The naturally radioactive elements occupy positions at the end of the periodic table. They have large nuclei which are made up of about 90 protons together with about 140 neutrons. Little is yet known for certain about the force which holds protons and neutrons together in the nucleus (much present-day research is devoted to this subject), but it seems to have reached its limit in uniting this number of nucleons, and from time to time an atom disintegrates.
Unable to display preview. Download preview PDF.