Question

What is the correct ionic formula when mg2+ and p3− react?

Answer 1

Any compound should have a neutral charge. So the compound you make should have a net charge of zero.

So you have:
Mg which is 2+
And P which is 3-

You want to balance out both their charges. If you simply made MgP it would have a negative charge (because 2-3 = -1.)

Now to balance this, you have to find the LCM: In this case it is 6. (2x3)

Mg: Mg x 3
P: P × 2

You'll finally get Mg3P2.

Answer 2

Answer: Ionic formula will be $Mg_3P_2$.

Explanation: $Mg^{2+}$ and $P^{3-}$ ions will form a ionic compound. Ionic compounds have both metals and non-metals.

Here $Mg^{2+}$ is a metal and $P^{3-}$ is a non-metal.

The net charge on any compound must be 0.

So we need 2 phosphate ions to balance the charge on $Mg^{2+}$ ions. Similarly we need 3 Magnesium ions to balance the charge on $P^{3-}$ ions.

Criss-crossing the charges, we will get the formula as $Mg_3P_2$

Criss-crossing is shown in the image below.