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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.

Sulfur-35 is a radioactive isotope of sulfur with 16 protons and 19 neutrons. It undergoes beta-minus decay to stable chlorine-35. The cited evaluation gives a half-life of $87.25(15)\mathrm{d}$, much longer than the roughly 14-day half-life of phosphorus-32.^[ref]

The 1952 Hershey-Chase experiment used sulfur-35 to follow the sulfur-containing protein component of Bacteriophage T2. Agitation released about 75 percent of the phage sulfur label, compared with about 15 percent of the phosphorus label, while the infected cells could still yield progeny. The complementary labels helped separate the fates of external protein and entering DNA.^[ref] A label identifies the material carrying it under the specified labelling scheme; detection alone does not establish the function of every component.

Using the quoted half-life, a closed sample observed 14 days later retains $2^{-14/87.25}\approx 0.895$ of its starting radioactivity. That is about 90 percent, rather than one half. Comparing sulfur-35 and phosphorus-32 signals over days therefore requires isotope-specific decay accounting. Detector efficiency and recovery of labelled material also belong to the measurement model; equal raw counts do not automatically mean equal numbers of labelled atoms.