Lawrencium (Lr) is a synthetic element, and thus a standard atomic mass cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was 258Lr in 1961. There are twelve known radioisotopes from 252Lr to 266Lr, and 1 isomer (253mLr). The longest-lived isotope is 266Lr with a half-life of 11 hours. Heavier isotopes are expected to have longer half-lives.