Isotopes of magnesium
naturally occurs as three stable isotopes:,, and. There are also 19 artificial radioisotopes that have been discovered, ranging from to . The longest-lived of them is with a half-life of 20.915 hours. The isotopes lighter than the stable ones mostly decay to isotopes of sodium, while those heavier decay to isotopes of aluminium.
A precise measurement of the neutron-rich 40Mg in 2019 showed an unexpected difference in its nuclear structure, compared to the lighter neighboring isotopes.