond is a directional bond. So they exhibit the phenomenon of isomerism.
Uses of covalent compounds:
* Covalent bonds make the diamond hard.
* These covalent bonds strengthen the hardness of Cement.
* Our body consists of 99% covalent compounds.
* We know the many uses of water which is a covalent Compound
* Many things around us are covalent. Even the air we breathe in contains covalent molecules of Oxygen and Nitrogen.
* The substances we take in like sugars, tea, coffee, food substances are all covalent substances.
V. COMMUNICATION THROUGH DRAWING, MODEL MAKING
10. Draw Simple diagrams to show how electrons are arranged in the following Covalent molecules.
a) Calcicum Oxide (CaO)
b) Water (H2O)
c) Chlorine (Cl2) (Each 2 Marks)
A: a) Calcium oxide
11. Represent the molecule H2O using Lewis notation. (2 Marks)
12. Represent each of the following atoms using Lewis notation. (Each 1 Mark)
a) Beryllium b) Calcium c) Lithium
A: a) Beryllium: Be
(Z = 4) Electronic Configuration: 1s2 2s2
b) Calcium: Ca
(Z = 20) Electronic Configuration: 1s2 2s2 2p6 3s2 3p6 4s2
Only two electrons are present in:
The valence shell (4s2)
c) Lithium: Li
(Z = 3) Electronic configuration: 1s2 2s1 :
13. Represent each of the following molecules using Lewis notation.
a) Bromine gas (Br2)
b) Calcium Chloride (CaCl2)
c) Carbon dioxide (CO2)
d) Which of the three molecules listed above contains a double bond? (Each 1 Mark)
A: a) Bromine gas (Br2)
b) Calcium Chloride (CaCl2)
c) Carbon dioxide (CO2)
O = C = O
d) CO2 contains double bond in the above list. Its structure is O = C = O
14. Two chemical reactions are described below.
a) Nitrogen and Hydrogen react to form Ammonia (NH3)
b) Carbon and Hydrogen bond to form a molecule of Methane (CH4)
For each reaction, give:
a) The valency of each of the atoms involved in the reaction
b) The Lewis structure of the product that is formed. (4 Marks)
A: a) 1) Nitrogen and Hydrogen react to form Ammonia. The reaction is
N2 + 3 H2→ 2 NH3
2) The valency of each atom involved in the reaction:
Valency of Nitrogen = 3
Valency of Hydrogen = 1
3) The Lewis structure of the product formed is [: NH3]
b) 1) Carbon and hydrogen bond to form a molecule of methane (CH4)
The reaction is C + 2 H2→ CH4 (Methane)
2) The valency of each atom involved in the reaction
Valency of Carbon = 4
Valency of Hydrogen = 1
3) The Lewis structure of the product formed is
VI: Appreciation and Aesthetic Sense, Values
15. How Lewis dot structure helps in understanding bond formation between atoms? (4 Marks)
A: * The valence electron with the atom of an element is represented in a short form as a dot structure by Lewis symbol.
* The nuclear and inner shell electrons of the atom is represented by the symbol of the element.
* Electrons in the outer shell of the atom are represented by dots or cross mark on the symbol of the element.
* Lewis dot structure helps in understanding the valence of an element. We can also predict its nature in forming either ionic bond or covalent bond.
16. What is octet rule? How do you appreciate the 'octet rule' in explaining the chemical properties of elements? (4 Marks)
A: Octtet rule: The atoms of elements tend to undergo Chemical changes that help to leave their atoms with eight outer - shell electrons.
* I appreciate this rule. We are left with the conclusion that eight electrons in the outershell of an atom of an element make it stable and inactive.
* Atoms of elements which do not have octet of electrons in their valence shell are very active elements to take part in a chemical reaction.
* The reactivity of these elements arises from their tendency to achieve the octet, by forming bonds either with atoms of their own type or with atoms of other elements.
17. Explain the formation of the following molecules using valence bond theory.
a) N2 molecule b) O2 molecule
A: Formation of N2 molecule
* 7N has electronic configuration 1s2 2s2 2px1 2py1 2pz1. Suppose that px orbital of one 'N' atom overlaps the 'px' orbital of the other 'N' atom giving σpx - px, bond along the inter- nuclear
* The py and pz orbitals of one 'N' atom overlap the py and pz orbital of other 'N' atom laterally, respectively perpendicular to inter-nuclear axis giving π py - py and π pz - pz bonds.
* Therefore, N2 molecule has a triple bond between two nitrogen atoms.
Formation of O2 molecule
* 8O has electronic configuration 1s2 2s2 2px2 2py1 2pz1.
* If the 'py' orbital of one 'O' atom overlaps the 'py' orbital of other 'O' atom along the inter nuclear axis, a sigma py - py bond (σ py - py) is formed.
* pz orbital of one 'O' atom overlaps the pz orbital of the other 'O' atom laterally, perpendicularly to the inter-nuclear axis giving a π pz - pz bond.
* O2 molecule has a double bond between two oxygen atoms (O = O)
18. What is hybridisation? Explain the formation of the following molecules using hybridisation.
a) BeCl2 b) BF3
A: Hybridisation: Hybridisation is a phenomenon of intermixing of atomic orbitals of almost equal eneregy which are present in the outer shells of the atom and their reshuffling or redistribution into the same number of arbitals but with equal properties like energy and shape.
Formation of BeCl2 :
* The atomic number of Beryllium = 4
* The electronic configuration of Beryllium in its ground state = 1s2 2s2
* The electronic configuration of Beryllium in its excited State = 1s2 2s1 2p1
* The two orbitals namely 2s1 and 2px1 intermix to give two equivalent 'sp' hybrid orbitals as shown below.
* The electronic configuration of chlorine is (Z = 17)
17Cl - 1s2 2s2 2p6 3s2 3px2 3py2 3pz1
* The half filled 3pz orbitals of two chlorine atoms overlap with sp hybrid orbitals of Beryllium atom in their axes to form two σ sp - p bonds.
* BeCl2 molecule formed has linear shape and the bond angle is 180º.
b) Formation of BF3 (Boron trifluoride):
* 5B has electronic configuration 1s2 2s2 2px1.
* As it has one unpaired electron (2px1) it should form only one covalent bond to give B - F molecule. But we get practically BF3 molecule.
* To explain this, it is suggested that
i. Boron (B) first undergoes excitation to get electronic configuration 1s2 2s1 2px1 2py1.
ii. As it forms three identical B - F bonds in BF3, it is suggested that excited 'B' atom undergoes hybridisation. There is an intermixing of 2s, 2px, 2py orbitals and their redistribution into three identical orbitals called sp2 hybrid orbitals.
* For three sp2 orbitals to get separated to have minimum repulsion the angle between any two orbitals is 120º at the central atom and each sp2 orbital gets one electron.
* Now three flourine atoms overlap their 2pz orbitals containing unpaired electrons (F9 1s2 2s2 2px2 2py2 2pz1 ) the three sp2 orbitals of 'B' that contain unpaired electrons to form three
σsp2 - p bonds.
Questions and answers given in the lesson
1. How do elements usually exist? (1 Mark)
A: Elements usually exist as a single atom or as a group of atoms.
2. Do elements exist as a single atom or as a group of atoms? (1 Mark)
A: Elements exist as a single atom, or as a group of atoms.
3. Are there elements which exist as atoms? (1 Mark)
A: Yes, there are elements which exists as atoms.
4. Why do some elements exist as molecules and some times as atoms? (2 Marks)
A: * Chemical compouds are formed as a result of combination of atoms or molecules of elements in different ways following different laws of chemical combination.
* So some elements exist as molecules and some times as atoms.
5. Why do some elements and compounds react vigorously while others are inert? (2 Marks)
A: * Some elements and compounds react vigorously because the outer shell of the atoms of these are not filled with eight electrons.
* Some elements and compounds are inert because the outer shell of the atoms of these are filled with eight electrons.
6. Why is the chemical formula for water H2O and for Sodium chloride is NaCl, why not HO2 and NaCl2? (1 Mark)
A: Chemical formula represents the valencies of atoms participating in forming molecules of the compound.
7. Why do some atoms combine while others do not? (1 Mark)
A: * Atoms which have eight outer-shell electrons do not combine.
* Atoms which do not have eight outer-shell electrons combine.
8. Are elements and compounds simply made up of separate atoms individually arranged? (1 Mark)
A: No. Elements and componds are not simply made up of separate atoms individually arranged.
9. Is there any attraction, between the atoms? (1 Mark)
A: Yes, there is attraction between the atoms.
10. What is that holding them (atoms)? (1 Mark)
A: Atoms are hold together by force of attraction.
11. Why there is absorption of energy in certain chemical reactions and release of energy in other reactions? (1 Mark)
A: This is because of the bond energy between the atoms in a molecule.
12. Where the absorbed energy goes? (1 Mark)
A: The absorbed energy is useful for breaking chemical bonds between atoms in a molecule.
13. Is there any relation to energy and bond formation between atoms? (1 Mark)
A: There is relation to energy and bond formation between atoms.
14. What could be the reason for the change in reactivity of elements? (1 Mark)
A: The number of electrons present in the outer shell of the atoms of the elements are responsible for the change in reactivity of the elements.
15. What could be the reason for the inertness of noble gases? (1 Mark)
A: The atoms of the elements of noble gases have eight outer-shell electrons. So they are inert.
16. What did you notice in Lewis dot structure of Noble gases and electronic configurations of the atoms of these elements? (1 Mark)
A: Except Helium, other noble gas elements Ne, Ar, Kr... etc have 8 electrons in their outer shell of their atoms.
17. What have you observed from the conclusions about the main groups? (2 Marks)
A: * Group IA elements try to lose one valence shell electron.
* Group IIA elements try to lose two valence shell electrons.
* Group IIIA elements try to lose two valence shell electrons.
* Group VIA elements try to gain two electrons into their valence shell electrons.
* Group VIIA elements try to gain one electron into the valence shell.
* Group VIIIA elements do not try to lose or gain electrons.
18. Why do atoms of elements try to combine and form molecules? (1 Mark)
A: Atoms of elements try to combine and form molecules to attain stable electronic configuration.
19. Is it accidental that IA to VIIA main group elements during chemical reactions get eight electrons in the outermost shells of their ions, similar to Noble gas atoms? (1 Mark)
A: It is not accidental.
20. How do cations and anions of an ionic compound exist in its solid state? (1 Mark)
A: Ionic compounds exist in crystalline solids. So cations and anions are present in the lattice of the crystal.
21. Do you think that pairs of Na+, Cl - as units would be present in the solid crystal? (2 Marks)
A: * In Sodium chloride crystal each Na+ is surrounded by 6 Cl - and each Cl - is surrounded by six Na+ ions.
* Ionic compounds in crystalline state consist of orderly arranged cations and anions held together by electrostatic forces of attractions in three dimensions.
22. Can you explain the reasons for all these? (1 Mark)
A: Ionic bond is formed between atoms of elements with electronegativity difference equal to or greter than 1.9
23. Can you say what type of bond exists between atoms of Nitrogen molecule? (1 Mark)
A: A triple bond exists between atoms of Nitrogen molecule.
24. What do you understand from bond lengths and bond energies? (1 Mark)
A: Bond lengths and bond energies are not the same when that atoms the form the bond are different.
25. Are the values of bond energy not different for the bonds between different types of atoms? (1 Mark)
A: The value of bond energy different for the bonds between different types of atoms.
26. What is the bond angle in a moelcule? (2 Marks)
A: Bond angle is the angle subtended by two imaginary lines that pass from the nuclei of two atoms which form the covalent bonds with the central atom through the nucleus of the central at the Central atom.
27. How is HCl molecule formed? (1 Mark)
A: The '1s' orbital of 'H' atom containing unpaired electron overlaps the 3p orbital of Chlorine atom containing unpaired electron of opposite spin.
28. Explain the formation of ionic compounds NaCl, MgCl2, Na2O and AlF3 through Lewis electron dot symbols. (Formulae) (4 Marks)
a) Formation of Sodium chloride (NaCl):
* Sodium chloride is formed from the elements sodium and chlorine. It can be explained as follows:
Na (s) + 1/2 Cl 2(g) NaCl (s)
When Sodium (Na) atom loses one electron to get octet electron configuration it forms a cation (Na+) and gets electron configuration that of Neon (Ne) atom.
11Na(g)→ 11Na+(g) + e-
Electronic configuration: 2, 8, 1 2, 8
[Ne] 3s1 [Ne]
Chlorine has shortage of one electron to get octet in its valence shell. So it gains the electron from Na atom to form anion and gets electron configuration as that of Argon (Ar).
17Cl(g) + e- → Cl-(g)
Electronic configuration: 2, 8, 7 2, 8, 8
[Ne] 3s23p5 [Ne] 3s2 3p6 or [Ar]
Formation of the compound NaCl from its ions:
Transfer of electrons between 'Na' and 'Cl' atoms, results in the formation of Na+ and Cl - ions. These oppositely charged ions get attracted towards each other due to electrostatic forces and form the compound Sodium chloride (NaCl)
Na+(g) + Cl -(g) → Na+ Cl -(s) or NaCl
b) Formation of Magnesium chloride (MgCl2):
* Magnesium chloride is formed from the elements Magnesium and Chlorine. The bond formation MgCl2 in brief using chemical equation is as follows:
Mg(s) + Cl2(g) → MgCl2(s)
11 Mg (g)→ 11Mg2+(g) + 2 e-
Electronic configuration: 2, 8, 2 2, 8
[Ne] 3s2 [Ne]
2 Cl(g) + 2 e- → 2 Cl - (g)
Electronic configuration: 2, 8, 7 2, 8, 8
[Ne] 3s2 3p5 [Ne] 3s2 3p6 or [Ar]
4) The compound MgCl2 formation from its ions:
Mg2+ gets 'Ne' configuration and each Cl- gets 'Ar' configuration
Mg2+(g) + 2 Cl -(g) MgCl2(s)
* One 'Mg' atom transfers two electrons one each to two 'Cl' atoms and so formed Mg2+ and 2 Cl - attract to form MgCl2.
c) Formation of di sodium monoxide (Na2O):
* Disodium monoxide formation can be explained as follows:
Cation formation (Na+ formation):
2 Na(g)→ 2 Na+(g) + 2 e-
Electronic configuration: 2, 8, 1 2, 8
[Ne] 3s1 [Ne]
Anion formation (O2-, the oxide formation):
O(g) + 2 e- → O2-(g)
Electronic configuration: 2, 6 2, 8
[Ne] 2s2 2p4 [He] 2s22p6 or [Ne]
* The compound Na2O formation from its ions is has shown.
2 Na+(g) + O2-(g) → Na2O(s)
Two 'Na' atoms transfer one electron each to one oxygen atom to form 2 Na+ and O2-.
* Each Na+ gets 'Ne' configuration and O2- gets 'Ne' configuration. These ions (2 Na+ and O2-) attract to form Na2O.
d) Formation of Aluminium chloride (AlCl3):
* Aluminium chloride formation can be explained as follows:
Formation of aluminium (Al3+), the cation:
13Al(g) → Al3+(g) + 3 e-
Electronic configuration: 2, 8, 3 2, 8
[Ne] 3s2 3p1 [Ne]
Formation of chloride ion (Cl-), the anion:
3 Cl(g) + 3 e- → 3 Cl-(g)
Electronic configuration: 2, 8, 7 2, 8, 8
[Ne] 3s2 3p5 [Ne] 3s2 3p6 OR [Ar]
* Each aluminium atom loses three electrons and three chlorine atoms gain them, one electron each.
* The compound AlCl3 is formed from its component ions by the electrostatic forces of attractions.
Al3+(g) + 3 Cl-(g) → AlCl3(g)
1. Write the Lewis structures of the given elements in the table. Also consult the periodic table and fill in the group number of the element. (4 Marks)
ADDITIONAL QUESTIONS AND ANSWERS
I. Conceptual Understanding
1. How many forces do Scientists know in early twentieth century? (1 Mark)
A: Scientists know about three types of forces. They are gravitational, magnetic and electrostatic forces.
2. What are the earlier views with regard to the formation of bonds between atoms? (4 Marks)
A: * Scientists knew about the existence of electrons and protons. So it was believed that electrostatic forces were the cause of attraction between atoms in a molecule.
* When two atoms come sufficiently close together the electrons of each atom experience the attractive force of the nucleus of the other atom.
* But the elecltrons which are negatively charged repel each other, and the positively charged also repel each other.
* The strength of attraction or repulsion will decide bond formation.
* If attraction is more than repulsion then the atoms combine.
* If repulsion is more than attraction then the atoms do not combine.
* The nucleus and the electrons in the inner shell remain unaffected when atoms come close together.
* But the electrons in the outer most shell (valence shell) of atom gets effected.
* Electrons in the valence shell are responsible for the bond formation between atoms.
3. How do the discovery of Noble gases helped us in understanding the formation of chemical bonds among the atoms of the elements? (2 Marks)
A: * The discovery of Noble gases and the understanding of their electronic configurations helped us in explaining the formation of chemical bonds among the atoms of the elements.
* Noble gases which belong to zero group (18th group or VIII A) are typical gases with almost negligible chemical activity when compared to other element.
* They undergo few or no chemical changes.
* They are more stable and do not form molecules by allowing their atoms to combine among themselves or with the atoms of other elements.
4. How do you write Lewis dot structure of Argon and Sodium? (2 Marks)
A: * First let us write the symbol of element Argon Ar.
* We put the valence electrons around the symbol. Put two dots at a time on each of the four sides of the symbol of the element till all are used up. So, we get
* Similarly, for Sodium, the number of valence electrons in Sodium is one and the symbol is Na, we can also use cross mark for the electrons. The Lewis structure for Sodium atom is therefore:
5. Write the Lewis dot structures of the atoms of Noble gases? (4 Marks)
A: The Lewis dot structures of the atoms of Noble gases are
6. Explain electronic theory of valence given by Lewis and Kossel with an example. (4 Marks)
A: * Kossel and Lewis in 1916 gave a satisfactory explanation to explain the chemical bond formation between atoms in terms of electrons.
* They provided logical explanation of valence on the basis of lack of chemical activity of Noble gases which led to the proposal of octet rule.
* When Group IA, IIA, IIIA, IVA, VA, VIA, VIIA are allowed to undergo chemical changes, they try to get octet elelctronic configuration in the outer shells.
e.g.: 11Na → 2, 8, 1; 11Na+ → 2, 8
* Group IIA elements (Mg to Ba) try to lose two valence electrons from their atoms during chemical changes and form di-positive ions with the octet in the outer shells.
e.g.: 12Mg → 2, 8, 2; 12Mg2+→ 2, 8
* Group IIIA elements try to lose three valence electrons from their atoms and form corresponding tri positive ions with octates in the outer shells.
13Al → 2, 8, 3; 13Al3+ → 2, 8
7. Explain how Lewis - Kossel electronic theory of valence can be applied to Group VIA, VIIA and VIIIA elements? (4 Marks)\
A: Group VIA elements try to gain two electrons into the valence shells of their atoms during the chemical changes and form corresponding di negative anions which get octet in
8O → 2, 6; 8O2-→ 2, 8
* Group VIIA elements try to gain one electron into the valence shells of their atoms during the chemical changes and form corresponding uni-anions which get octet in their outer shells.
9F → 2, 7; 9F- → 2, 8
* Group VIIIA elements, the Noble gases do not try to lose or gain electrons. Generally, Helium and Neon do not participate in chemical changes. Even other elements of VIIIA do not gain or lose electrons from their atoms when they participate in a very few chemical changes.
8. Give in a tabular form mentioning the gain electrons of non metals and lose electrons of metals to get magic octet of Noble gases. (2 Marks)
9. What are the proposals of Kossel about the formation of Ionic bond? (4 Marks)
A: Kossel proposed the ionic bond (electrostatic bond) based on the following facts.
i. Ionic bond is formed between atoms of two dissimilar elements due to transfer of electrons from the atom of one element to the other.
ii. There are highly reactive metals like alkali metals (IA) and highly reactive non-metals like halogens (VIIA) on the left side and right side of the periodic table respectively.
iii. Noble gases except Helium have eight electrons in the valence shells of their atoms. They are chemically inactive and stable.
iv. To attain eight electrons in the outermost shell similar to noble gases metal atoms that have one, two or three electrons in the valence shells generally lose those electrons and form
stable positive ions called cations.
10. How is an Ionic bond formed? Define Ionic bond. (2 Marks)
A: * The positive ions (cations) and negative ions (anions) that are formed due to the transfer of electrons from the metal atoms to the non-metal atoms experience the electrostatic forces and get attracted to form chemical bond.
* As this bond is between charged particles known as ions, it is called Ionic bond.
* Sometimes based on the forces being electrostatic, the bond is also called the electrostatic bond.
* As the valence concept has been explained in terms of electrons, it is also called the electrovalent bond. Now, we can define ionic bond as follows;
* The electrostatic attractive force that keeps cation and anion (which are formed from metal atoms and non-metal atoms due to transfer of electrons) together to form a new electricallyneutral compounds is called Ionic bond.
11. What are the factor that affect the formation of cation and anion? (4 Marks)
A: * Generally elements of metals have tendency of losing electron to attain the octet in their valence shell. This property is called as the metallic character or electropositivity. Elements withmore electropositive character form cations. Similarly non mentals like Oxygen (8O), fluorine (9F) and chlorine (17Cl) acquire electron configuration of element of inert gases by gaining electrons. This property is called the non-metallic character or electronegativity of the element. Elements with more electronegative character form anions.
* The tendency of losing electrons to form cations (or) gaining electron to form anions depends on the following factors:
i. Atomic size
ii. Ionisation potential
iii. Electron affinity
* The atoms of elements with low ionisation energy, low electron affinity high atomic size and low electronegativity form cations.
* The atoms of elements with high ionisation potential, high electron affinity, small atomic size and high electronegativity form anions.
12. How is a covalent bond formed? (2 Marks)
A: * G.N. Lewis (1916) proposed that atoms of some elements could achieve an octet in their valance shells without transfer of electrons between them. They can attain octet configuration in their valance shells by sharing the valence electrons with one or more atoms.
* The electrons shared between two atoms belong to both the atoms and the sharing of electrons between them leads to the formation of a chemical bond known as covalent bond.
13. Define 'Bond length' and 'Bond energy'. (2 Makrs)
A: * Bond length or bond distance is the equilibrium distance between the nuclei of two atoms which form a covalent bond. It is generally given in nm (nano meter) or Aº (Angstrom Unit).
* Bond energy or Bond dissociation energy is the energy needed to break a covalent bond between two atoms of a diatomic covalent compound in its gaseous state.
14. What are the drawbacks of electronic theory of valance? (4 Marks)
A: Drawbacks of electronic theory of valance:
a) When a covalent bond is formed between any two atoms, irrespective of the nature of the atoms, the bond lengths and bond energies are expected to be the same.
* This is because any covalent bond between any two atoms is a result of mere sharing of two idential electrons.
* But, practically it was observed that bond lengths and bond energies are not same when the atoms that form the bond are different.
b) The theory could not explain why Cl Cl in BeCl2 is 180º, F F in BF3 is 120º, H
15. What is VSEPRT? Who proposed it? Who developed it? (2 Marks)
A: To explain the bond angles in the molecules with three or more than three atoms with all atoms attached to a central atom through covalent bonds a theory called the valence - shell - electron - pair repulsion - theory (VSEPRT) was proposed by Sidgwick and Powell (1940). It was further improved by Gillespie and Nyholm (1957).
16. Explain the salient features of VSEPRT theory. (4 Marks)
A: VSEPRT theory suggests the following points.
a) VSEPRT consider electrons in the valence shells which are in covalent bonds and in lone pairs as charge clouds the repel one another and stay as far apart as possible.
* This is the reason why molecules get specific shapes.
b) If we know that total number of electron pairs in the valence shell as covalent bonds and lone pairs in the central atom, it will help us to predict the arrangement of those pairs around the nucleus of the central atom and from that the shape of the molecule.
c) Lone pairs occupy more space around the central atom than bond pairs.
* Lone pair means unshared electron pair or non - bonding electron pair.
* These are attracted to only one nucleus where as the bond pair is shared by two nuclei.
* Thus, the presence of lone pairs on the central atom causes light distortion of the bond angles from the expected regular shape.
* If the angle between lone pair and bond pair increases at the central atom due to more repulsion, the actual bond angles between atoms must be decreased.
d) If two bond pairs are present in two covalent bonds around the nucleus of the central atom without any lone pairs in the valence shell, they must be separated by 180º to have minimum repulsion between them. Thus, the molecule would be linear.
e) If three bond pairs are there in three covalent bonds around the nucleus of the central atom, without any lone pairs they get separated by 120º along three corners of a triangle. Therefore, the shape of the molecule is trigonal planar.
f) If there are four bond pairs in the valence shell of the central atom, the four bond pairs will orient along the four corners of a tetrahedron (three dimensional arrangement) and the bond angle expected is 109º 28'.
g) If there are three bond pair and one lone pair i.e. unshared electron pair, then the lone pair occupies more space around the nucleus of the central atom. The remaining three bond pairs come relatively closer as in NH3 molecule.
h) If there are two bond pairs and two lone pairs of electrons around the nucleus of the central atom in its valence shell, lone pair - lone pair repulsion is greater than lone pair - bond pair repulsion. Therefore, the angle between bond pairs further decreases.
17. What is bond angle in a molecule? (1 Mark)
A: It is the angle subtended by two imaginary lines that pass from the nuclei of two atoms which form the covalent bonds with the central atom through the nucleus of the central atom at the central atom. (see figure 'α' is the bond angle)
18. Why does VSEPRT fail in explaining the strength of the bonds? (1 Mark)
A: * Valence Shell Electron Pair Repulsion Theory (VSEPRT) mainly fails in explaining the strengths of the bonds.
* This is because; the theory still depends on the Lewis concept of covalent bond formation.
* It could not say anything extra about the electronic nature of covalent bonds.
19. Why Linus Pauling suggested valence bond theory? (1 Mark)
A: To describe covalent bonding a quantum mechanical model called valence bond theory has been suggested by Linus Pauling (1954).
20. How is valence bond theory explained? (4 Marks)
A: To describe covalent bonding a quantum mechanical model called valence bond theory has been suggested by Linus Pauling (1954). It is explained as follows:
a) A covalent bond between two atoms is formed when the two atoms approach each other closely and one atom overlaps its valence orbital containing unpaired electron, the valence orbital of the other atom that contians the unpaired electron of opposite spin.
* The so formed paired electrons in the overlapping orbitals are attracted to the nuclei of both the atoms. This bonds the two atoms together.
e.g.: In the formation of H2 molecule, the 1s orbital of one 'H' atom containing an unpaired electron overlaps the '1s' orbital of the other 'H' atom containing unpaired electron of opposite spin giving H-H bond and H2 molecule.
b) The greater the overlapping of the orbitals that form the bond, the stronger will be the bond. This gives a directional character to the bond when other than 's' orbitals are involved.
c) Each bonded atom maintains its own atomic orbitals but the electron pair in the overlapping orbitals is shared by both the atoms involved in the overlapping.
d) If two atoms form multiple bonds between them the first bond is due to the overlap of orbitals along the inter-nuclear axis giving a stronger sigma (σ) bond.
* After formation of (σ) bond the other bonds are formed due to the overlap of orbitals side wise or laterally giving weaker π bonds.
* The 'σ' bond is stronger because the electron pair shared is concentrated more between the two nuclei due to end-end or head on overlap and attracted to both the nuclei.
* The π bond overlap gives a weaker bond due to the lateral overlap of 'p' orbitals which is not to greater extent.
21. Explain the formation of Ammonia molecule according to valence bond theory - hybridisation.
* One nitrogen atom and three hydrogen atoms are present in one ammonia molecule. All the three N - H bonds are of same strength and HH = 107º 48' .
* 7N has electronic configuration 1s2 2s2 2px1 2py1 2pz1.
* If three hydrogen atoms overlap their 1s orbitals on the three 'p' orbitals of nitrogen atom, they give identical σ p - s bonds but, then the HH should be equal to 90º where as 107º 48'.
* To explain the discrepancy in the bond angle 'N' atom is said to undergo sp3 hybridisation.
* In this process '2s' and 2px, 2py, 2pz orbitals of nitrogen intermix and redistribute into four identical sp3 orbitals.
* One of the four sp3 orbitals get a pair of electrons and the other three sp3 orbitals get one electron each.
* Now hydrogen atoms overlap their 1s orbitals containing unpaired electrons the sp3 orbitals of 'N' atom containing unpaired electrons to give three σ s - sp3 bonds.
* HH should be 109º 28' for sp3 hybridisation. As there is a lone pair in one of the sp3 orbitals, there is a greater lone pair-bond pair repulsion which decreases the bond angle HH to 107º48'.
A: * It is found that HH is 104º 31'.22. How do you explain the shape of water molecule keeping in view valence bond theory.
* The electronic configuration of 8O is 1s2 2s2 2px2 2py1 2pz1 and 1H is 1s1.
* Therefore, there should be two σ s-p bonds due to the overlap of 's' orbitals of two hydrogen atoms, the 'p' orbitals of oxygen atom which contain unpaired electrons. HH should be 90º.
* But HH observed is 104º31'
* To explain this, sp3 hybridisation is suggested for the valence orbitals of 'O' atom.
* One s-orbital (2s) and three 'p' orbitals (2px, 2py, 2pz) intermix and redistribute into four identical sp3 orbitals.
* As there are six electrons and two sp3 orbitals get pairs and two sp3 orbitals get one electron each.
* Now, the two sp3 orbitals of 'O' atom overlap the 's' orbitals of two hydrogen atoms to give σ sp3-s bonds.
* Due to the lone pair-lone pair repulsions and lone pair-bond pair repulsions H
23. Distinguish between a σ bond and a π bond. (2 Marks)
24. Differentiate between Ionic bond and Covalent bond. (2 Marks)
25. What is polar covalent bond? Give an example. (2 Marks)
A: * If the covalent bond is between atoms of two different elements, the shared electron pair shift more towards the atom of more electronegative element.
* Thus within the molecule the more electronegative atom bears a partial negative charge and the less electronegative atom bears a partial positive charge.
* A molecule of this type which is neutral but possesses partial charges on the atoms within the molecule is called a polar molecule and the bond is called a polar covalent bond or partial ionic and partial covalent bond.
26. Write a note on Ionic compounds. (2 Marks)
A: * In Ionic compounds there exist stronger electrostatic forces of attractions between the oppositely charged ions of these compounds.
* Therefore, they are solids with high melting points and boiling points.
* Based on the principle "like dissolves in like", they being highly polar, soluble in polar solvents.
* In chemical reaction of solution of ionic compounds simply rearrangement of ions take place in the solutions, the reactions are instantaneous or very fast.
27. Write about the properties of covalent compounds. (4 Marks)
A: * The forces of attractions among covalent molecule are weak.
* Therefore, the covalent compounds are gases or liquid at room temperature.
* They have low melting points and low boiling points.
* Based on the principle that like dissolve in like these covalent compounds are soluble in non-polar solvents because of non-polar nature of solvent molecules.
* In chemical reactions of covalent compounds there exist bond breaking and bond forming to get products.
* Therefore, these reactions are moderate or very slow.
28. What do you understand when we say 'Like dissolves in like'. (1 Mark)
A: Like dissolves in like means what type of chemical bonds are there in the solute particles that solute could be soluble in that solvent which has the similar type of chemical bonds in its molecules.
IV: Information Skills and Projects
1. Collect the information about the bond lengths and bond energies of different molecules.
2. Write the physical properties of Ionic and Covalent compounds by collecting information.
A: Properties of ionic and covalent compounds.
* From the above table we understand that ionic compounds like NaCl are solids at room temperature.
* Polar compounds like HCl possess properties like melting point, boiling point, reactivity, solubility etc, between those of ionic compounds and covalent compounds.
* If the covalent bond is between atoms of two different elements, the shared electron pair shift more towards the atom of more electronegative element.
V. COMMUNICATION THROUGH DRAWING, MODEL MAKING
1. Draw the crystal structure of Sodium Chloride and explain it. (4 Marks)
A: * NaCl is said to possess face centred cubic lattice Crystal structure.
* The number of ions of opposite charge that surrounds a given ion of given charge is known as the coordination number of that given ion.
* In Sodium Chloride Crystal the coordination number of Na+ is 6 and that of Cl - is also 6.
* The crystal structure of Sodium Chloride is shown in the diagram.
2. Explain the formation of Covalent bond in flourine molecule with Lewis dot structures. (4 Marks)
A: * Take two fluorine atoms which form a stable molecule.
* Each fluorine atom contributes one electron for bonding and the electron pair that is formed in this way is mutually shared by both the fluorine atoms.
* Each atom in the F2 molecule has an octet of valence electrons.
* The dots around fluorine atom shows the valence electrons of respective atoms.
* The chemical bond formed between two atoms by mutual sharing of a pair of valence shell electrons so that both of them can attain octet or duplet in their valence shell is called the covalent bond.
3. Describe the formation double covalent bond in Oxygen molecule with Lewis dot structures. (4 Marks)
A: * The electronic configuration of 8O is 2, 6. Oxygen atom has six electrons in its valence shell.
* It requires two more electrons to get octet in its valence shell.
* Therefore oxygen atoms come close and each oxygen atom contributes two electrons for bonding.
* Thus, there exist two covalent bonds between two oxygen atoms in O2 molecule as there are two pairs of electrons shred between them.
* We can say that a double bond is formed between two oxygen atoms in O2 molecule. Observe the following figures. Both the Oxygen atoms have octet in the valence shell.
4. Describe with the help of Lewis dot structures how a triple covalent bond is formed in a Nitrogen molecule. (4 Marks)
A: * The electronic configuration of 'N' atom is 2, 5 and to have octet in the valence shell it requires three more electrons.
* When two nitrogen atoms approach each other, each atom contributes 3 electrons for bonding.
* There are six electrons shared between two nitrogen atoms in the form of three pairs.
* Therefore, there is a triple bond between two nitrogen atoms in N2 molecule.
5. With the help of Lewis dot structures describe the formation of covalent bonds in a Methane molecule.
A: * In the formation of Methane, CH4 molecule, carbon contributes 4 electrons, (one electron to each hydrogen atom) and 4 hydrogen atoms contribute one electron each.
* Thus in CH4 molecule, there are four C-H covalent bonds as shown below.
6. Write the Lewis dot structures of Ammonia molecule during its formation. (4 Marks)
A: * In ammonia molecule, three N-H single covalent bonds are present. Electron configuration of 7N is 2, 5 and 1H is 1.
* Nitrogen atom contributes three electrons for bonding. Three hydrogen atoms at the same time contribute one electron each for bonding.
* Six electrons form three pairs and each pair is shared between nitrogen and one hydrogen atom as shown below:
7. Explain the formation of water molecule with Lewis dot structures.
A: In water molecule (H2O) there are two O-H single covalent bonds. Electron configuration of 8O is 2, 6 and 1H is 1. Oxygen atom needs '2' electrons to attain octet in its valence shell. Therefore it shares '2' electron with two hydrogen atoms to form water (H2O) molecule. The total number of covalent bonds that an atom of an element forms is called its covalency.
8. If two bonds pairs are present in two covalent bonds around the nucleus of the central atom without any lone pairs in the valence shell, they must be separated by 180º to have minimum repulsion between them. Represent a molecule diagramatically to explain this.
9. Draw the formation of BF3 molecule and explain its shape. (2 Marks)
* If three bond pairs are there in three covalent bonds around the nucleus of the central atom without any lone pairs they get separated by 120º along three corners of a triangle.
* Therefore the shape of BF3 molecule is trigonal - Planar.
10. Draw Methane Molecule. (2 Marks)
● In Methane molecule (CH4)
11. Draw the shape of NH3 (Ammonia) molecule and explain how the bond angle is formed. (2 Marks)
Ammonia (NH3) molecule
* In Ammonia molecule there are three bond pairs in covalent bonds (3 N - H) around the nucleus of the nitrogen atom.
* Lone pair- bond pair repulsion is greater than bond pair- bond pair repulsion.
* Therefore, NH3 which is expected to be tetrahedral with four electron pairs in the valence shell and = 109º.28', it has
* The shape of the NH3 molecule is triagonal pyramidal with N at the apex of the Pyramid.
12. Explain with a neat diagram the formation of water molecule.
A: * In water molecule (H2O) there are four electron parts around the nucleous of oxygen atom, but, two of them are lone pairs and two bond pairs.
* Therefore, H2O molecule gets 'V' shape or bent shape or angular instead of tetrahedral shape as that of CHdue to lone pair- lone pair and lone pair- bond pair repulsions.
* HH is 104º 31'.
13. Explain the formation of Chlorine molecule with reference to Valence bond theory. (2 Marks)
A: 17Cl 1s2 2s2 2p6 3s2 3px2 3py2 3pz1
In the formation of Cl2 molecule, the 3pz orbital of one chlorine atom containing an unpaired electron overlaps the 3pz orbital of other Chlorine atom that contains unpaired electron of opposite spin.
14. On the basis of Valence bond theory explain the formation of N2 molecule. (2 Marks)
A: * 7N has electronic configuration 1s2 2s2 2px1 2py1 2pz1
* Suppose that px orbital of one N atom overlaps the px orbital of the other 'N' atom giving σ px - px bond along the inter-nuclear axis.
* The py and pz orbitals of one N-atom. Overlap the py and pz orbital of other 'N' atom laterally, respectively perpendicular to inter-nuclear axis giving π py - py and π pz - pz bonds.
* Therefore, N2 molecule has a triple bond between two nitrogen atoms.
Do you know?
1. Davy's Experiment
Humphry Davy (1778-1819), a Professor of Chemistry at the Royal Institution in London, constructed a battery of over 250 metallic plates. In 1807, using electricity from this battery, he was able to extract highly reactive metals like Potassium and Sodium by electrolysis of fused salts.
It was seen that the metal part of the compound migrated towards the negative electrode and the non-metal part towards the positive electrode. So it was proposed that metals are responsible for positively charged particles and non-metals are responsible for negatively charged particles. The oppositely charged particles are held together by electrostatic forces in a compound. Do you agree with this explanation? Why? While this explanation could explain bonding in NaCl, KCl etc. it could not explain bonding in carbon comopounds or diatomic molecules of elements.
2. The number of electrons lost from a metal atom is the valence of its element which is equal to its group number.
e.g.: Na and Mg have valence 1 and 2 respectively.
3. The number of electrons gained by a non-metal for its atom is its valency, which is equal to (8- its group number).
e.g.: The valency of Chlorine is (8 - 7) = 1
4. * An Angstrom (Aº) is a unit of length equal to 10-10 metre, or 0.1 nanometre, or 100 picometre.
* 1 nanometre = 10-9 metre.
Writer: C.V. Sarveswara Sarma