Write electron configurations for the most stable ion formed by each of the elements Br, Cs, Sr, and O (when in stable ionic compounds).

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