A chemical element is a pure substance consisting only of atoms that all have the same numbers of protons in their nuclei. Elements represent the fundamental building blocks of matter and are defined by their unique atomic number, which determines the type of atom. The organization of elements is systematically presented in the periodic table, which groups them according to recurring chemical properties. Elements combine to form compounds, the vast majority of substances encountered in nature and technology. They are essential to physical and biological processes, forming the structural components of minerals, alloys, and biological macromolecules.