If the atomic number of an element is 5 and its mass number is 20, how many neutrons are in the atom's nucleus? neutrons

The mass of an atom comes from the protons and neutrons that are found in the nucleus. The number of protons is the atomic number of an element. To find the number of neutrons, subtract the atomic number from the mass of an atom. So the number of neutrons are 20 (mass number) - 5 (atomic number) = 15 neutrons.