A substance that cannot be broken down into simpler substances by chemical means. An element is composed of atoms that have the same atomic number, that is, each atom has the same number of protons in its nucleus as all other atoms of that element. Today 115 elements are known, of which 92 are known to occur in nature, while the remainder have only been made with particle accelerators. Eighty-one of the elements have isotopes that are stable. The others, including technetium, promethium, and those with atomic numbers higher than 83, are radioactive. See Periodic Table.
Our Living Language: When Russian chemist Dmitri Mendeleev devised the periodic table in 1869, there were 63 known elements. Mendeleev classified the known elements by atomic weight, and arranged a table listing them with vertical rows corresponding to shared chemical characteristics. Gaps in the table suggested the possibility of elements not yet discovered, and indeed elements were later discovered, or in some cases, artificially created, that filled the gaps and had the expected chemical properties. The striking correlation between the atomic weight of an element and its chemical properties was later explained by quantum mechanical theories of the atom. The weight of an atom of any given element depends on the number of protons (and neutrons) in its nucleus, but the number of protons also determines the number and arrangement of electrons that can orbit the nucleus, and it is these outer shells of electrons that largely determine the element's chemical properties. Currently, 115 distinct elements are known.