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.