Which isotope of magnesium has mass number 24 and 12 protons?

The magnesium isotope with mass number 24 and 12 protons is magnesium-24.

An isotope’s mass number equals the total number of protons and neutrons in its nucleus. Magnesium always has 12 protons because the element’s atomic number is 12. Magnesium-24 therefore contains 12 protons and 12 neutrons, giving it a mass number of 24.

Magnesium-24 is one of magnesium’s three naturally occurring stable isotopes, alongside magnesium-25 and magnesium-26. It is the most abundant of the three, making up about 79% of naturally occurring magnesium. The other two stable isotopes have the same 12 protons but different neutron counts.

The wording helps distinguish mass number from atomic number. The number 12 identifies magnesium itself; the number 24 identifies this particular isotope. Magnesium-25, by contrast, has 13 neutrons, while sodium-24 has mass number 24 but only 11 protons, so it is a different element.