Does covalency fail in explaining the formation of BeCl2 and PCl5? Explain

Yes , covalency fails to explain the formation of BeCl₂ and PCl₅.

Explanation:

covalent bonds are formed between two atoms  by sharing of outermost electrons in order to attain octet for attaining stability . To octet means to attain 8 electrons in their outermost orbit .

BeCl₂ :

Be has two electrons in its outermost orbit and each of Cl has 7 electrons .

Each of Cl shares one electron each with each of two electrons of Be . Cl atoms attain 8 electrons as octet . But Be attains only 4 electrons ( 2 of Be and 2 from Cl atoms ) . Hence octet of Be is not attained . So covalency fails in the formation of BeCl₂ .

PCl₅ :

P has 5 electrons in the outermost orbit . Each of these electrons shares with one  electron from  each of Cl atoms . Thus Cl attains octel state ( 8 electrons ) but P attains 10 electrons ( 5 + 5 ) , 5 of P and 5 from 5 Cl atoms .

Hence Octet of P is not attained . Hence covalency fails in the formation of PCl₅ .