1. Atoms, elements and isotopes

An atom is the smallest constituent of chemical compounds, capable of combining chemically with others. This notion of the atom, known since antiquity, is the basis of modern material sciences, principally chemistry. Although in chemistry, the atom is regarded as the basic unbreakable entity, in other fields it is no longer always considered as such, since the nuclear physics experiments that revealed its structure at the beginning of the 20th century. An atom is made up of a nucleus concentrating over 99.9% of its mass, around which electrons are distributed to form a cloud 40,000 times larger than the nucleus itself. The nucleus is made up of positively-charged protons and electrically-neutral neutrons. Electrons interact with the nucleus, while nucleons are held together within the nucleus by the strong nuclear interaction. The electron cloud is distributed over quantized energy levels...