Science, Tech, Math › Science Helium Isotopes, Radioactive Decay and Half-Life Share Flipboard Email Print This is a periodic table tile for the element helium. Todd Helmenstine Science Chemistry Periodic Table Basics Chemical Laws Molecules Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry in Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by Anne Marie Helmenstine, Ph.D. Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. Updated January 29, 2019 It takes two protons to make a helium atom. The difference between isotopes is the number of neutrons. Helium has seven known isotopes, ranging from He-3 to He-9. Most of these isotopes have multiple decay schemes where the decay type depends on the overall energy of the nucleus and its total angular momentum quantum number.This table lists helium isotopes, half-life, and type of decay:IsotopeHalf-LifeDecayHe-3StableN/AHe-4Stable≈ 0.5 x 10-21 sec - 1 x 10-21 secN/Ap or nHe-51 x 10-21 secnHe-60.8 sec5 x 10-23 sec - 5 x 10-21 secβ-nHe-73 x 10-22 sec - 4 x 10-21 secnHe-80.1 sec0.5 x 10-21 sec - 1 x 10-21 secβ-n/αHe-9unknownunknownpnαβ-proton emissionneutron emissionalpha decaybeta- decay Continue Reading