What is defined as a pure substance made of only one kind of atom?

An element is defined as a pure substance that consists of only one kind of atom. Elements are the basic building blocks of matter and cannot be broken down into simpler substances by chemical methods. Each element is characterized by a unique number of protons in its atomic nucleus, which determines its chemical properties and its position on the periodic table.

For example, carbon (C), gold (Au), and oxygen (O) are all elements, each composed solely of atoms of that particular type. Compounds, on the other hand, are substances formed when two or more different types of atoms bond together, such as water (H2O), which contains hydrogen and oxygen. Molecules refer to two or more atoms bonded together, but they can involve the same kind of atoms (like in O2) or different kinds (like in H2O). Ions are charged particles that have lost or gained electrons, making them charged atoms or groups of atoms but not pure substances solely comprised of one type of atom. Thus, the definition of an element clearly matches the characteristics of a pure substance made of only one type of atom.