Strontium (Sr) lies in the second group of the periodic table, which means it has two valence shell electrons so it forms the Sr⁺² ion. Meanwhile, bromine (Br) has seven valence shell electrons, which means it forms the Br⁻ ion. Each strontium atom is therefore able to donate electrons to two bromine atoms. So the ratio of the ions in neutral strontium bromate (SrBr₂) will be 1 : 2.