This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Isotopes of chlorine" – news ·newspapers ·books ·scholar ·JSTOR(May 2018) (Learn how and when to remove this message)

Chlorine (₁₇Cl) has two stableisotopes,³⁵Cl (75.8%) and³⁷Cl (24.2%), giving chlorine astandard atomic weight of 35.45. Artificicalradioisotopes are known ranging from²⁸Cl to⁵²Cl, and there are also twoisomers,^34mCl and^38mCl. The longest-lived radioactive isotope is³⁶Cl, which has a half-life of 301,000 years. All other isotopes and isomers have half-lives under an hour, and most under 10 seconds.

The representative terrestrial abundance of chlorine contains about is 24.2% of³⁷Cl, with a normal range of 23.9–24.5% of chlorine atoms.^[2] When measuring deviations in isotopic composition, the usual reference point is "Standard Mean Ocean Chloride" (SMOC), although aNISTStandard Reference Material (975a) also exists. SMOC is known to have a³⁷Cl/³⁵Cl ratio of 0.319627 ± 0.000199 (24.221% ± 0.0015% chlorine-37)^[9] and to have an atomic weight of 35.4525.

There is known variation in the isotopic abundance of chlorine. This heavier isotope tends to be more prevalent in chloride minerals than in aqueous solutions such as seawater, although the isotopic composition oforganochlorine compounds can vary in either direction from the SMOC standard in the range of severalparts per thousand.^[2]

Trace amounts ofradioactive³⁶Cl exist in the environment, in a ratio of about 7×10⁻¹³ to 1 with stable isotopes.³⁶Cl is produced in the atmosphere byspallation of³⁶Ar by interactions withcosmic rayprotons. In the subsurface environment,³⁶Cl is generated primarily as a result ofneutron capture by³⁵Cl ormuon capture by⁴⁰Ca.³⁶Cl decays to either³⁶S (1.9%) or to³⁶Ar (98.1%), with a combinedhalf-life of 308,000 years. The half-life of thishydrophilic nonreactive isotope makes it suitable forgeologic dating in the range of 60,000 to 1 million years. Additionally, large amounts of³⁶Cl were produced by neutron irradiation ofseawater during atmospheric detonations ofnuclear weapons between 1952 and 1958. The residence time of³⁶Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water insoil andground water,³⁶Cl is also useful for dating waters less than 50 years before the present.³⁶Cl has seen use in other areas of the geological sciences, forecasts, and elements. In chloride-basedmolten salt reactors the production of³⁶
Cl byneutron capture is an inevitable consequence of using natural isotope mixtures of chlorine (i.e. Those containing³⁵
Cl). This produces a long lived radioactive product which has to be stored or disposed off.Isotope separation to produce pure³⁷
Cl can vastly reduce³⁶
Cl production, but a small amount might still be produced by (n,2n) reactions involvingfast neutrons.

Besides being a component of natural stable chlorine, the chief notability of this isotope is its use to detectsolar neutrinos through inverse electron capture (producingthe gas³⁷Ar). This was used in the first detection at theHomestake experiment. Subsequentlygallium-71 was found more suitable for this purpose, and used inGALLEX/GNO andSAGE.

Daughter products other than chlorine

• Isotopes of argon
• Isotopes of sulfur
• Isotopes of phosphorus
• Isotopes of silicon

• Isotopes of chlorine
• Chlorine
• Lists of isotopes by element

• Articles with short description
• Short description with empty Wikidata description
• Articles needing additional references from May 2018
• All articles needing additional references