(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., B2H6
(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., CH4) 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., NH3 has one lone pair, H2O has two lone pairs and HF has three lone pairs.