Lewis concept of covalency of an element involved octet rule .Later on it was found that many elements in their compoinds `e.g, BeF_2,BF_3` etc . Have incomplete octet whereas `PCl_5 ,SF_6` etc , have expanded octet . This classical conept also failed in predicting the geometry of molecules .Modern concept of covalence was propsed in terms of valence bond theory proposed by Heitler and London and later on modified by Pauling nad Slater . Hybridisation concept alongwith valence bond theory although successfully explained the geometry of various molecules but failed in many molecules . the geometry of such molecules was explained by VSEPR concept . Finally molecular orbital theory was porosed by Hund -Mulliken to explain many other anomalies . Ratio of lone pair-bond pair electrons on central atom in ` i_3^-` and `XeF_4` are respectively :A. ` 2. 66, 0. 5`B. `1. 5 , 0.5`C. ` 2, 0. 5`D. ` 0 5, 2`