Nucleon number

The nucleon number is the total number of protons and neutrons in the nucleus of an atom. It is also called the mass number. Malay: nombor nukleon · Chinese: 核子数.

The name comes from nucleon, the general word for a particle in the nucleus, that is, a proton or a neutron. Because protons and neutrons each have a relative mass of about 1 and the electrons are almost massless, the nucleon number is a good measure of the atom’s mass. To find the number of neutrons, subtract the proton number from the nucleon number.

Example. Chlorine-35 has a nucleon number of 35, made up of 17 protons and 18 neutrons. Sodium has a nucleon number of 23, made up of 11 protons and 12 neutrons, so it has 23 − 11 = 12 neutrons. A carbon-12 atom has a nucleon number of 12: 6 protons and 6 neutrons.

Confusion to avoid. Do not confuse the nucleon number with the proton number, and do not confuse it with relative atomic mass. The proton number counts only protons; the nucleon number counts protons and neutrons. Relative atomic mass is an average over an element’s isotopes, so it is often not a whole number, while a single atom’s nucleon number always is. Remember, too, that electrons are never counted in the nucleon number.

A useful check is that the nucleon number can never be smaller than the proton number, because an atom cannot have a negative number of neutrons; for most atoms it is roughly twice the proton number. If your neutron count comes out negative, you have almost certainly swapped the two numbers, so put the larger figure as the nucleon number and try again.

In the exam the nucleon number appears in the standard atomic notation, in questions asking for numbers of neutrons, and in defining isotopes, which share a proton number but differ in nucleon number. Together with the proton number it lets you describe any atom in the Matter and the Atomic Structure chapter completely.