Lanthanum

Discovered by Carl Gustav Mosander of Stockholm Sweden in 1839, lanthanum is a soft metal that can be easily cut with a knife. Lanthanum also is the most reactive of all rare earth metals- it reacts with carbon, nitrogen, boron, selenium, silicon, phosphorus, sulfur and the halogens (such as iodine). It's name is derived from the Greek word for "to lie hidden".

Fun Fact: It is one of the four rare earth elements.

Lanthanum is mildly toxic, and while a small exposure cannot kill you or make you sick, a large exposure to it can so it should be handled with care. It oxidizes quickly to oxygen and hot water, but barely reacts at all to cold water.

Fun Fact: Lanthanum is often used as an alloy, but its compounds are more often used for lighting by cinemas for stage lights.