Answer
Magnesium has a smaller atomic radius and a higher ionization energy than sodium. This means that it requires more energy to remove an electron from magnesium and its outer electrons are held more tightly. As a result, magnesium is less likely to react with water or other substances compared to sodium. Additionally, the electron configuration of magnesium has a full outer shell, making it more stable and less likely to react. Therefore, magnesium is much less reactive than sodium.