Mendelevium is a synthetic element with chemical symbol Md (formerly Mv) and atomic number 101. A metallic radioactive transuranic element in the actinide series, it is the first element that currently cannot be produced in macroscopic quantities through neutron bombardment of lighter elements. It is the actinide and the ninth transuranic element. It can only be produced in particle accelerators by bombarding lighter elements with charged particles. A total of sixteen mendelevium isotopes are known, the most stable being ²⁵⁸Md with a half-life of 51 days; nevertheless, the shorter-lived ²⁵⁶Md (half-life 1.27 hours) is most commonly used in chemistry because it can be produced on a larger scale.