There are 13 neutrons in an atom of the isotope magnesium-25. The number of the isotope refers to the number of nuclear particles in general; subtracting 13 neutrons means that there are 12 protons at the nucleus.
Magnesium-25 is one of three stable, non radioactive, naturally-occurring isotopes of the element Magnesium. It is found in approximately 10 percent of these isotopes, located in minerals like brucite, dolomite and talc. The most abundant form of the isotopes is magnesium-24, which is the basic form of the element, making up approximately 79 percent of magnesium isotopes. The element is the twelfth on the periodic table, with the atomic symbol Mg.