(Z = 1 → 118, Strict IUPAC Stable Definition) Element (Z) Isotopes Known Stable Unstable Longest-Lived / Notable Isotope Notable Uses H (1) 7 2 5 ³H (12.3 y) Nuclear fusion, tracers He (2) 9 2 7 ⁸He (119 ms) Cryogenics, shielding Li (3) 11 2 9 ¹¹Li (8.6 ms) Nuclear halo studies Be (4) … Continue reading Isotope Master Summary