a white metallic transuranic element artificially produced from plutonium. It is used as an alpha-particle source. Symbol: Am; atomic no: 95; half-life of most stable isotope,243Am: 7.4 × 103 years; valency: 2,3,4,5, or 6; relative density: 13.67; melting pt: 1176°C; boiling pt: 2607°C (est)