# What is the name and formula for the compound formed from magnesium, 2+, and phosphide, 3-, ions?

$\text{Magnesium phosphide.........}$
And to form the salt, we draft in anions and cations to form a neutral entity, i.e. here $M {g}_{3} {P}_{2}$........This would be nasty stuff; so if you have an old, unopened bottles lurking in the back of a cupboard exercise due care. Of course, an opened bottle might have hydrolyzed long ago....
So how did I know the formula was $M {g}_{3} {P}_{2}$? Well, the constituent ions are $M {g}^{2 +}$ and phosphide, ${P}^{3 -}$. The salt MUST be electrically neutral, so we cross mulitply to give $2 \times {P}^{3 -} + 3 \times M {g}^{2 +} = M {g}_{3} {P}_{2}$. Capisce?