Thallium

Thallium is a soft, gray metal, so soft it can be cut with a knife. William Crookes discovered it in 1861 when he spotted a bright green line in the light from a heated residue left over from making sulfuric acid. He named it after the Greek thallos, a green shoot. Today it is recovered in small amounts as a by-product of smelting zinc and lead ores.

Thallium is extremely poisonous, tasteless and odorless. It was once used in rat poisons and, infamously, by murderers; most countries have banned those uses. Today small amounts go into heart scans using thallium-201, into sodium iodide crystals that detect radiation, and into glass and lenses for infrared light.