Chlorine, (Chemical Element, Symbol Cl, Atomic Number 17)




(The other halogens - that is, members of the same group are Fluorine, Bromine, Iodine and Astatine. These have some properties in common with chlorine and there are also certain "trends" or patterns in the variation of properties across this group of elements.)

Each chlorine atom consists of 17 protons, 17 electrons plus a quantity of neutrons whose exact number depends on the particular isotope of chlorine (in most cases there are 18 neutrons in each atom of chlorine).

History and Discovery of Chlorine

The most common compound of chlorine is sodium chloride, which is also known as 'common salt' or 'table salt'. Sodium chloride has been known and used for thousands of years. Archaeologists have found evidence of rock salt being used as early as 3000 BC and brine since even earlier.

Elemental chlorine has only been recognised more recently. Swedish pharmacist Carl Wilhem Scheele is credited with the 'discovery' (first recorded observation) of the element chlorine gas (Cl2) because in 1774 he released a few drops of hydrochloric acid onto a piece of the mineral pyrolusite, whose chemical name is manganese dioxide. The result was a greenish-yellow gas that he called 'dephlogisticated muriatic acid air' and which later became known as 'chlorine'. Initially Scheele thought that the greenish-yellow gas he observed was an oxide produced as a result of a reaction of the mineral with the hydrochloric acid. The identity of this greenish-yellow gas as an element was not established until 1810 when the English chemist Sir Humphrey Davy tried to decompose the 'dephlogisticated muriatic acid air' by reacting it with charcoal to release the free element 'muriaticum' (which is not a real element but rather the name given by Scheele to the element whose oxide he thought had been released in the form of the greenish-yellow gas) and carbon dioxide. Sir Davy concluded from his experiements that the greenish-yellow gas was not, in fact, an oxide or any other compound but rather an element and suggested the name 'chloric gas' or 'chlorine' based on the Greek word khloros, which means 'greenish-yellow'.

Chlorine was liquefied for the first time in 1823 by the English chemist and physicist Michael Faraday.

Chlorine at standard temperature and pressure ('standard conditions')

Chlorine exists in nature as a toxic gas (Cl2) that irritates the respiratory system.
Chlorine gas is approx. 2.5 times heavier than air so it tends to accumulate at the bottom of poorly ventilated spaces. It is also a strong oxidizer, that can react with flammable materials.

Chlorine gas consists of molecules, rather than atoms, of chlorine. Two atoms of chlorine form each chlorine molecule, hence molecules of chlorine are said to be diatomic and the chemical symbol for chlorine gas is Cl2.

Health effects of Chlorine Gas

The toxicity of chlorine it due to its oxidizing power.

When chlorine is inhaled at concentrations above 30 ppm, it begins to react with water and the cells of the body, which change it into hydrochloric acid (HCl) and hypochlorous acid (HClO). Coughing and vomiting may occur at 30 ppm and lung damage at 60 ppm. Chlorine gas is said to 'sting' the back of the throat and chest and reacts with the water present in the mucosa of the lungs to form hydrochloric acid, which can be lethal. Approx 1000 ppm of chlorine can be fatal after just a few deep breaths. Even breathing in only lower concentrations can irritate the respiratory system, while exposure to the gas can irritate the eyes.

Does chlorine have any positive effects on health ?

Certain compounds of chlorine are important for good health in all animals, including humans.
For example:

  • Hydrochloric acid (HCl) in the stomach, helps in with the process of digesting of foods.
  • Salts such as sodium chloride (NaCl) which is also known as 'table salt', and potassium chloride (KCl) are necessary for the nervous system, that is how nerves transmit messages around body. Because humans eat so much salt, a lack of chlorine compounds is seldom a health problem.

Compounds of Chlorine (in general)

Chlorine combines with most other elements, forming chlorides.
The Cl - ('chlorine') anion is one of the most common anions in nature.

Hydrogen chloride and its aqueous solution, hydrochloric acid, are produced on a large scale.

Isotopes of Chlorine

Chlorine has several isotopes, the most common and stable of which are 35Cl, commonly referred to as "chlorine 35" and 37Cl, commonly referred to as "chlorine 37". Typical proportions of these are 75.77% 35Cl and 24.23% 37Cl, hence the atomic mass of chlorine is often stated as 35.5. (That is, 35 x 75.77/100 + 37 x 24.23/100 = 35 x 0.7577 + 37 x 0.2423 = 26.5195 + 8.9651 = 35.4846, which is approximately equal to 35.5)

Isotopes of chlorine include:


Content of Nucleus:



17 protons, 18 neutrons

Chlorine-35 is a stable isotope that accounts for approx. 76% of known chlorine by weight.


17 protons, 19 neutrons

Trace amounts of the radioactive isotope chlorine-36 exist in nature. Larger amounts of chlorine-36 were produced by irradiation of seawater during atmospheric detonations of nuclear weapons in the 1950s. Chlorine-36 has been used in various applications within the geological sciences - including dating ice and sediments.


17 protons, 20 neutrons

Chlorine-37 is a stable isotope that accounts for approx. 24% of known chlorine by weight.

Uses of Chlorine (Gas)

  • Chemical Weapons (Historically): Chlorine gas, also known as 'bertholite', was used as a chemical weapon during World War I. The damage caused by chlorine gas (see the section 'Health effects of Chlorine Gas' above) can be prevented by a gas mask or other filtration method. After its first use, chlorine was used as a chemical weapon by both sides in WWI but it was soon replaced by the more deadly gases.

Uses of Chlorine (including chlorine combined with other elements to form compounds of chlorine)

  • Bleaching and Disinfectants (in general): Disinfectants that include the chemical element chlorine are used against many different dangerous germs in both domestic house and public places such as hospitals, swimming pools, hotels and restaurants. Chlorine's powerful disinfectant qualities are due to its ability to bond with and destroy the outer surfaces of bacteria and viruses.
    Historically: Chlorine was first used as a germicide in the maternity wards of Vienna General Hospital in Austria in 1847 and became one of humanity's fiercest weapons against a wide range of life-threatening infections, viruses, and bacteria.
  • Disinfecting Water: Chlorine is often present in chemicals used to kill bacteria in drinking water and swimming pools. Chlorine is very effect against e coli bacteria.
  • Organic Chemistry: Chlorine has been found to be very useful for studying and synthesizing organic compounds (definitions at What is Organic Chemistry ? and What is a compound ?).
  • Oxidant: Chlorine is a powerful oxidant.
  • Pharmaceuticals: Chlorine chemistry is used in the production of a large proportional of all prescription drugs.
  • Swimming Pools: Chlorine is present in chemicals commonly used to keep swimming pools safe by killing potentially harmful bacteria that might otherwise thrive and multiply in the water, causing health hazards.

There are many uses of specific well-known compounds of chlorine.

Examples include:

  • Chloroform: used to be used in medicine and is still in common use a solvent in science laboratories.
  • Methyl chloride: has several applications. It is used as an anesthetic. It is also used to make certain silicone polymers and to extract certain greases, oils and resins.
  • PVC (polyvinyl chloride): has many and various uses, incl. to make clothing, flooring, electrical cables, flexible hoses and tubings, waterbeds and inflatable structures from furniture to outdoor play-equipment, even some ceiling tiles.
  • Salt (Table Salt): Sodium Chloride (NaCl) is widely used to season food - both during food preparation and at the table, and has also been used in the past as a method of food preservation. In the past this was such an important substance that some people were even paid in for their work in 'salt', leading to the common English word 'salary'.
  • Trichloroethylene: is used to degrease metal parts, e.g. in some engineering situations.



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