What is the name of #Ba_3P_2#?
We're dealing with an ionic compound, so what you want to do is name the cation (metal) first. The name of the metal is taken straight from the periodic table. Ba is the atomic symbol for the element barium. Thus that is what comes first.
Now the anion (nonmetal) is phosphorus, but you have to take the root word and change the ending to -ide. Therefore, phosphorus becomes phosphide.
In summary, when naming ionic compounds, the procedure goes like this:
name of first element + root name of second element that has the suffix -ide
I hope this explanation is clear!