What is the formula of the compound formed when Mg reacts with P?

The formula is Mg₃P₂.

This is a reaction between a metal and a nonmetal. The product will be the ionic compound, magnesium nitride.

Step 1. Identify the atomic symbols of the cation and anion.

Magnesium is Mg and phosphide is P

Step 2. Identify the charge on each ion and place it on each symbol.

Mg is in Group 2, so we write Mg²⁺.

P is in Group 15, so it needs 3 electrons to complete its octet. We write the ion as P³⁻.

Step 3. Determine the least common multiple of the charges.

The lowest common multiple of 2 and 3 is 6. We need 6 + charges and 6 – charges. Thus, we need 3 Mg²⁺ ions and 2 P³⁻ ions.

Step 4. Write the formula using the numbers as subscripts.

The formula is Mg₃P₂.