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Abundance of the major isotopes of the halogens.
Isotope Natural abundance (%)
Fluorine-19 100
Chlorine-35 75.77
Chlorine-36 trace
Chlorine-37 24.23
Bromine-79 50.69
Bromine-81 49.31
Iodine-127 100%

While 19 F is the only naturally abundant isotope of fluorine, the synthetic isotope, 18 F, has half life of about 110 minutes, and is commercially an important source of positrons for positron emission tomography (PET). PET is a nuclear medicine imaging technique that produces a 3-D image of processes within the body. The system detects pairs of γ-rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule.

Trace amounts of radioactive 36 Cl exist in the environment at about 7 x 10 -11 %. 36 Cl is produced in the atmosphere by the interaction of cosmic rays with 36 Ar. In the ground 36 Cl is generated through neutron capture by 35 Cl or muon (an elemental particle similar to an electron) capture by 40 Ca. 36 Cl decays with a half-life of 308,000 years making it suitable for geologic dating in the range of 60,000 to 1 million years. However, due to the large amounts of 36 Cl produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958, it is also sued as an event marker for 1950s water in soil and ground water.

Iodine has 37 isotopes of iodine, but only one, 127 I, is stable. Of the radioactive isotopes, 129 I (half-life 15.7 million years) is used for radiometric dating of the first 85 million years of solar system evolution. 129 I is also a product of uranium and plutonium fission, and as a consequence of nuclear fuel reprocessing and atmospheric nuclear weapons tests, the natural signal has been swamped. As a consequence it can now be used as a tracer of nuclear waste dispersion into the environment. 129 I was used in rainwater studies to track fission products following the Chernobyl disaster.

Due to preferential uptake of iodine by the thyroid, isotopes with short half lives such as 131 I can be used for thyroid ablation, a procedure in which radioactive iodine is administered intravenously or orally following a diagnostic scan. The lower energy isotopes 123 I and 125 I are used as tracers to evaluate the anatomic and physiologic function of the thyroid.

Industrial production.

Industrial production of fluorine involves the electrolysis of hydrogen fluoride (HF) in the presence of potassium fluoride (KF) during which fluorine gas is formed at the anode and hydrogen gas is formed at the cathode ( [link] ). The potassium fluoride (KF) is converted to potassium bifluoride (KHF 2 ), [link] , which is the electrolyte and intermediate to the fluorine and hydrogen, [link] .

The fluorine cell room at F2 Chemicals Ltd, Preston, UK.

The HF is formed as a byproduct of the production of phosphoric acid, since phosphate-containing minerals contain significant amounts of calcium fluorides, which upon treatment with sulfuric acid release hydrogen fluoride, [link] .

Chlorine is generally manufactured by electrolysis of a sodium chloride solution (brine). The production of chlorine results in the co-products caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H 2 ). Chlorine can also be produced by the electrolysis of a solution of potassium chloride, in which case the co-products are hydrogen and caustic potash (potassium hydroxide). There are three industrial methods for the extraction of chlorine by electrolysis of chloride solutions, all proceeding by the same reaction at the cathode, [link] , and anode, [link] , which lead to the overall reaction, [link] , where M = Na or K.

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Source:  OpenStax, Chemistry of the main group elements. OpenStax CNX. Aug 20, 2010 Download for free at http://cnx.org/content/col11124/1.25
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