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An elementary component of inert or living matter.
The atom is the smallest component that gives matter its chemical properties. It is itself made up of particles (protons, neutrons and electrons). There are about a hundred types of atom. They combine in very different ways (although without losing their specific identity) to form all known matter.
Each atom is made up of a nucleus, itself consisting of interacting neutrons (particles with no electric charge) and protons (positively charged). This nucleus is surrounded by electrons (negatively charged), which occupy a volume of space a million billion times greater than the nucleus, and are governed by the laws of quantum physics. The radius of an atom with its electron cloud is of the order of a tenth of a nanometre. Each atom is neutral overall (with no electric charge), having an equal number of electrons and protons. It is this number - known as the atomic number - that characterises the atom's nature and governs its chemical properties. The simplest atom, made up of 1 proton and 1 electron, is the hydrogen atom.