A compound is a substance formed when two or more different chemical elements are chemically bonded together in a fixed ratio. NaCl is formed by the chemical combination of the elements sodium (Na) and chlorine (Cl) in a 1:1 ratio. An element is a pure substance consisting only of atoms that all have the same number of protons in their atomic nuclei. A mixture is a substance comprising two or more components not chemically bonded. An alloy is a mixture of metals.