Magnesium (12Mg) naturally occurs as three stable isotopes: 24Mg, 25Mg, and 26Mg. There are also 19 artificial radioisotopes that have been discovered, ranging from 18Mg to 40Mg. The longest-lived of them is 28Mg 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.