The core of the atom

Almost all matter in the Universe is concentrated within atomic nuclei. Some 100,000 times smaller than the atom, they contain 4,000 times more mass than their orbiting electrons. All nuclei are made up of neutrons and protons, collectively known as nucleons, which play similar roles in maintaining the structure of the nucleus. Whereas electrons can be said to be ‘fundamental’ (or indivisible) particles in their own right, nucleons are made up of quarks, the smallest known units of matter, and as a result are not considered to be fundamental particles.

The existence of quarks has imposed itself since the 1970s. In the field of nuclear physics which is that of radioactivity, the habit before was to consider the nucleons as the elementary constituents of the nucleus. For simplicity, we will keep this convention, referring to the internal structure of nucleons and quarks only when necessary.

Composition of an atomic nucleus
The nucleus of aluminium comprises 27 nucleons: 13 protons and 14 neutrons. As protons have a positive electric charge, equal and opposite to that held by electrons, 13 electrons are needed to overcome the nucleus charge, and form one atom of aluminium. As protons and neutrons have approximately the same mass, each weighing almost 2,000 times as much as an electron, the mass of the orbiting electron body is negligible when compared to that of the nucleus.

The classical representation of the nucleus is a dense grouping of protons and neutrons, defined by three mathematical numbers: Z, N and A. Z represents the number of protons, N the number of neutrons, and A the total number of nucleons. As both protons and neutrons are referred to as nucleons, A is simply the sum of N and Z: A=N+Z. The two fundamental properties of a nucleus are its mass and its electric charge. As protons and neutrons have approximately the same mass, A is a convenient measure of the mass of the nucleus.

The electrical charge of the proton (taken as a conventionnal unit), however, is different from that of the neutron: a proton has a charge of +1, and a neutron, being neutral, has a charge of 0. As a result, Z is used to calculate a nucleus’s charge.

For all nuclei naturally present in the Universe, Z varies from 1 to 92, and A from 1 to 238. The heaviest natural element is Uranium-238, which contains 92 protons for 146 neutrons. The nucleus, therefore, has a combined mass of 238 nucleons, and a charge of +92.

Light nuclei
The beginning of the map shows how protons and neutrons agglomerate in order to form light nuclei, hydrogen, helium, lithium and berylium isotopes. All the nucleons have been drawn visible. The number of protons has been limited to 4, that of neutrons to 3. Radioactive isotopes are unstable and are thus absent from our environment. The proton (ordinary hydrogen) and helium-4 (alpha particle), which are stable, together form more than 99% of the mass of the universe.