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.However, in discussing radioactivity.
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.
It may seem unusual, at first glance, that the nucleus has any sort of stability whatsoever. Being comprised entirely of positive and neutral particles, and remembering the law of electromagnetism that ‘like charges repel’, it would be expected that any nucleus would fly apart very quickly. The paradox is irresolvable in terms of classical physics, and was only explained away in the middle of the 20th Century, with the discovery of a very strong force which exists over very short distances inside the nucleus. This ‘strong’ force acts like a nuclear glue, overcoming the electrostatic repulsion between the protons and maintaining the nucleus’s stability.
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