Oxygen has an atomic number of 8, which means it has 8 electrons. When it gains 2 electrons to become $\mathrm{O}^{2-}$, it will have a total of 10 electrons. The electron configuration for $\mathrm{O}^{2-}$ is 1s$^2$ 2s$^2$ 2p$^6$.
b. Bromine has an atomic
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