A chemical element, Po, atomic number 84. Marie Curie discovered the radioisotope ²¹⁰Po in pitchblende. This isotope is the penultimate member of the radium decay series. All polonium isotopes are radioactive, and all are shortlived except the three α-emitters, artificially produced ²⁰⁸Po (2.9 years) and ²⁰⁹Po (100 years), and natural ²¹⁰Po (138.4 days).