(i) Electron-deficient hydrides : The hydrides of group 13 elements have an incomplete octet and hence are electron deficient molecules. They act as Lewis acids e.g., $B_2H_6$ (ii) Electron-precise hydrides : These compounds have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (e.g., $CH_4$) which have tetrahedral structure. (iii) Electron rich compounds of hydrogen : These compounds have excess electrons which are present as lone pairs, elements of group 15-17 form such compounds. e.g., $NH_3$ has one lone pair, $H_2O$ has two lone pairs and HF has three lone pairs.