On the periodic table, Magnesium has the symbol ₁₂Mg²⁴, where 24 is the mass number and 12 is the atomic number. The atomic number is the number of protons found inside an atom, while the mass number tells us how many neutrons and protons are found in an atom. As the atomic number is 12, we can see we have 12 protons. To calculate the number of neutrons, we do 24-12, which gives us 12. Therefore, the mass number is 24 because Magnesium has 12 protons and 12 neutrons.