a substance in which all atoms are identical

A substance in which all atoms are identical is called an element. An element is a pure substance made up of only one kind of atom, where every atom has the same number of protons and similar properties. This distinguishes elements from compounds, which contain atoms of different elements chemically bonded together, and from mixtures, which contain different types of substances. Examples include oxygen (O), hydrogen (H), and gold (Au) where all atoms in those substances are the same type of atom.