Dimitri Ivanovitch Mendeleev was born on 8th february 1834 as the 17th child in the family of the head master of a local high school in Tobolaok. He could not succeed in the entrance examination to Moscow university.
Mendeleev started working on periodic law in late 1860's. He was exhorted that he could not survive for more than six months as he had a defect in the lung at his 21st year.
Mendeleev passed away just five days before his 73rd birthday on 2nd february 1907. The 101st element was named 'Mendeelevium' in 1955 after 50 years of his death.
KEY WORDS
* Triad * Octave
* Periodic law * Periodic table
* Period * Group
* Lanthanides * Actinides
* Metalloids * Periodicity
* Ionization energy * Electron affinity
* Element family * Metalloids
* Electronegativity * Electropositivity
KEY WORDS - EXPLANATION
Triad: In a group of three elements the atomic weight of the middle element is the average of the atomic weights of the first and third elements.
Octave: 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.
Periodic law: The periodic law states that the physical and chemical properties of elements are a periodic function of their atomic weights.
Periodic table: The charts of elements arranged in a systematic order of increasing atomic weight or atomic number is called periodic table.
Period: The horizontal rows in Mendeleev's periodic table are called periods.
Group: The vertical columns in Mendeleev's periodic table are called groups.
Lanthanides: '4f' elements are called Lanthanides or Lanthanoids. Elements from 58Ce to 71Lu possess almost the same properties as 57La . So the name Lanthanoids is most appropriate one for these elements.
Actinides: The 5f elements are called Actinoids or as Actinides. They are from 90Th to 103Lr.
Element family: Group of elements is also called element family or chemical family. For example group 1 (I A) has from Li to Fr with outershell electronic configuration ns1 and is called alkali metal family.
Metalloids: Metalloids or semi-metals are elements which have properties that are intermediate between the properties of metals and non-metals. They are generally semi conductors.
e.g.: B, Si, Ge.
Periodicity: In the periodic table, the properties of elements change gradually with a change in their electronic configurations. This trend repeats itself at regular intervals. This repetition of characters is called 'periodicity'.
Atomic radius: Atomic radius of elements may be defined as the distance from the centre of the nucleus of the atom to its outermost shell.
Ionization energy: The energy required to remove an electron from the outermost orbit or shell of a neutral gaseous atom is called ionization energy.
Electron affinity: The electron affinity of an element is defined as the energy liberated when an electron is added to its neutral gaseous atom.
Electronegativity: The electronegativity of an element is defined as the relative tendency of its atom to attract electrons towards itself when it is bonded to the atom of another element.
Electropositivity: Metals generally show less electronegative character. In compounds they generally show a tendency to remain as a positive ion. This property is often termed as electropositive character. Metals are electropositive elements.
SYNOPSIS
* At the time of Robert Boyle (1661) only 13 elements were known.
* Towards the end of the eighteenth century, by the time of Lavoisier another 11 elements were discovered.
* By 1865 about 63 elements were known.
* By 1940, a total of 91 elements from natural sources and another seventeen elements synthetically were obtained.
* By now, including synthetic elements, there are more than 115 elements.
* There was a need to classify these elements.
* Chemists started to frame ways to group these elements and compounds on the basis of their physical and chemical properties.
* In the beginning of 18th century Joseph Louis Proust stated that hydrogen atom is the building material and atoms of all other elements are simply due to the combination of number of hydrogen atoms.
* A German chemist Johann Wolfgang Dobereiner (1829) noted that there were groups of elements with three elements known as triads in each group with similar chemical properties.
* According to Dobereiner the atomic weight of the middle element is the average of the atomic weights of the first and third element.
* This law failed for very low mass and for very high mass elements.
* Newlands a British chemist in 1865 found that when elements are arranged in ascending order of their atomic weights, they appeared to 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. This is law of octaves.
* 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.
* Mendeleev arranged the elements known at that time in a chart in a systematic order in the increasing order of their atomic weights.
* Mendeleev divided the chart into 8 vertical columns known as groups.
* The horizontal rows of Mendeleev'stable are called periods.
* The Mendeleev's periodic law states that the physical and chemical properties of the elements are a periodic function of their atomic weights.
* Based on the arrangement of the elements in the table Mendeeleev predicted that some elements were missing and left blank spaces at the appropriate places in the table.
* He predicted the properties of these new additional elements in advance purely depending upon his table.
* He named those elements as eka-boron, eka-aluminium and eka-silicon. Later these were discovered as scandium, gallium and germanium respectively.
* There were limitations of Mendeleev's periodic table. Anomalous pair of elements: certain element of highest atomic weights precede those with lower atomic weights.
* Dissimilar elements placed together in the table.
* In 1913 H.J. Moseley found that each element emits a characteristic pattern of X- rays when subjected to bombardment by high energy electrons.
* The periodic law is changed from atomic weight concept to atomic number concept and it is now called the modern periodic law.
* The modern periodic law is stated as 'the properties of elements are the periodic function of their atomic numbers'.
* The modern periodic table is the extension of the original Mendeleev's periodic table known as short form of the periodic table.
* The modern periodic table is called the long form of the periodic table.
* 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 arrangement (electronic configurations) in atoms.
* 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'.
* The modern periodic table has 18 vertical columns known as groups and 7 horizontal rows known as periods.
* 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.
* The vertical columns known as groups are represented by using roman numerals 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 7 periods in the modern periodic table. These periods are represented by the arabic numerals 1 through 7.
* Each period starts with a new main shell. 's' sub-shell and ends when the main shell is filled with respect to the 's' and 'p' sub-shells (except the first period).
* The first period starts with K - shell. The first main shell (K) contains only one sub - shell. (1s). For this sub - shell 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.
* Third period starts with third main shell (M). This shell (M) 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. Therefore the 3rd period contains again 8 elements, which includes 2 s - block elements and 6 p - block elements.
* Fourth main shell (N). This shell (N) has 4 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 2 s-block, 10 elements from d-block and 6 elements from p - block. There are altogether 18 elements in fourth period.
* On the same lines, we can explain why there are 18 elements in the 5th period.
* There are 32 elements in the 6th period which includes 2 elements from s- block (6s), 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.
* 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.
* The 'f' block elements known as lanthanoids and actinoids are shown, separately at the bottom of the periodic table.
* 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.
* Metalloids or semi metals are elements which have properties that are intermediate between the properties of metals and non-metals. They are generally semi-conductors.
* Valance (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.
* 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 valency is 8-group number. For example, Chlorine valence is 8 - 7 = 1
* 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.
* Atomic radius of an element may be defined as the distance from the centre of the nucleus of the atom to its outermost shell.
* Atomic radius is measured in 'pm' (pico meter) units. 1 pm = 10-12 m.
* Atomic radii of elements decrease across a period from left to right.
* The energy required to remove an electron from the outermost orbit or shell of a neutral gaseous atom is called ionization energy.
* 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.
* The electronegativity of an element is defined as the relative tendency of its atom to attract electrons towards itself when it is bonded to the atom of another element.
* 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.
* 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'.
* 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.
Writer: C.V.Sarveswara Sarma
