Magnesium (12Mg) naturally occurs in three stable isotopes: 24 Mg, 25 Mg, and 26 Mg. There are 19 radioisotopes that have been discovered, ranging from 18 Mg to 40 Mg (with the exception of 39 Mg). The longest-lived radioisotope is 28 Mg with a half-life of 20.915(9) h. The lighter isotopes mostly decay to isotopes of sodium while the heavier isotopes decay to isotopes of aluminium. The shortest-lived is proton-unbound18 Mg with a half-life of 4.0(3.4) zeptoseconds.
A precise measurement of the neutron-rich 40Mg in 2019 showed the unexpected difference in its nuclear structure, compared to the lighter neighboring isotopes.[4][5]