Magnesium-26
Magnesium (Mg) naturally occurs in three stable isotopes, 24Mg, 25Mg, and 26Mg. There are 19 radioisotopes that have been discovered, ranging from 19Mg to 40Mg. The longest-lived radioisotope is 28Mg with a half-life of 20.915 hours. The lighter isotopes mostly decay to isotopes of sodium while the heavier isotopes decay to isotopes of aluminium. The shortest-lived is 39Mg with a half-life shorter than 180 nanoseconds, or possibly 40Mg, with a half-life longer than 170 nanoseconds (the half-life of 19Mg is unknown).
Relative atomic mass: 24.3050(6)
...
Wikipedia