High Quality Content by WIKIPEDIA articles! Selenium (Se) has six naturally occurring isotopes, five of which are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last three also occur as fission products, along with 79Se which has a half-life of 327,000 years, and 82Se which has a very long half-life (~1020 yr, decaying via double beta decay to 82Kr) and for practical purposes can be considered to be stable. 23 other unstable isotopes have been characterized, the longest-lived being 79Se with a half-life 327,000 years, 75Se with a half-life of 120 days, and 72Se with a half-life of 8.40 days. All other isotopes have half-lives less than 8 hours, most less than 38 seconds, which of these, 73Se is the most stable, with a half-life of 7.15 hours.