N mass of one electron - 9 c 10j28. N elements j pure substance containing only on kind of atom. N the number of protons identifies an element. N all elements except hydrogen have one or more neutrons. N mass number is the number of protons + number of neutrons. N mass number = mass of atom if daltons. Isotopes j forms of an element with different numbers of neutrons, thus different mass numbers. N atomic weight: average of mass numbers of isotopes in their normally occurring proportions. N radioisotopes are unstable; they give off energy in the form of alpha, beta, and gamma radiation from the nucleus. S this radioactive decay transforms the atom, including changes in the number of protons. S energy from radioactive decay can interact with surrounding material. S radioisotopes can be incorporated into molecules and.