outer shell electron of sodium would occupy one corner of the cube, while in the case of a noble gas all the eight corners would be occupied. This octet of electrons, represents a particularly stable electronic arrangement. Lewis postulated that atoms achieve the stable octet when they are linked by chemical bonds. In the case of sodium and chlorine, this can happen by the transfer of an electron from sodium to chlorine thereby giving the Na + and Cl – ions.
In the case of other molecules like Cl , H , F , etc., the bond is formed by the sharing of a pair of electrons between the atoms. In the process each atom attains a stable outer octet of electrons . Lewis Symbols: In the formation of a molecule, only the outer shell electrons take part in chemical combination and they are known as valence electrons . The inner shell electrons are well protected and are generally not involved in the combination process.
G.N. Lewis, an American chemist introduced simple notations to represent valence electrons in an atom. These notations are called Lewis symbols . For example, the Lewis symbols for the elements of second period are as under: Significance of Lewis Symbols : The number of dots around the symbol represents the number of valence electrons.
This number of valence electrons helps to calculate the common or group valence of the element. The group valence of the elements is generally either equal to the number of dots in Lewis symbols or minus the number of dots or valence electrons. Kössel, in relation to chemical bonding, drew attention to the following facts: In the periodic table, the highly electronegative halogens and the highly electropositive alkali metals are separated by the noble gases; The formation of a negative ion from a halogen atom and a positive ion from an alkali metal atom is associated with the gain and loss of an electron by the respective atoms; The negative and positive ions thus formed attain stable noble gas electronic configurations. The noble gases (with the exception