Chlorine (Cl)

Chlorine is a chemical element with an atomic number of 17 in the periodic table of elements. It’s the twentieth most abundant element found in Earth’s crust, amounting to 130 ppm or 0.01 percent. Being a member of the halogen family of periodic table elements, chlorine is a highly reactive chemical substance with seven valence electrons in its outer shell.

Chemical and Physical Properties of Chlorine

Atomic number17
Atomic weight35.4527 g.mol-1
ColorPale greenish-yellow gas at room temperature / Clear, amber-colored liquid under applied pressure or at temperatures below -30°F (-34°C)
Physical stateGas / Liquid / Solid
Crystal StructureOrthorhombic
Density0.003214 g/cm³
Melting point172.17 °C
Boiling point239.1 °C
Ionic radius1.81 (-1) Å
Most characteristic isotope35Cl, 36Cl, 37Cl
Electronic shell[Ne] 3s2 3p5
The energy of the first ionization1255.7 kJ.mol-1
DiscovererCarl Wilhelm Scheele
Oxydation States(±1),3,5,7
UsesUsed in water purification, bleaches, acids and many other compounds such as chlorofluorocarbons (CFC)
DescriptionColorless, odorless, tasteless noble gas. It is the third most abundant element in the earth’s atmosphere and makes up about 1%.

Diatomic in nature (occurring as Cl2), this yellow-green gaseous element with a pungent, irritating odor carries the symbol Cl and the atomic number 17 in the periodic table of elements. Having an atomic mass of 35.45 g.mol -1 and electron configuration [Ne] 3s2 3p5, chlorine is a chemical element that is heavier than air.

It changes from a gaseous aggregate state into a liquid under pressure. The solid form of chlorine occurs as calcium hypochlorite [Ca(OCl)2].

Furthermore, chlorine reaches its boiling point at -34.6 °C, while the melting point of this gas is achieved at -101 °C. This member of the halogen family of elements in the periodic table has an electronegativity of 3.16 according to Pauling, whereas the atomic radius according to van der Waals is 0.127 nm.   

Despite being a non-flammable gas in its elemental form, chlorine reacts in a volatile and explosive manner due to its combustion-enhancing properties. Namely, when chlorine is compounded with bases, turpentine, ammonia, fuel gas, hydrogen, fluorine, and organic compounds, it poses a potential risk of fire or explosion.

Highly reactive with water (H2O), chlorine also becomes very corrosive when combined with H2O molecules. Apart from adopting highly corrosive properties, chlorine makes for a strong acid and an oxidizer in the presence of water molecules that readily attacks rubber, plastic, and various metals. 

How Was Chlorine Discovered?

Traces of the chlorine compounds that are nowadays extensively used in the industries (pentachlorophenol, polychloropyrols, polychlorinated biphenyls (PCB), and tetrachlorodibenzodioxins (TCDD)) have been found in rock sediments as old as 8,000 years. And, like almost all great inventions and discoveries of the modern times, chlorine was discovered after an oversight.

Namely, in 1774, the Swedish pharmacist Carl Wilhelm Scheele (1942 – 1786) attempted an experiment by causing a reaction between hydrochloric acid with manganese dioxide. In an instant, a greenish-yellow gas was released from the chemical mixture. However, Scheele didn’t realize that he had just discovered a new chemical element.

He firmly believed that what he had in front of him was a compound of oxygen, which he called ‘a dephlogisticated marine acid’. 

Several decades later, in 1810 to be more precise, the English chemist Sir Humphry Davy (1778 – 1829) gave more attention to this compound, and by conducting his experiments on what was believed to be dephlogisticated marine acid, Sir Davy succeeded in affirming chlorine as a new chemical element by proving that there is no oxygen in the compound. The English chemist also managed to determine the properties of the newly established chemical element. 

How Did Chlorine Get Its Name?

The name of the chemical element chlorine comes from the Greek word “khloros” that literally depicts the green color of chlorine’s pure elemental form. This chemical element was named in 1810 by Sir Humphry Davy.  

Where Can You Find Chlorine?

Chlorine belongs to the group of chemical elements that can be found in their elemental form in nature, counting approximately a hundred of them. These elements are considered to be the basic building blocks of our planet, as well as the life on it.  

This chemical element can be naturally found in inorganic chlorinated substances (such as sodium chloride, or table salt), as well as in organochlorines. Chlorine is released in the atmosphere from the ocean algae in a form of methyl chloride. This yellow-green gas is also released by volcanoes, forest fires, and some plants. Seawater and the Sun also contain certain quantities of chlorine in its elemental form. 

Today, chlorine is mostly obtained by electrolysis of sodium chloride solutions. This chemical element not only has a versatile use in everyday life, but it also has an efficient process of production. Namely, sodium hydroxide and hydrogen as by-products of the chlorine production process, have an equally wide application as chlorine in various industrial processes and in everyday life. 

Chlorine in Everyday Life

Due to the strong bactericidal properties, chlorine gas and its compounds are frequently used in everyday life as bleach, disinfectants, table salts, cleaning products, or insecticides. This gaseous chemical substance also finds extensive application in the industry. It has a wide variety of applications:

  • Production of solar energy;
  • Disinfection of hospitals;
  • Water treatment;
  • Manufacturing of paper; 
  • Production of chlorine bleach and bleaching products; 
  • In the construction industry (floors, energy-efficient window and door frames, bolts, electrical insulation, garden hoses, caulking, etc.);
  • Fabrication of environmentally-friendly air-conditioning refrigerants; 
  • Chlorinated solvents; 
  • Hybrid car batteries;
  • Synthetic rubber;
  • Vinyl flooring;
  • Making of paints;
  • Life-saving airbags for cars and airplanes;
  • PVC (plastic);
  • In the military and protection;
  • Manufacturing of pesticides;
  • Production of table salt;
  • For extraction of bromine;
  • In organic chemistry as an oxidizing agent;
  • Household cleaning products and disinfectants. 

In the past, this toxic gas was also used as anesthetic chloroform and a dry-cleaning solvent labeled as carbon tetrachloride. Today, these chemicals are produced in strictly controlled quantities and surroundings due to the fact that prolonged exposure to both of these chlorine compounds can lead to liver cancer

Table Salt

Sodium chloride (NaCl), or popularly known as table salt, is one of the chemical compounds we can hardly imagine life without. This inorganic compound comes in the form of colorless and odorless cube-shaped crystals that appear white due to their reflection. It not only enhances the flavor of our food (especially enriched with the trace nutrient iodine), but it also plays a vital role in some bodily mechanisms. 

The contraction of muscles, regulation of blood pressure, and transmission of sensory information between the neuron cells are only a few of the vital processes supported by NaCl. Sodium chloride also enhances bodily absorption of water, glucose, and amino acids.  

However, despite being responsible for some of the most vital functions in our body, salt intake can also have some adverse health effects. While high levels of sodium chloride in the body may lead to retention of water, swelling, and high blood pressure, lower, below-average levels of NaCl can result in diarrhea, vomiting, extreme thirst, and even dysfunctional kidneys. 

The Use of Chlorine as a Disinfectant of Waters and Surfaces

As a disinfectant, chlorine is often used in drinking water and swimming pools as a chemical agent for destroying harmful germs, bacteria, and viruses, such as the norovirus (widely known as the ‘stomach bug’), Escherichia coli, salmonella, etc. It’s also applied in the process of sanitation of sewerage and disposal of industrial waste. 

By destroying waterborne pathogens that may cause adverse health effects, the strong disinfectant properties of chlorine keep both the drinking and recreational waters safe, as well as the kitchen and bathroom surfaces. In this way, chlorine serves as chemical protection against water-borne diseases that include cholera, dysentery, Legionnaires’ disease, typhoid fever, and hepatitis A.

It’s worth noting that chlorine is not present in chlorinated water in its elemental form since this gaseous substance changes its chemical structure almost immediately upon contact with water. The resulting hypochlorous acid and hypochlorite anion from the reaction between chlorine and water molecules retain the disinfecting properties. In the United States, chlorine is almost exclusively used as a means for appropriate chlorination of water. 

The Use of Chlorine in Medicine

This chemical substance is contained in numerous pharmaceutical products and medicines. The extensive list includes medicines administered as a treatment and regulation of the following health conditions:

  • Anemia;
  • Cancer;
  • High cholesterol levels;
  • Epilepsy;
  • Depression;
  • Stomach ulcers.

As one of the main components, chlorine can be found in the production of:

  • Soft contact lenses;
  • Antiseptics;
  • Respiratory inhalers;
  • Protective glasses and corrective eyeglasses;
  • Blood bags;
  • Artificial human joints (in surgery).

Furthermore, saline solutions based on sodium chloride are applied in the following medical cases:

  • As an intravenous infusion n case of dehydration and imbalance of electrolytes;
  • For flushing and disinfection of the IV system after administration of the medicine;
  • Rinsing and cleaning of wounds;
  • Treating dryness of the eyes.

The Use of Chlorine for the Production of Solar Energy

The application of chlorine in the production of innovative energy sources, such as solar energy panels and wind turbines, is probably one of the most significant uses of this chemical element. 

But, let’s take a closer look at this procedure in order to see the role of chlorine in this chemical process. To begin with, the solar cells used for the accumulation of solar energy are made of purified silicone. In order to get a pure form of silicone, the substance undergoes a reaction with hydrochloric acid at 300°C (572°F). The resulting product of this chemical reaction, trichlorosilane, HSiCl3, is further exposed to a temperature of 1150°C (2102°F) in order for the pure form of silicone to be isolated. 

The Use of Chlorine in the Military

The military forces use bullet-proof vests that are made via processes that include chlorine as one of the main chemicals. Also, chlorine is used in the manufacturing processes of surveillance cameras, night-vision goggles, parachutes, and other advanced technologies serving the purpose of protection and communication.   

How Dangerous Is Chlorine?

Most cases of chlorine poisoning occur as a result of an occupational hazard, i.e. prolonged exposure to high levels of chlorine and its high toxicity. The exposure may occur as a result of improper handling of cleaning products, accidental release of chlorine gas, drinking contaminated water or food, or from an intentional terrorist attack with chlorine as a chemical weapon. 

However, since chlorine adopts a gaseous form at room temperature, exposure to this toxic chemical via ingestion is less likely. Regardless of the levels of chlorine exposure, the people who come into direct contact with chlorine or chlorine chemicals are at an extremely high risk of acute or chronic chlorine poisoning. 

Moreover, chlorine poisoning may result from mixing chlorine-based cleaning products with other cleaning products. In this way, the reaction of the mixed chemical compounds triggers the release of chlorine gas which is extremely dangerous when inhaled.

According to the WHO guidelines for drinking water quality, heavily chlorinated waters (above the prescribed level and prolonged exposure) can trigger the existing chronic pulmonary diseases. Also, skin exposure to both chlorine and hypochlorite may result in dermatitis. 

On the other hand, the EPA (U.S. Environmental Protection Agency), in collaboration with the National Institute for Occupational Safety and Health, has proposed lowering the chlorine levels allowed in drinking water supplies and when disinfecting swimming pool water, in order to avoid adverse effects of this chemical upon human health. 

How Does Chlorine Damage Bodily Tissues?

Chlorine’s strong and pungent smell serves as a warning of exposure to this toxic substance. At the moment of inhalation, physical contact, or ingestion, this irritant gas triggers severe inflammation of the moist bodily tissues, especially of the eyes, lungs, skin, and throat. When chlorine comes into contact with water molecules, the reaction produces hypochlorous acid with a highly corrosive property that damages cells of the irritated tissue or organ of the body. 

Hypochlorous acid is most easily absorbed through the gastrointestinal tract and it accumulates in the lungs, testes, bone marrow, skin, bones, and the liver, where it’s further metabolized to trichloroethanoic acid, dichloroethanoic acid, chloroform, and dichloroacetonitrile. In this way, what begins as a temporary irritation may lead to more serious medical issues with adverse health effects. 

What Symptoms and Medical Conditions Occur After Chlorine Exposure?

According to the New York State Department of Health, and the Agency for Toxic Substances and Disease Registry, the exposure to compressed liquid chlorine or chlorine gas may lead to severe injuries of bodily tissues or organs, such as:

  • Acute chlorine gas inhalation injury;
  • Frostbites of both skin and eyes;
  • Loss of eyesight;
  • Skin necrosis;
  • Pulmonary edema.

The symptoms and signs that point out to a chlorine exposure include:

  • Throat irritation (burning);
  • Sneezing bouts;
  • Severe nausea and vomiting;
  • Shortness of breath;
  • Blurred vision;
  • Blood in the stool;
  • Severe headaches;
  • Loss of consciousness;
  • Cardiovascular problems;
  • Blood pressure variations and sudden drops;
  • Pain in the chest;
  • Fatigue;
  • Difficulty breathing;
  • Severe eye irritation;
  • Severe coughing (with or without blood); 
  • Severe airway irritation;
  • Severe skin irritation.

Chlorine Gas as a Chemical Weapon in World War I

Labeled as a potential chemical weapon by the Centers for Disease Control and Prevention (CDC), chlorine is classified as one of the most toxic and hazardous chemicals. During World War I, chlorine gas was used for chemical warfare as a chemical weapon that has the properties of a lung-damaging agent. Inhalation of this irritant gas would trigger suffocation and choking, constriction of the chest, throat irritation, and lung edema (swelling).  

Phosgene (COCl2)

Little did the Cornish chemist John Davy (1790–1868) know that by producing this chlorine compound, he would manage to synthesize a chemical weapon that was massively used in World War I. Phosgene is an odorless and highly toxic gas produced by the reaction of chlorine with carbon dioxide.

Its devastating effect was especially notable in the trench warfare of WWI since this toxic gas would fill in the trenches with no way out for the soldiers due to its heavier-than-air weight. After WWI, chemical weapons were banned by the Geneva Protocol due to their brutality, signed on June 17th, 1925.

Environmental Effects of Chlorine

As a result of its unstable chemical structure, chlorine easily reacts with other chemical elements found in nature. The high reactivity and the half-life of less than an hour prevent this gas from accumulating in the deeper layers of the soil or waters. The Sun doesn’t contribute to a longer life of chlorine, either. Sunrays degrade this chemical in only a few minutes, making it evaporate in a fast manner. When released in larger concentrations from the industries, it’s swiftly carried away by the wind or becomes dissolved in the waters equally fast. 

Despite the fact that chlorine does not do much harm to the environment by itself, its compounds typically have a hazardous effect upon it. By polluting the waters, the toxicity of the chlorine compounds may contaminate both the environment and consequently – wildlife and fish. In the natural food chain, humans are the final consumers of the contaminated products obtained from nature. Also, when the air is contaminated by this toxic gaseous substance, it can be easily inhaled and lead to adverse effects on human health.    

Isotopes of Chlorine

Chlorine isotopes range from 28Cl to 51Cl. With a half-life of 301,000 years, chlorine-36 is the longest living isotope of this chemical element, while 35Cl and 37Cl are the only two stable isotopes of this chemical element. The combination of chlorine-35 (75.53 percent) and chlorine-37 (24.47 percent) make the pure elemental form of chlorine. 



[n 1]

ZNIsotopic mass (Da)[3]


[n 2][n 3]



[n 4]




[n 5]




[n 6]

Spin and



[n 7][n 4]

Natural abundance (mole fraction)
Excitation energyNormal proportionRange of variation
28Cl[4]171128.02954(64)# p27S1+#  
29Cl[4]171229.01413(20)<10 psp28S(1/2+)  
30Cl[4]171330.00477(21)#<30 nsp29S3+#  
31Cl171430.992448(4)190(1) msβ+ (97.6%)31S3/2+  
β+, p (2.4%)30P
32Cl171531.9856846(6)298(1) msβ+ (99.92%)32S1+ 
β+, α (.054%)28Si
β+, p (.026%)31P
33Cl171632.9774520(4)2.5038(22) sβ+33S3/2+  
34Cl171733.97376249(5)1.5266(4) sβ+34S0+  
34mCl146.360(27) keV31.99(3) minβ+ (55.4%)34S3+  
IT (44.6%)34Cl
36Cl[n 8]171935.96830682(4)3.013(15)×105 yβ (98.1%)36Ar2+Trace[n 9]approx. 7×10−13
β+ (1.9%)36S
38Cl172137.96801042(11)37.24(5) minβ38Ar2−  
38mCl671.365(8) keV715(3) msIT38Cl5−  
39Cl172238.9680082(19)56.2(6) minβ39Ar3/2+  
40Cl172339.97042(3)1.35(2) minβ40Ar2−  
41Cl172440.97068(7)38.4(8) sβ41Ar(1/2+,3/2+)  
42Cl172541.97334(6)6.8(3) sβ42Ar   
43Cl172642.97406(7)3.13(9) sβ (>99.9%)43Ar(3/2+)  
β, n (<.1%)42Ar
44Cl172743.97812(15)0.56(11) sβ (92%)44Ar(2-)  
β, n (8%)43Ar
45Cl172844.98039(15)413(25) msβ (76%)45Ar(3/2+)  
β, n (24%)44Ar
46Cl172945.98512(22)232(2) msβ, n (60%)45Ar2-#  
β (40%)46Ar
47Cl173046.98950(43)#101(6) msβ (97%)47Ar3/2+#  
β, n (3%)46Ar
48Cl173147.99541(54)#100# ms [>200 ns]β48Ar   
49Cl173249.00101(64)#50# ms [>200 ns]β49Ar3/2+#  
50Cl173350.00831(64)#20# msβ50Ar   
51Cl173451.01534(75)#2# ms [>200 ns]β51Ar3/2+#  
52Cl[5]1735  β52Ar   

Source: Wikipedia

List of Chlorine Compounds 

Chlorine forms the Cl2O, ClO2, O2O6, Cl2O7, and Cl2O8 oxides when compounded with oxygen molecules. Being highly soluble in water, this chemical element reacts with H2O to form hypochlorous acid and hypochlorites, thus expressing its highly corrosive properties. In addition, chlorine forms chloramines when reacted with ammonia or amines in water. Also, helium, neon, and argon are the only elements that do not produce halides in reaction with chlorine. 

Chlorine-Releasing Compounds

  • Sodium hypochlorite, NaOCl
  • Calcium hypochlorite, Ca(OCl)2
  • Potassium hypochlorite, KOCl
  • Chloramine, NH2Cl
  • Chloramine-T, or tosylchloramide sodium salt, [(H3C)(C6H4)(SO2)(NHCl)]−Na+
  • Sodium dichloroisocyanurate [((ClN)(CO))2(NCO)]−Na+
  • Halazone, or 4-((dichloro-amino)sulfonyl)benzoic acid, (HOOC)(C6H4)(SO2)(NCl2)
  • Chlorine dioxide, ClO2

5 Interesting Facts And Explanations

  1. Chlorine occurs as a trace element in about 0.35% or less in the human body.
  2. In the family of halogen elements, only fluorine is more reactive than chlorine.
  3. The medical condition characterized by a build-up of fluids in the lungs is known as pulmonary edema. 
  4. Despite the fact that swimming pool chemicals do not impose a great health hazard, it should be noted that the toxic chlorine gas accumulates and builds up in high concentrations under the covers of swimming pools with chlorinated water. This owes to the fact that chlorine reacts strongly when combined with water. 
  5. In most cases, chlorine poisoning among children occurs due to the mishandling of cleaning products in the household. For the safety of children, all chemicals must always be put out of reach.