Thorium (Th) is a radioactive chemical element with atomic number 90 and a mass of 232.038 u. It has 90 electrons, of which four are valence electrons, and four atomic orbitals are theoretically available for the valence electrons to occupy: 5f, 6d, 7s, and 7p. Despite its position in the f-block of the periodic table, thorium has an anomalous [Rn]6d27s2 electron configuration in the ground state.