A synthetic transuranic radioactive element having isotopes with mass numbers from 235 to 254. Its longest-lived isotopes, Bk-247 and Bk-248, have half-lives of 1,380 years and more than 9 years, respectively; the isotope produced in greatest quantity, Bk-249, has a half-life of 330 days. Atomic number 97; melting point 996°C; specific gravity 14 (estimated); valence 3, 4. See Periodic Table.