Atoms, the building blocks of matter, are made of elementary particles. Most particles carry what is called an electric charge: they are each surrounded by their own electric field and exert (produce) an electric force. The strength of the electric force between two particles depends on the strength of their electric charges and the distance between them. The French engineer, Charles Augustin de Coulomb (1736–1806) proposed an "inverse-square law" called Coulomb's Law, which says that the strength of this electric force decreases with the square of the distance between the particles. The basic unit of electric charge is named after him: the coulomb (C).

Positive and negative

There are two kinds of electric charge, labelled "positive" and "negative". Two charges of the same kind repel—the force they exert pushes them apart—but two charges of the opposite kind attract—they are pulled together. In an atom, its positively charged nucleus is surrounded by negatively charged electrons, so the atom is electrically neutral: it has no charge. When, however, the atom loses or gains an electron, it becomes electrically charged, either positively (if it loses one) or negatively (if it gains one). The atom is then known as an ion.