Which of the following elements has the highest second ionization energy?

• A. Na
• B. K
• C. Al
• D. Mg

Answer: (A)

Verified step by step guidance

1. Understand what second ionization energy means: it is the energy required to remove the second electron from a singly charged ion (M⁺ → M²⁺ + e⁻).
2. Recall that ionization energy generally increases as you remove electrons closer to the nucleus and from a more positively charged ion, but it also depends on the electron configuration after the first electron is removed.
3. Look at the electron configurations of the neutral atoms and their ions after the first electron is removed: for example, Na (neutral: 1s² 2s² 2p⁶ 3s¹) loses its 3s electron first, resulting in a noble gas configuration (Ne-like) for Na⁺.
4. Recognize that removing a second electron from Na⁺ means removing an electron from a stable, full shell (Ne configuration), which requires significantly more energy compared to the other elements where the second electron is removed from a less stable configuration.
5. Compare the other elements (K, Al, Mg) similarly, noting that their second ionization energies are lower because their second electron is removed from a less stable or more shielded orbital, so conclude that Na has the highest second ionization energy.