To determine the number of neutrons in an atom, you subtract the atomic number (number of protons) from the mass number. In the case of magnesium-25 (Mg-25), the atomic number of magnesium is 12.
Therefore, to find the number of neutrons in Mg-25:
Neutrons = Mass number - Atomic number Neutrons = 25 - 12 Neutrons = 13
So, an atom of magnesium-25 (Mg-25) has 13 neutrons.