Why PCl5 exist but PH5 does not exist?

In PH3 and PCl3  orbital overlapping  Is different.  

 We know hydrogen has only one orbital and is s orbital and phosphorus has P orbital in its ultimate shell 

 

So the overlapping of orbitals   in PH3 is   S-P  which is less energetically favoured  and  mismatches in orbitals. 

And in PCl3 molecule both phosphorus and chlorine contains P orbital in ultimate shell so overlapping is P-P orbital and is energetically favoured     

 As a result PH3 is not formed but PCl3 is formed. 💙   

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