(physics) A long-lived radioactive isotope of chlorine, ³⁶₁₇Cl, having seventeen protons and nineteen neutrons, and a half-life of 3x10⁵ years; it can be used to date samples of groundwater

(physics) A minor stable isotope of sulfur, ³⁶₁₆S, having sixteen protons and twenty neutrons; it amounts to only 0.01% of the element in nature

(physics) A rare stable isotope of argon, ³⁶₁₈Ar, having eighteen protons and eighteen neutrons; it amounts to about 0.3% of the element in nature