Here's why:

* Calcium (Ca) is in group 2 of the periodic table, meaning it forms a +2 cation (Ca²⁺).

* Bromine (Br) is in group 17 of the periodic table, meaning it forms a -1 anion (Br⁻).

To balance the charges and create a neutral compound, we need two bromide ions (2 x -1 = -2) to balance the +2 charge of the calcium ion.