Question

Explain electronegativity in depth. This is part of a discussion board and I need to know all aspects of electronegativity.

Answer 1

Linus Pauling was the first to define the term electronegativity in the year 1932. It is defined as the tendency of an atom to attract shared electron pair to itself when combined in a molecule. The electronegativity is thus not a property of an isolated atom but of an atom in a molecule under the influence of the surrounding atoms. Greater the electronegativity of an element more will be electrons attracting tendency of its atom.

According to G.N.Lewis, the formation of a covalent bond involves the sharing of valence electrons between the combining atoms. When a covalent bond is formed between two similar atoms,the shared electron pair lies between the two atoms and contributes equally to both the atoms and thus no polarity is formed on the combining atoms in a molecule.But in the case of hetero nuclear molecules the electron pair of the bond is not shared equally by the 2 combining atoms.

Various attempts have been made to evolve a quqntitative scale of the electronegativity.

a) pauling scale: The larger the electronegativity difference,the higher would be the polarity developed and the greater would be the stability of the electronegativity difference of the bond.

Pauling electronegativities values ot he main group elements are Group1,2,13,14,15,16,17.

b) Mulliken approach(1934) : The electronegativity of an atom is the arithmetic mean of its first ionisation energy & electron affinity.

Factors of electronegativity :

1) Effective nuclear charge

2) Atomic radius

3) Charge on the atom

4) State of hybridisation

5) Effect of substituents

variation of electronegativity in the Periodic Table: Electronegativity varies regularly across a period or on going down a group.

a) Variation across a period: The electonegativity increases on moving from left to right in a period due to decrease in atomic size. In each period, the alkali metal(group1 element) possesses the least vale of electronegativity. This element has no tendency to pull the shared electron pairs and it is said to be an electropositive element. The halogens (i.e. group 17 element) are having the highest values of electronegativity because they have greater tendency to attract electrons.

b) Variation in a group : The electronegativity decreases on going down a group. This is due to a decrease in the effective nuclear charge with increase in atomic number as well as increase in the shielding effect.