Atomic structure — practice question

Which equation shows the second ionisation energy of an element X?

• A$X(g) \rightarrow X^{2+}(g) + 2e^-$
• B$X^+(g) \rightarrow X^{2+}(g) + e^-$
• C$X(g) + 2e^- \rightarrow X^{2-}(g)$
• D$X^-(g) + e^- \rightarrow X^{2-}(g)$