Selenium-74

Main isotopes of selenium

Iso­tope Decay abun­dance half-life (t1/2) mode pro­duct 72Se syn 8.4 d ε 72As γ – 74Se 0.86% stable 75Se syn 119.8 d ε 75As γ – 76Se 9.23% stable 77Se 7.60% stable 78Se 23.69% stable 79Se trace 3.27×105 y β− 79Br 80Se 49.80% stable 82Se 8.82% 1.08×1020 y β−β− 82Kr
Standard atomic weight (Ar, standard)	78.971(8)

The chemical element selenium (₃₄Se) 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 years, decaying via double beta decay to ⁸²Kr) and for practical purposes can be considered to be stable. There are 23 other unstable isotopes that 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. Of the other isotopes, ⁷³Se has the longest half-life, 7.15 hours; most others have half-lives not exceeding 38 seconds.

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