Asubstance 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 117 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.