1. A reminder of the structure of matter

1.1 Atom

Atoms are made up of elementary particles: protons and neutrons combined in a very dense nucleus carrying a positive electric charge, and electrons, very light negatively-charged particles in motion around the nucleus.

Atoms have a total radius of the order of 0.1 nm, and atomic nuclei have dimensions of the order of a few femtometers (10 ^–15 m). Electrons, on the other hand, are point particles with no dimensions. The mass of a proton or neutron is approximately 1.7 × 10 ^{– 27} kg, that of an electron 9.1 × 10 ^{– 31} kg.

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