mass number

The mass number is the total count of protons plus neutrons in one atom's nucleus. It is written with an isotope's name — sodium-23 — and is not the whole number on an element's box, which is the atomic number; the box's other figure is the decimal average atomic mass. The two are close because a proton and a neutron each weigh about one atomic mass unit. Because the atomic number already counts the protons, the neutrons are the difference: neutrons = mass number − atomic number. For example, sodium with atomic number 11 and mass number 23 has 23 − 11 = 12 neutrons. The mass number is not a charge and not an identity — two atoms of the same element can have different mass numbers if they have different numbers of neutrons. The classic error is to report the mass number when the question asked for the proton count or the neutron count.