Question

Which of the following is the correct order of ionisation energy?

• A. $O^{2-}<F^{-}<N{a}^{+}<M{g}^{2+}$
• B. $F^{-}<O^{2-}<N{a}^{+}<M{g}^{2+}$
• C. $O^{2-}<N{a}^{+}<F^{-}<M{g}^{2+}$
• D. $M{g}^{2+}<N{a}^{+}<F^{-}<O^{2-}$

Answer 1

The correct option is A $O^{2-}<F^{-}<N{a}^{+}<M{g}^{2+}$

All are isoelectronic species. As number of protons i.e. atomic number increases, the attraction between electron and nucleus increases.
Thus, to remove an electron from the atom, more energy has to be supplied.
Hence, ionisation energy increases with increasing atomic number for isoelectronic species.