Fr is the chemical symbol for the element francium, which is a highly radioactive alkali metal. It is the second rarest naturally occurring element in the Earth's crust and was discovered in 1939 by Marguerite Perey. Francium is known for its extreme instability and has a very short half-life, making it difficult to study. Due to its radioactivity, francium is not found in significant amounts in nature and is primarily produced in laboratories. It is estimated that there are only about 30 grams of francium in the entire Earth's crust at any given time. Its most stable isotope, francium-223, has a half-life of just 22 minutes.