It Is Easier To Find The Valencies Of Monoatomic Ions i.e To Recite Them, However I Am Confused How To Find Valencies Of Polyatomic Ions And Bigger? For Ex. How To Find Valency Of NH4 And PO4

As we know that the electrons filling takes place as is 2,8,18,32

 

Now in case of NH4

The total electrons in the valence shell is =5+4=9

To get 8 in valence shell we have to remove one electron hence it has NH4+

 

In case of PO4

electrons in valence shell is 5+24=29

 

Now to complete the octet we require 32 we need to add 3 electrons to get stable electronic configuration . Hence the charge is -3.