A chemical element, symbol Sm, atomic number 62, belonging to the rare-earth group. Its atomic weight is 150.35, and the naturally occurring isotopes are ¹⁴⁴Sm (3.09%), ¹⁴⁷Sm (14.97%), ¹⁴⁸Sm 11.24%, ¹⁴⁹Sm 13.83%, ¹⁵⁰Sm 7.44%, ¹⁵²Sm 26.72%, and ¹⁵⁴Sm 22.71%. ¹⁴⁷Sm, ¹⁴⁸Sm, and ¹⁴⁹Sm are radioactive and emit alpha particles. The half-lives are 1.06 × 10¹¹ years, 1.2 × 10¹³ years, and 4 × 10¹⁴ years, respectively.