Electronic Theory of Valency and Bonding

INTRODUCTION

Electronic theory of valency was given by Gilbert N. Lewis and Walther Kossel. This theory of valency explains the main cause of chemical combination and combining capacity of different elements on the basis of number of electrons present in the outermost orbit and tendency to lose or gain the electron. The terms involved in electronic theory of valency are discussed below.

Valence electrons and Lewis dot symbol

The electrons present in outermost orbit of each and every element are called valence electrons and that shell is called valence shell. The valency of every element is explained on the basis of number of electrons present in it. All the valence electrons can be represented by dots or crosses and are known as Lewis dot symbol or Lewis electron dot symbol.

Fig: Lewis structure of hydrogen atom                                  Fig: Lewis structure of oxygen atom

OCTET RULE

Lewis and Kossel, 1916 observed that all the atoms of noble gases ( except H or He ) contain 8 electrons in their outermost orbit. All other atoms during chemical combination try to adjust their electronic configuration like nearest noble gas by losing or gaining or sharing the electron. Due to which chemical combination occur and this rule is called octet rule.

Though, octet rule successfully explain about the bonding in most cases it is unable to explain the bonding in PCl₅ , BF₃ , etc.

POSTULATES OF ELECTRONIC THEORY OF VALENCY

In order to explain the combining capacity of atoms and the main cause of chemical combination, Lewis and Kossel, in 1916, put forward a theory called electronic theory of valency. The major postulates/assumptions of electronic theory of valency are -

• The electrons present in outermost orbit are called valence electrons and the corresponding shell is called valence shell.


• The combining capacity of each and every atom of an element is determined with respect to number of electrons present in valence shell.


• Atoms having 8 electrons in their outermost orbit are stable ( inert gas except helium have octet ) and all other atoms try to adjust their electronic configuration like nearest noble gas. Due to this tendency chemical combination occurs. This rule is called octet rule.


• Atoms that undergo combination achieve the electronic configuration ( 8 electrons ) like nearest noble gases by loss or gain or share of electrons.