What do you call a substance made up of only one type of atom?

An element is defined as a pure substance that consists entirely of one type of atom, which means it cannot be broken down into simpler substances using chemical methods. Each element is characterized by a unique number of protons in its atomic nucleus, known as the atomic number, which distinguishes it from other elements. For example, oxygen (O) is an element made up solely of oxygen atoms, while gold (Au) consists only of gold atoms.

In contrast, compounds are substances formed when two or more different types of atoms chemically bond together. A mixture contains two or more substances that are not chemically combined and can usually be separated by physical means. Solutions are homogeneous mixtures of two or more substances, where one substance (the solute) is dissolved in another (the solvent). Understanding these distinctions helps clarify why an element is specifically defined as comprising only one type of atom.