Explanation:
(1) \(\mathrm{Mg}^{+2}=1 \mathrm{~s}^{2}\, 2 \mathrm{~s}^{2} \,2 \mathrm{p}^{6}\)
Penultimate shell electron \(=2\)
Outermost shell electron \(=8\),
(2) \(\mathrm{O}^{-2}=1 \mathrm{~s}^{2}\, 2 \mathrm{~s}^{2} \,2 \mathrm{p}^{6}\)
Penultimate shell electron \(=2\)
Outermost shell electron \(=8\),
(3) \(\mathrm{F}^{-}=1 \mathrm{~s}^{2} \,2 \mathrm{~s}^{2} \,2 \mathrm{p}^{6}\)
Penultimate shell electron \(=8\)
Outermost shell electron \(=8\),
(4) \(\mathrm{Ca}^{+2}=1 \mathrm{~s}^{2} \,2 \mathrm{~s}^{2} \,2 \mathrm{p}^{6} \,3 \mathrm{~s}^{2} \,3 \mathrm{p}^{6}\)
Penultimate shell electron \(=\)
Outermost shell electron \(=8\)
Hence, \(\mathrm{Ca}^{2+}\) has 8 electrons in outermost as well as in penultimate shell.