where, S: bonding strength Zt. number of valence electron CNT: C.N. of cation ion if s=s S. Then Z=CN)×S=(CN)×(zCN+)CN :C.N. of cation ion : number of valence electron where, : bonding strength CN + + + + = Z S Z S if S1＝S2＝……＝S, Then Z ＝(CN－)×S＝（CN－）×（Z + /CN+）   + − = − = + = = CN 1 CN 1 , i i i Z Si Z S