Element : substance that cannot be broken down to other substances by chemical reactions. Dalton : unit of measurement (it is the atomic mass unit or amu ) Atomic mass : total mass of an atom (protons + neutrons) Isotope : different atomic forms of the same element. Radioactive isotopes : is one in which the nucleus decays spontaneously, giving off particles + energy. Half-life : the time it takes for 50% of the parent isotope to decay (useful for dating things i. e. fossils) Radiometric dating : a process that scientists use to measure the ratio of different isotopes and calculate how many half-lives (in years) have passed since an organism was fossilized or a rock was formed. Energy : the capacity to cause change by doing work. Potential energy : the energy that matter posses because of its location or structure.