* Sodium (Na) is in group 1 of the periodic table, meaning it forms +1 ions (Na+).

* Phosphorus (P) is in group 15 of the periodic table, meaning it forms -3 ions (P3-).

To achieve a neutral compound, we need three sodium ions (3 x +1 = +3) to balance the charge of one phosphorus ion (-3). Therefore, the correct formula is Na3P.