Dating With Radioactivity It is possible to measure numerical values for the age of Earth?s geologic activities - in billions and millions of years - by using radiometric dating. A. Reviewing Basic Atomic Structure An atom has a nucleus with protons (positive) and neutrons (proton+electron combined = neutral) orbited by electrons (negative charge). Atomic Number = number of protons in the nucleus; each element?s Atomic Number differs. Atoms of the same element have the same number of protons. 99.9% of an atom?s mass is in the nucleus - electrons have hardly any mass. By adding together the number of protons and neutrons, the atomic mass of an atom is obtained. The number of neutrons varies (called isotopes; have different atomic masses).