Magnesium 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. The longest-lived radioisotope is ²⁸ Mg with a half-life of 20.915 h. Wikipedia