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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.

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.

The nuclear reactionconserves electric charge and total nucleon count: $3+1=2+2$ and $7+1=4+4$. Its nuclear reaction Q-value is the initial minus final rest energy. It is additional to the incoming beam kinetic energy; the accelerator triggers a rearrangement whose products can carry much more kinetic energy than the incident proton.

Use one consistent set of neutral-atom isotope masses, in unified atomic mass units, from the National Institute of Standards and Technology:These are isotope masses, not element-averaged atomic weights.^[ref] With $\mathrm{u}{c}^2=931.49410372\mathrm{MeV}$,^[ref]The four electron rest masses cancel between sides. Differences of total electronic binding energies do not cancel exactly: this atomic-mass result approximates the bare-nucleus nuclear reaction Q-value at the displayed $0.001\mathrm{MeV}$ scale, not at arbitrary precision. No mixture of rounded nuclear binding energies is needed. At chosen incident kinetic energy $T=0.500\mathrm{MeV}$ on a stationary target, the two outgoing alpha particles together carry approximately $Q+T=17.846\mathrm{MeV}$ before losses in the target and detector window.

Sulfur-32 is a stable isotope of sulfur whose atomic nucleus contains 16 protons and 16 neutrons. Its mass number is $32$, the sum of those counts. The measured relative atomic mass is approximately $31.9720711744$; a mass number is an integer count, whereas a relative atomic mass is a mass ratio.^[ref]

The same NIST table gives a representative isotopic abundance of $0.9499$, or $94.99\%$, making sulfur-32 the predominant natural sulfur isotope. This is a representative composition, not an immutable percentage in every sample. Using the listed sulfur-34 fraction $0.0425$, the corresponding isotope ratio is $0.0425/0.9499\approx 0.04474$: about 4.47 sulfur-34 atoms per 100 sulfur-32 atoms.^[ref] An isotope ratio and a percentage of all sulfur atoms have different denominators.

Sulfur-32 is also the daughter of phosphorus-32 beta-minus decay. One nuclear neutron becomes a proton, changing $Z=15$ to $Z=16$ while retaining mass number $32$.^[ref] This connects a radioactive tracer to a stable daughter without turning their chemical identities into the same thing.

Sulfur-34 is a stable isotope of sulfur with 16 protons and 18 neutrons in its atomic nucleus. Its mass number is $34$, while its relative atomic mass is approximately $33.967867004$. NIST lists a representative isotopic abundance of $0.0425$, or $4.25\%$, alongside $0.9499$ for sulfur-32.^[ref] The representative composition does not require every natural sample to have exactly those fractions.

Comparing sulfur-34 with sulfur-32 illustrates how an isotope ratio differs from a fraction of all sulfur. The representative ratio isIn an idealized measurement that has already corrected differences in response and interferences, suppose the inferred atom counts are $N_{34}=450$ and $N_{32}=10000$. Their ratio is $0.045$, about $0.58\%$ higher than that representative ratio. This compares two isotopes; calculating the sulfur-34 fraction of all sulfur additionally needs the other isotopes in the denominator. Raw mass spectrometer signals are not automatically corrected atom counts.

Sulfur-34 and sulfur-32 have the same atomic number, so replacing one with the other preserves the element while changing nuclear mass. This makes isotope composition an additional descriptor of a sulfur-containing sample, distinct from its chemical formula. A stable isotope also provides a different kind of label from radioactive sulfur-35: its usefulness does not depend on detecting radioactive decay.