Strontium (Sr) is an alkaline earth metal belonging to Group II of the periodic table. The most stable valency of elements of this group is 2. Bromine (Br), is a halogen having a stable valency of 1. When a compound is formed between any two elements, the ratio of atoms in that compound will be in the cross- ratio of their valencies.
Therefore, in this case, the ratio of valencies of Sr and Br is 2:1, hence, the ratio of the atoms in the compound formed between Sr and Br will be 1:2, and the formula of the chemical compound will be `SrBr_2` .