Isotope periodic table. Scientists Peter Armbrus- ter & Sigurd Hofmann discuss a periodic table of the different forms (isotopes) of each chemical element. The table is in 4 sections. The lightest elements are at top left, whilst the heaviest elements are in the poster's bottom right corner. An element's identity is determined by the number of protons it has in its nucleus (rows); each element's isotopes have different numbers of neutrons (columns). Isotopes coloured black are stable, the others are radioactive. Armbruster discovered elements 107-112, the 6 heaviest- known in 1997, during 1981-96 at the Society for Heavy Ion Research (GSI) centre, Darmstadt, Germany.