The number of protons in an atom is also referred to as the atomic number, so an atom of iron has 26 protons in its nucleus. The mass number of an atom is the sum of the number of protons and neutrons in the nucleus. So, to find the number of neutrons in the atom, we can subtract the number of protons (26) from the mass number (56):
56 - 26 = 30 neutrons.
So, the number of protons plus neutrons in the nucleus is:
26 protons + 30 neutrons = 56.