Thorium

radioactive · element · metal · rare

Atomic number 90. Thorium sits near the bottom of the periodic table with some of the heaviest elements, making it part of an exclusive group called the actinides.

Weakly radioactive. Unlike some radioactive elements that are dangerously powerful, thorium is only mildly radioactive, which means it releases energy very slowly over millions of years.

Can spontaneously ignite. When thorium powder is extremely finely divided, it can actually catch fire in air all by itself because of chemical reactions happening on its surface.

Silvery and malleable. Fresh thorium looks like a shiny silver metal, but when exposed to air it gradually turns a dull olive-grey color as it forms a protective coating called thorium dioxide.

High melting point. Thorium can stay solid at incredibly hot temperatures, making it useful in materials that need to handle extreme heat without melting.