Radiation can be stopped by an intervening material. The items don"t appear green because it"s radioactive, but because it contains uranium. Not harmful unless an acidic drink sits overnight in the item and is then consumed. Process of fossilization, minerals in the bone were replaced by minerals of uranium. Electrons (-) orbiting nucleus of protons (+) and neutrons. Same number of electrons as protons; net charge = 0. Isotopes: atoms with the same number of protons, different number of neutrons. Decay creates new unstable elements that also decay. Half life (t1/2) - time for 1/2 unstable nuclei to decay. After one t1/2 half of the original parent remains. After three t1/2 an eighth of the original parent remains. As the parent disappears, the daughter "grows in" Radioactive decay also creates heat in the crust. Higher energy electromagnetic waves (x-ray or gamma) or heavy particles (beta and alpha) High enough energy to pull electron from orbit.