Atomic Structure: Practice Problem & Solution

Solution Explained:

To solve this problem, we apply the core principles of Atomic Structure. Understanding the underlying formula is key to arriving at the correct answer below:

The energy of an electron in a hydrogen-like ion is $E_n = -13.6 \frac{Z^2}{n^2} eV$. For a $He^+$ ion, $Z=2$. The first excited state corresponds to $n=2$. Thus, $E_2 = -13.6 \frac{2^2}{2^2} = -13.6 eV$.