= Chlorine-35
{title2=$^{35}\mathrm{Cl}$}
<Chlorine-35> is a stable <isotope> of <chlorine>, with 17 <protons> and 18 <neutrons> in its <atomic nucleus>. Its <mass number> is $35$, while its measured relative <atomic mass> is approximately $34.968852682$. NIST lists representative natural fractions $p=0.7576$ for <chlorine-35> and $q=0.2424$ for <chlorine-37>.<Atomic Weights and Isotopic Compositions for All Elements by National Institute of Standards and Technology>[Chlorine rows 35 and 37, masses and compositions]{ref}
Those fractions explain a characteristic <mass spectrometry> pattern. In an idealized sample of $\mathrm{Cl_2}$ with independently paired isotope identities, the three molecular combinations have probabilities
$$
p^2\approx0.57396,\qquad 2pq\approx0.36728,\qquad q^2\approx0.05876.
$$
The middle term has a factor of two because either atom can be the heavier isotope. For singly charged molecular ions, the combinations give nominal mass peaks at 70, 72 and 74. A <mass spectrometer> separates ions by <mass-to-charge ratio>, so the two-unit spacing encodes isotope composition rather than three different chemical elements. These are ideal abundance predictions; actual peak intensities also depend on fragmentation and instrumental response.
<Chlorine-35> is the stable daughter in <sulfur-35> <beta-minus decay>: $Z$ increases from 16 to 17 while the <mass number> remains 35.<Sulfur-35 in LNE-LNHB Table de Radionucleides>[Page 1, decay scheme]{ref} Neither that origin nor the presence of <chlorine-35> makes an ordinary chlorine-containing compound radioactive.
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