Solution to 2001 Problem 18

The energy required to remove the second electron after the first electron has been removed can be calculated using the formula for the Bohr energies.
\begin{align*}E_1 = 4 \cdot 13.6 \mbox{ eV} = 54.4 \mbox{ eV}\end{align*}
Therefore, the energy required to remove the first electron is
\begin{align*}79.0 \mbox{ eV} - 54.4 \mbox{ eV} = \boxed{24.6 \mbox{ eV}}\end{align*}
Therefore, answer (A) is correct.

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