Lawrencium (Lr) is a radioactive, metallic element that is a member of the actinide series and also a transuranic element. It was first made by A. Ghioro and coworkers in 1961 at the University of California at Berkeley by bombarding californium with boron nuclei; it is named after the physicist Ernest Lawrence. The element has been made in only trace amounts but has been identified chemically. Its most stable isotope is 256Lr (half-life 27 seconds). Atomic number 103, relative atomic mass 262.