The mass number of an atom is the sum of its protons and neutrons. In this case, the atom has a mass number of 25 and 12 neutrons.
To find the atomic number (which is the number of protons), we subtract the number of neutrons from the mass number:
Atomic number = Mass number - Number of neutrons Atomic number = 25 - 12 Atomic number = 13
Therefore, the atomic number of this atom is 13.