Astatine (At, Z=85) is the second-rarest naturally occurring element (after francium). All isotopes of astatine are short-lived and highly radioactive. Francium (Fr, Z=87) is an alkali metal. It is the most unstable of the naturally occurring elements. All isotopes of francium are highly radioactive and have very short half-lives. Tritium (³H) is a radioactive isotope of hydrogen, with a half-life of about 12.3 years. It decays by beta emission. Zirconium (Zr, Z=40) is a transition metal. Natural zirconium is composed of five stable isotopes (⁹⁰Zr, ⁹¹Zr, ⁹²Zr, ⁹⁴Zr) and one extremely long-lived radioisotope (⁹⁶Zr, half-life > 2.0 × 10¹⁹ years). In general contexts, Zirconium is considered a stable element and is used in applications like nuclear reactors due to its low neutron absorption cross-section, not its radioactivity.