How do you write binary forumlas like Mg and P?

Answer 1

Just the way you did.

Answer 2

You are really asking: How does one write formulas for binary compounds such as the compound between Mg and P? The answer is to consider that Mg is a metal in Group IIA of the Periodic Table, and so will have oxidation state +2 and valence 2. P is a nonmetal in Group VA, like nitrogen. The valence of nitrogen is 3 in ammonia, NH3. So the most likely valence of P is also 3. A good guess for the formula is Mg3P2. 3Mg's of valence 2 balance 2 P's of valence 3.

It is good to think about valences and oxidation numbers when solving problems. But one must remember that valence and oxidation number are not the same thing. Valence is real. Valence is a number of bonds: Count 'em. Oxidation numbers are an arbitrary system of bookkeeping whose only use is balancing oxidation-reduction equations.

Answer 3

You write each ion with its charge:

Mg 2+ P 3-

Then you "criss cross" the absolute value of the charges to get the correct subscripts:

Mg 3 P 2 = Mg3P2