10 Chlorine Facts (Cl or Atomic Number 17)

liquid chlorine
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Chlorine (element symbol Cl) is an element you encounter every day and need in order to live. Chlorine is atomic number 17 with element symbol Cl.

Fast Facts: Chlorine

  • Symbol: Cl
  • Atomic Number: 17
  • Appearance: Greenish-yellow gas
  • Atomic Weight: 35.45
  • Group: Group 17 (Halogen)
  • Period: Period 3
  • Electron Configuration: [Ne] 3s2 3p5
  • Discovery: Carl Wilhelm Scheele (1774)

Chlorine Facts

  1. Chlorine belongs to the halogen element group. It is the second lightest halogen, after fluorine. Like other halogens, it's an extremely reactive element that readily forms the -1 anion. Because of its high reactivity, chlorine is found in compounds. Free chlorine is rare but exists as a dense, diatomic gas.
  2. Although chlorine compounds have been used by man since ancient times, pure chlorine was not produced (on purpose) until 1774 when Carl Wilhelm Scheele reacted magnesium dioxide with spiritus salis (now known as hydrochloric acid) to form chlorine gas. Scheele did not recognize this gas as a new element, instead believing it to contain oxygen. It wasn't until 1811 that Sir Humphry Davy determined the gas was, in fact, a previously unidentified element. Davy gave chlorine its name.
  3. Pure chlorine is a greenish-yellow gas or liquid with a distinctive odor (like chlorine bleach). The element name comes from its color. The Greek word chloros means greenish-yellow.
  4. Chlorine is the 3rd most abundant element in the ocean (about 1.9% by mass) and 21st most abundant element in the Earth's crust.
  5. There is so much chlorine in the Earth's oceans that it would weigh 5x more than our present atmosphere if it were somehow suddenly released as a gas.
  6. Chlorine is essential for living organisms. In the human body, it's found as the chloride ion, where it regulates osmotic pressure and pH and aids digestion in the stomach. The element is usually obtained by eating salt, which is sodium chloride (NaCl). While it's needed for survival, pure chlorine is extremely toxic. The gas irritates the respiratory system, skin, and eyes. Exposure to 1 part per thousand in air may cause death. Since many household chemicals contain chlorine compounds, it's risky to mix them because toxic gases may be released. In particular, it's important to avoid mixing chlorine bleach with vinegar, ammonia, alcohol, or acetone.
  7. Because chlorine gas is toxic and because it's heavier than air, it was used as a chemical weapon. The first use was in 1915 by the Germans in World War I. Later, the gas was also used by the Western Allies. The effectiveness of the gas was limited because its strong odor and distinctive color alerted troops to its presence. Soldiers could protect themselves from the gas by seeking higher ground and breathing through damp cloth since chlorine dissolves in water.
  8. Pure chlorine is obtained primarily by electrolysis of saltwater. Chlorine is used to make drinking water safe, for bleaching, disinfection, textile processing, and to make numerous compounds. The compounds include chlorates, chloroform, synthetic rubber, carbon tetrachloride, and polyvinyl chloride. Chlorine compounds are used in medicines, plastics, antiseptics, insecticides, food, paint, solvents, and many other products. While chlorine is still used in refrigerants, the number of chlorofluorocarbons (CFCs) released into the environment has dramatically declined. These compounds are believed to have contributed significantly to the destruction of the ozone layer.
  9. Natural chlorine consists of two stable isotopes: chlorine-35 and chlorine-37. Chlorine-35 accounts for 76% of the natural abundance of the element, with chlorine-37 making up the other 24% of the element. Numerous radioactive isotopes of chlorine have been produced.
  10. The first chain reaction to be discovered was a chemical reaction involving chlorine, not a nuclear reaction, as you might expect. In 1913, Max Bodenstein observed a mixture of chlorine gas and hydrogen gas exploded upon exposure to light. Walther Nernst explained the chain reaction mechanism for this phenomenon in 1918. Chlorine is made in stars via the oxygen-burning and silicon-burning processes.


  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
  • Weeks, Mary Elvira (1932). "The discovery of the elements. XVII. The halogen family". Journal of Chemical Education. 9 (11): 1915. doi:10.1021/ed009p1915
  • Winder, Chris (2001). "The Toxicology of Chlorine". Environmental Research. 85 (2): 105–14. doi:10.1006/enrs.2000.4110
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Helmenstine, Anne Marie, Ph.D. "10 Chlorine Facts (Cl or Atomic Number 17)." ThoughtCo, Aug. 26, 2020, thoughtco.com/facts-about-the-element-chlorine-3860219. Helmenstine, Anne Marie, Ph.D. (2020, August 26). 10 Chlorine Facts (Cl or Atomic Number 17). Retrieved from https://www.thoughtco.com/facts-about-the-element-chlorine-3860219 Helmenstine, Anne Marie, Ph.D. "10 Chlorine Facts (Cl or Atomic Number 17)." ThoughtCo. https://www.thoughtco.com/facts-about-the-element-chlorine-3860219 (accessed March 30, 2023).