Obscure Elements

In the final programme in our Elements series, Justin Rowlatt looks at the rarest and oddest members of the periodic table. Selenium, bismuth, molybdenum, antimony, rhenium, hafnium, zirconium, tellurium, thallium, barium. What are they? And what are they used for? Minor metals merchant Anthony Lipmann explains how he made a fortune tracking down a stockpile of one toxic element sufficient to kill millions of people - and sold it to Japanese camera manufacturers. We set chemistry professor Andrea Sella a musical challenge to round off his elucidation of the periodic table, going out with a pyrotechnic bang. And cosmologist Martin Rees explains why 85% of the matter in the universe isn't made up of chemical elements at all, but instead of "dark matter", whatever that is. (Picture: Elements series planning board; Credit: Laurence Knight/BBC)