Schematic description of oxygen atom.

The building block of matter. The name “atom” comes from the Greek “atomos”, which means uncuttable, but an atom is not the smallest part of matter. It consists of a nucleus surrounded by an envelope of electrons. The nucleus is composed of neutrons and protons; in the lightest element, hydrogen, the atom’s nucleus is made up of only one single proton. When the number of electrons and protons match, the atom has a neutral charge. If there are fewer electrons than protons, the atom is called an ion and has a positive charge. The number of neutrons in a nucleus can vary. The atoms with different number of neutrons are called isotopes. Some isotopes can be unstable and thus radioactive. The number of protons in the nucleus distinguishes different chemical elements. To 2023, atoms of 118 elements are known.