The first ionization energy of Mg is lower than

Explanation

The first ionization energy generally increases across a period and decreases down a group. Magnesium (Mg) has a lower first ionization energy than Beryllium (Be) because Mg is located to the right of Be in the periodic table, meaning Mg has a larger atomic size and its valence electron is further from the nucleus. In contrast, Be has a smaller atomic size, and its valence electrons are held more tightly, resulting in a higher ionization energy. Sodium (Na) has a lower ionization energy than Mg because Na is to the left of Mg in the same period. Calcium (Ca) has a lower ionization energy because it is below Mg in the same group, where atomic size and shielding increase. Although Aluminum (Al) is to the right of Mg, it has a slightly lower ionization energy than Mg due to a new sublevel being filled, but it is not the correct answer in this context where a clear higher ionization energy example is required.