lawrencium, artificially produced radioactive chemical element; symbol Lr; at. no. 103; mass number of most stable isotope 262; m.p. about 1,627°C; b.p. and sp. gr. unknown; valence +3. Lawrencium is the last member of the actinide series of elements found in Group 3 of the periodic table. Lawrencium was the 11th transuranium element to be discovered. It was first prepared in 1961 (after three years of preliminary work) by Albert Ghiorso, Torbjørn Sikkeland, Almon E. Larsh, and Robert M. Latimer at the Univ. of California at Berkeley; a sample of californium isotopes was bombarded with a beam of boron nuclei from the heavy-ion linear accelerator. The resulting isotope, lawrencium-258, had a half-life of 4.2 sec. The element was named for Ernest O. Lawrence, the inventor of the cyclotron. The symbol Lw was used at first, but it was changed to Lr in 1963. Ten isotopes, all of which are radioactive, are known; the most stable, lawrencium-262, has a half-life of 3.6 hours.