Why is the formula for magnesium phosphate not Mg3PO42? Since 3 and 42 have a GCF, the formula needs to be reduced. The correct formula is MgPO13. Since 3 and 42 have a GCF, the formula needs to be reduced. The correct formula is MgPO, 13, . Magnesium phosphate is just an ionic compound, it doesn't contain a polyatomic ion. The correct formula is Mg3P2. Magnesium phosphate is just an ionic compound, it doesn't contain a polyatomic ion. The correct formula is Mg, 3, P, 2, . Magnesium phosphate is just an ionic compound, it doesn't contain a polyatomic ion. The correct formula is MgP. Magnesium phosphate is just an ionic compound, it doesn't contain a polyatomic ion. The correct formula is MgP. Since there was a number brought down after the phosphate, there need to be parentheses around it. The correct formula is Mg3(PO4)2.