Here's why:

* Magnesium (Mg) is in group 2 of the periodic table, meaning it forms ions with a +2 charge (Mg²⁺).

* Bromine (Br) is in group 17, meaning it forms ions with a -1 charge (Br⁻).

To balance the charges and create a neutral compound, you need two bromide ions (2 x -1 = -2) for every one magnesium ion (+2).