Thorium - Th
Thorium is one of the weakly radioactive elements. Many isotopes exist, but only one of them occurs in nature, being 232Th, with a half-life of 1.405×1010 years. The half-life of the 232Th isotope is so large (approximately the age of the universe), that the radioactivity only is marginal. For this reason, thorium can be handled safely, just as any other chemical. Thorium, however, is a toxic heavy metal and as such it is best to have it around in a completely sealed glass ampoule, as shown in the picture above.
The piece of metal in the ampoule has a size of approximately 1.5x5 cm2, and it has a thickness of almost 1 mm. The ampoule itself is quite large. Unfortunately, the piece of metal is not ampouled in an inert atmosphere, apparently it was ampouled with plain air. The sample is old (expected age approximately 50 years) and in those years, the oxygen inside the ampoule has oxidized the surface of the thorium metal, and a thin layer of white thorium dioxide is formed. The picture nicely shows the layer of oxide, and some of the grey metal can be seen below the irregular oxide layer.
Inside the same ampoule, also some thorium oxide has fallen off the metal piece. The picture below shows the piece of metal, and some powder below the metal. So, the thorium oxide does not form a protective layer and in air, a piece of thorium metal slowly would be converted to nothing more than a heap of thorium dioxide dust.