Lawrencium is an artificially produced radioactive element. It has an atomic number (number of protons) of 103. Its chemical symbol is Lr. The most stable form of lawrencium has an atomic mass number (total number of protons and neutrons) of 262. This isotope has a half-life of 3.6 hours--that is, due to radioactive decay, only half the atoms in a sample of isotope 262 would still be atoms of that isotope after 3.6 hours.