Look at the periodic table. Do you see any relation between the number of valence electrons and group numbers? We find that for groups 1-2 and 13 –18, we can use the periodic table to find the number of valence electrons. Group1 has one outer electron, group 2 has two, and group 13 has three, group 14 has four and so on.

• What did you notice in Lewis dot structure of noble gases and electronic configurations of the atoms of these elements shown in table - 2.

It was found that the elements which participate in chemical reactions get octet or ns2 np6 configuration similar to that of noble gas elements. It is to be noted that octet rule is still a rule not the law, because there are considerable exceptions for this rule.

There were number of attempts to explain the chemical bond formation between atoms in terms of electrons but a satisfactory explanation for this concept was given by Kossel and Lewis in 1916. They gave this concept independently. The basis for their theory was valence in terms of electrons. They provided logical explanation of valence on the basis of the lack of chemical activity of noble gases which led to the proposal of octet rule.

Observe the practical behaviour of atoms of main group elements (Group IA, IIA, IIIA, IVA, VA, VIA, VIIA and zero or VIIIA group elements). When they are allowed to undergo chemical changes, they try to get octet electronic configuration in the outer shells.

Let us understand this with the following illustrations.

Group IA elements (Li to Cs) try to lose one valence shell electron from their atoms to form corresponding uni-positive ions which get octet in their outer shells.

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.

Group IIIA elements try to lose three valence electrons from their atoms and form corresponding tri positive ions with octates in the outer shells.