(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

(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 minor stable isotope of silicon, ³⁰₁₄Si, having fourteen protons and sixteen neutrons; it amounts to 3% of the element in nature

(physics) The minor stable isotope of chlorine, ³⁷₁₇Cl, having seventeen protons and twenty neutrons; it amounts to about 24% of the element in nature