The mass number of magnesium is 24.
The mass number is the sum of the number of protons and neutrons in an atom’s nucleus. Magnesium has an atomic number of 12, which means it has 12 protons. The most common isotope of magnesium has 12 neutrons, resulting in a mass number of 24 (12 protons + 12 neutrons = 24). There are other isotopes of magnesium, such as magnesium-25 and magnesium-26, which contain 13 and 14 neutrons respectively, but the most prevalent isotope is magnesium-24.