in the formula XSO4 the symbol X could represent the element
X represents group 5A by my periodic table. P2O5 is an example. Since there's charge specified, it must be a neutral atom. For this, each of the X would need a +5 charge. On the other hand, if it's in group 15, then it can form a covalent bond, since each atom in group 15 can share 5 atoms.
Answer: X could represent the element of oxidation state (+2) such as (Mg2+, Pb2+, Ba2+, Ca2+, Ba2+, Zn2+, ....etc)
Explanation: