Composition of the nucleus. Isotopes

The idea of the atom was voiced by Democritus; al-Razi held that atoms are divisible and contain voids, and Beruni also wrote that the atom can be divided. In 1911 E. Rutherford passed α-particles through a very thin metal foil, saw that most went straight through while very few were deflected or bounced back, and proposed the planetary model: a positively charged nucleus in the centre with negative electrons moving around it. The nucleus consists of protons (charge +1, mass ≈ 1 amu) and neutrons (no charge, mass ≈ 1 amu); an electron is about 1836 times lighter than a proton and has charge –1. The nuclear charge Z equals the number of protons and the atomic number; the atom is neutral, so the number of electrons is also Z. The mass number is A = Z + N (N is the number of neutrons) and is approximately the relative atomic mass, so N = A – Z.