Sodium (Na) is more reactive than magnesium (Mg) due to differences in their atomic structures and electron configurations. Sodium has a single valence electron in its outermost energy level, making it easier to lose that electron and form a stable positive ion. Magnesium, on the other hand, has two valence electrons, requiring more energy to remove them and become reactive. Additionally, the larger atomic size of sodium compared to magnesium contributes to the ease of electron loss in sodium.