Explanation:
Bromine belongs to group 7. It's most stable ion is the bromide, Br-.
Electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6
Caesium belongs to group 1. It's most stable ion is Cs+
Electronic configuration = 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6
Strontium belongs to group 2. It's most stable ion is the Sr2+
Electronic configuration = 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6
Oxygen belongs to group 6, it's most stable ion is O2-
Electronic configuration = 1s2 2s2 2p6
