Astatine is an element in the halogen group, which includes elements like chlorine and iodine. It’s a highly radioactive and rare element with a half-life of only a few hours. It’s typically a solid, black substance but it’s so rare that there’s never more than a few milligrams of it on Earth at any given time. Because of its radioactivity and rarity, astatine has mainly been used for scientific experiments in the field of nuclear chemistry and medicine.