Before starting to draft this chapter, I gave myself a short revision of school physics and chemistry. It could be useful to pass it on now to any reader whose memory is as poor as mine.

Fundamentals

The nucleus of every atom of an element has a fixed number of protons, which is equivalent to its atomic number. For example, the nuclei of hydrogen, helium, oxygen and uranium have 1, 2, 8, and 92 protons, respectively. Each proton carries a positive electric charge. Surrounding the nucleus are orbiting electrons, each of which carries a negative electric charge. Because the number of protons is nearly always equivalent to the number of electrons, the atom is neutral (carries neither a positive nor a negative charge). It is the number and arrangement of electrons (especially those in the outer orbits) that determine the chemical characteristics of the element.

In addition to its protons, the nucleus also contains neutrons. These have the same mass as the proton but do not carry an electric charge. Therefore, the same element can come in a number of different varieties depending on its total number of neutrons. These varieties are called isotopes or nuclides. There is a subtle difference in the definition of these two words, but we need not bother about it here. Isotopes (nuclides) of a substance are chemically identical but have a different mass. For example, uranium-238 (238U) has 92 protons and 146 neutrons, giving it an atomic mass of 238. Uranium-235 (235 U) has the same number of 92 protons but has 143 neutrons.

Some nuclides are stable, but others are said to be unstable because they constantly show a tendency to change (decay) into some- thing else, which may then itself also undergo further decay.