Here's why:

* Barium (Ba) is an alkaline earth metal in Group 2 of the periodic table, meaning it has a +2 charge when it forms an ion (Ba²⁺).

* Bromine (Br) is a halogen in Group 17 of the periodic table, meaning it has a -1 charge when it forms an ion (Br⁻).

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