Terbium is a chemical element that has the symbol Tb on the periodic table. It has the atomic number 65 which means it has 65 protons in an atom. Its compounds fluoresce green in ultraviolet light. The metal can burned in air to form an oxide. It is a lanthanide and, along with yttrium, erbium, and ytterbium, is one of the four elements to be named after the town of Ytterby in Sweden, all of which are rare earth elements.