The formula for terbium phosphate is ${\mathrm{TbPO}}_4$. The formula for terbium sulphate is:

The formula for terbium sulphate is ${\mathrm{Tb}}_2{\left({\mathrm{SO}}_4\right)}_3$.

Three electrons on the atom of terbium can either be removed or shared during a chemical reaction. Terbium's ionic formula is thus ${\mathrm{Tb}}^{+3}$. In contrast, sulphate is an ion with the formula ${{\mathrm{SO}}_4}^{-2}$. When they are coupled, they just swap the charges and turn it into a subscript.