So we we've got a selection of anions, say #PO_4^(3-)#, #SO_4^(2-)#, and #P^(3-)#. You simply have to know these species as common ions.
And now, these make music with a selection of common CATIONS, #Ca^(2+)#, #Fe^(3+)#, #Na^+#. Now all matter is electrically neutral, so when these ions combine, they do so in a way as to make the overall electric charge #0#, i.e. the salt is NEUTRAL. Are we clear on this? If no, voice your objections, and we will have another go.
So with phosphate ion, #PO_4^(3-)#, we would form #Ca_3(PO_4)_2#, #Fe(PO_4)#, and #Na_3(PO_4)#. All I have done here is to cross-multiply to give an ionic salt in which negative and positive charges are EQUAL such that the charge on the SALT is neutral.
And with sulfate, we get #CaSO_4, Fe_2(SO_4)_3, Na_2SO_4#.
You try this, and make neutral salts with the phosphide anion, #P^(3-)#. I grant that this appears abstract, but all we are doing is making sure that the ionic solid is NEUTRAL. Again, come back for another serve if you don't see it.
