# Exceptions of Octet Rule

Exceptions of Octet Rule

Some compounds such as $BeCl_2, BF_3, PCl_5, IF_7$ etc. do not follow octet rule, because in these compounds Be, B, P, S, I etc. possess 4, 6, 10, 12, 14 electrons, respectively.

According to Sidgwick, the maximum covalence of an element is limited according to the period, in the periodic table, in which it occurs, e.g. it is 2 for hydrogen, 4 for the elements of the second period (Li to F), 6 for those of the third and fourth periods and 8 for the rest. Sidgwick’s covalence rule is purely empirical. This rule explains satisfactorily the formation of $SF_6 \text{and} PCl_5$ since maximum capacity of the valency orbits of sulphur and phosphorus is 12 to 10 respectively.

Sudgen believes contrary to the views of Sidgwick, that the maximum number of electrons in the valency orbit cannot exceed eight. He introduced a new type of union involving the sharing of one electron between two atoms which is termed singlet linkage or simply singlet. He also named it single electron linkage or half bond. Singlet linkage is indicated by a dotted  line arrow (……………….>)or half

Arrow (  ).

Singlet linkage results between atoms, one of which possesses octet of electrons and can donate some of the electrons (single in each case) to other atoms each of which is short of only one electron form inert gas configuration.

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