Here's why:

* Rubidium (Rb) is an alkali metal in group 1 of the periodic table, meaning it has a +1 charge when it forms an ion.

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

To form a neutral compound, one rubidium ion (Rb+) combines with one bromide ion (Br-). This results in the formula RbBr.