Why is proton number of an atom of an element different from the nucleon number of an atom of an element?

The difference between proton number (which is atomic number) and nucleon number, which is the number of protons and neutrons in an atom's nucleus, is the number of neutrons in the nucleus of a given nuclide. Nucleons are the components of an atomic nucleus. We know that both protons and neutrons make up an atom's nucleus. So the number of nucleons, which is the number of protons and neutrons, minus the number of protons, will equal the number of neutrons in the atomic nucleus.