Reminders about the atomic nucleus

In the model first developed by Niels Bohr and Ernest Rutherford and extended by Arnold Sommerfeld, the atom is composed of two entities: a central nucleus, containing most of the mass, and an array of orbiting electrons. The nucleus carries a positive charge of + Ze, which is balanced by the electron cloud's negative charge of −Ze. The number of protons, Z, is matched in an electrically neutral atom by the number of electrons. Each of these particles carries a negative electric charge e.

As a rough description, the nucleus of any element is made up of two types of particle, neutrons and protons. A neutron is slightly heavier than a proton with a mass of mn = 1.674 95 · 10−27 kg compared with mp = 1.672 65 · 10−27 kg for the proton. While of similar masses, then, the two particles differ above all in their charges. The proton has a positive charge (+ e) while the neutron is electrically neutral. The number of protons (Z) is the atomic number. The sum A = N + Z of the number of neutrons (N) plus the number of protons (Z) gives the mass number. This provides a measure of the mass of the nuclide in question if we take as our unit the approximate mass of the neutron or proton.