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Argon-37 is a radioactive isotope with $18$ protons and $19$ neutrons. It undergoes electron capture to chlorine-37, with an approximately $35$-day half-life:The mass number is unchanged while the nuclear charge decreases by one. Atomic vacancies left by the captured electron can produce detectable low-energy radiation during atomic relaxation.

Argon-37 made from chlorine was the counted product in the radiochemical solar-neutrino experiment. The method separated rare radioactive argon atoms from a much larger chlorine target, connecting nuclear counting to a very weak astronomical signal.^[ref]

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.^[ref]

Those fractions explain a characteristic mass spectrometry pattern. In an idealized sample of $\mathrm{C}{\mathrm{l}}_2$ with independently paired isotope identities, the three molecular combinations have probabilitiesThe 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.^[ref] Neither that origin nor the presence of chlorine-35 makes an ordinary chlorine-containing compound radioactive.

Chlorine-37 is a stable isotope of chlorine containing $17$ protons and $20$ neutrons. Its mass number is $37$, which is distinct from its measured relative atomic mass and from the average atomic weight of a natural chlorine sample.

NIST's representative isotopic composition lists about $24.24\%$ chlorine-37 and $75.76\%$ chlorine-35.^[ref] The two stable isotopes explain characteristic peaks separated by two nominal mass units in chlorine-containing mass spectra. Exact peak patterns also depend on the number of chlorine atoms and other isotopes present.

Raymond Davis Jr. developed a chemical method for observing solar neutrinos. In the chlorine experiment, rare interactions converted chlorine-37 into radioactive argon-37; extraction and counting of the argon atoms made a very weak interaction observable. The unexpectedly low measured rate became a central part of the solar neutrino problem.^[ref]

A chemist.^[ref]