Chlorine Facts (Cl or Atomic Number 17)

Chlorine Chemical & Physical Properties

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Chlorine is a chemical element with atomic number 17 and element symbol Cl. It is a member of the halogen group of elements, appearing between fluorine and bromine moving down the periodic table. At ordinary temperature and pressure, chlorine is a pale. greenish-yellow gas. Like other halogens, it is an extremely reactive element and strong oxidizer.

Fast Facts: The Element Chlorine

  • Element Name: Chlorine
  • Atomic Number: 17
  • Element Symbol: Cl
  • Appearance: Pale greenish-yellow gas
  • Element Group: Halogen

Chlorine Facts

Atomic Number: 17

Symbol: Cl

Atomic Weight: 35.4527

Discovery: Carl Wilhelm Scheele 1774 (Sweden)

Electron Configuration: [Ne] 3s2 3p5

Word Origin: Greek: khloros: greenish-yellow

Properties: Chlorine has a melting point of -100.98°C, boiling point of -34.6°C, density of 3.214 g/l, specific gravity of 1.56 (-33.6°C), with a valence of 1, 3, 5, or 7. Chlorine is a member of the halogen group of elements and directly combines with almost all of the other elements. Chlorine gas is a greenish yellow. Chlorine figures prominently in many organic chemistry reactions, particularly in substitutions with hydrogen. The gas acts as an irritant for respiratory and other mucous membranes. The liquid form will burn the skin. Humans can smell as low an amount as 3.5 ppm. A few breaths at a concentration of 1000 ppm is usually fatal.

Uses: Chlorine is used in many everyday products. It is used for disinfecting drinking water. Chlorine is used in the production of textiles, paper products, dyes, petroleum products, medicines, insecticides, disinfectants, foods, solvents, plastics, paints, and many other products. The element is used to manufacture chlorates, carbon tetrachloride, chloroform, and in the extraction of bromine. Chlorine has been used as a chemical warfare agent.

Biological Role: Chlorine is essential for life. Specifically, the chloride ion (Cl-) is key to metabolism. In humans, the ion is obtained mainly from salt (sodium chloride). It is used in cells to pump ions and is used in the stomach to make hydrochloric acid (HCl) for gastric juice. Too little chloride produces hypochloremia. Hypochloremia can lead to cerebral dehydration. Hypochloremia may be caused by hypoventilaton or chronic respiratory acidosis. Too much chloride leads to hyperchloremia. Usually, hyperchloremia is asymptomatic, but it can present much like hypernatremia (too much sodium). Hyperchloremia affects oxygen transport in the body.

Sources: In nature, chlorine is only found in the combined state, most commonly with sodium as NaCl and in carnallite (KMgCl3•6H2O) and sylvite (KCl). The element is obtained from chlorides by electrolysis or via the action of oxidizing agents.

Element Classification: Halogen

Chlorine Physical Data

Density (g/cc): 1.56 (@ -33.6 °C)

Melting Point (K): 172.2

Boiling Point (K): 238.6

Appearance: greenish-yellow, irritating gas. At high pressure or low temperature: red to clear.

Isotopes: 16 known isotopes with atomic masses ranging from 31 to 46 amu. Cl-35 and Cl-37 are both stable isotopes with Cl-35 as the most abundant form (75.8%).
Atomic Volume (cc/mol): 18.7

Covalent Radius (pm): 99

Ionic Radius: 27 (+7e) 181 (-1e)

Specific Heat (@20°C J/g mol): 0.477 (Cl-Cl)

Fusion Heat (kJ/mol): 6.41 (Cl-Cl)

Evaporation Heat (kJ/mol): 20.41 (Cl-Cl)

Pauling Negativity Number: 3.16

First Ionizing Energy (kJ/mol): 1254.9

Oxidation States: 7, 5, 3, 1, -1

Lattice Structure: Orthorhombic

Lattice Constant (Å): 6.240

CAS Registry Number: 7782-50-5

Interesting Trivia

  • Chlorine leaks in containers are detected using ammonia. Ammonia will react with the chlorine and form a white mist above the leak.
  • The most common natural chlorine compound on Earth is sodium chloride or table salt.
  • Chlorine is the 21st most abundant element in the Earth's crust
  • Chlorine is the third most abundant element in the Earth's oceans
  • Chlorine gas was used as a chemical weapon during World War I. Chlorine is heavier than air and would form a deadly layer in low-lying foxholes and trenches.


  • Emsley, John (2011). Nature's building blocks: An A-Z Guide to the Elements. Oxford University Press. pp. 492–98. ISBN 978-0-19-960563-7.
  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  • Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN 978-0-8493-0485-9.
  • Levitin, H; Branscome, W; Epstein, FH (December 1958). "The pathogenesis of hypochloremia in respiratory acidosis." J. Clin. Invest. 37 (12): 1667–75. doi:10.1172/JCI103758
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
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Helmenstine, Anne Marie, Ph.D. "Chlorine Facts (Cl or Atomic Number 17)." ThoughtCo, Feb. 16, 2021, Helmenstine, Anne Marie, Ph.D. (2021, February 16). Chlorine Facts (Cl or Atomic Number 17). Retrieved from Helmenstine, Anne Marie, Ph.D. "Chlorine Facts (Cl or Atomic Number 17)." ThoughtCo. (accessed March 27, 2023).