Mendelevium-250

Main isotopes of mendelevium

iso NA half-life DM DE (MeV) DP 257Md syn 5.52 h ε 0.406 257Fm α 7.558 253Es SF – – 258Md syn 51.5 d ε 1.230 258Fm 260Md syn 31.8 d SF – – α 7.000 256Es ε – 260Fm β− 1.000 260No

Mendelevium (₁₀₁Md) is a synthetic element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was ²⁵⁶Md (which was also the first isotope of any element produced one atom at a time) in 1955. There are 16 known radioisotopes, ranging in atomic mass from ²⁴⁵Md to ²⁶⁰Md, and 5 isomers. The longest-lived isotope is ²⁵⁸Md with a half-life of 51.3 days, and the longest-lived isomer is ^258mMd with a half-life of 58.0 minutes.

...
Wikipedia