The element symbol "Mg" represents magnesium, and the number 25 in "Mg-25" refers to the mass number, which is the total number of protons and neutrons in the nucleus of an atom.
To determine the number of neutrons in an atom of Mg-25, we need to subtract the atomic number (number of protons) from the mass number. The atomic number of magnesium is 12 because it is the number of protons in the nucleus of a magnesium atom.
Number of neutrons = Mass number - Atomic number Number of neutrons = 25 - 12 Number of neutrons = 13
Therefore, an atom of Mg-25 contains 13 neutrons.