chemical element with symbol Cl and atomic number 17

Chlorine (chemical symbol Cl) is a chemical element. Its atomic number (which is the number of protons in it) is 17, and its atomic mass is 35.45. It is part of the 7th column (halogens) on the periodic table of elements.

Chlorine, 00Cl
A glass container filled with chlorine gas
Pronunciation/ˈklɔːrn, -n/ (KLOR-een, -⁠eyen)
Appearancepale yellow-green gas
Standard atomic weight Ar°(Cl)
Chlorine in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson


Groupgroup 17 (halogens)
Periodperiod 3
Block  p-block
Electron configuration[Ne] 3s2 3p5
Electrons per shell2, 8, 7
Physical properties
Phase at STPgas
Melting point171.6 K ​(−101.5 °C, ​−150.7 °F)
Boiling point239.11 K ​(−34.04 °C, ​−29.27 °F)
Density (at STP)3.2 g/L
when liquid (at b.p.)1.5625 g/cm3[2]
Critical point416.9 K, 7.991 MPa
Heat of fusion(Cl2) 6.406 kJ/mol
Heat of vaporization(Cl2) 20.41 kJ/mol
Molar heat capacity(Cl2)
33.949 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 128 139 153 170 197 239
Atomic properties
Oxidation states−1, 0, +1, +2, +3, +4, +5, +6, +7 (a strongly acidic oxide)
ElectronegativityPauling scale: 3.16
Ionization energies
  • 1st: 1251.2 kJ/mol
  • 2nd: 2298 kJ/mol
  • 3rd: 3822 kJ/mol
  • (more)
Covalent radius102±4 pm
Van der Waals radius175 pm
Color lines in a spectral range
Spectral lines of chlorine
Other properties
Natural occurrenceprimordial
Crystal structureorthorhombic
Orthorhombic crystal structure for chlorine
Speed of sound206 m/s (gas, at 0 °C)
Thermal conductivity8.9×103 W/(m⋅K)
Electrical resistivity>10 Ω⋅m (at 20 °C)
Magnetic orderingdiamagnetic[3]
Molar magnetic susceptibility−40.5·10−6 cm3/mol[4]
CAS Number(Cl2) 7782-50-5
Discovery and first isolationCarl Wilhelm Scheele (1774)
Recognized as an element byHumphry Davy (1808)
Isotopes of chlorine
Main isotopes[5] Decay
abun­dance half-life (t1/2) mode pro­duct
35Cl 76% stable
36Cl trace 3.01×105 y β 36Ar
ε 36S
37Cl 24% stable
 Category: Chlorine
| references
Chlorine gas in a tube



Physical properties


Chlorine is a very irritating and greenish-yellow gas. It has a strong, bleach-like smell. It is toxic and bad for you. It can be made into a liquid when cooled. It is heavier than air.

Chemical properties


Chlorine is highly reactive. It is more reactive than bromine but less reactive than fluorine. It reacts with most things to make chlorides. It can even burn things instead of oxygen. It dissolves in water to make a mixture of hypochlorous acid and hydrochloric acid. The more acidic it is, the more chlorine is made; the more basic it is, the more hypochlorous acid (normally turned into hypochlorite) and hydrochloric acid (normally turned into chlorides) are there. Chlorine reacts with bromides and iodides to make bromine and iodine.

Chlorine compounds


Chlorine exists in several oxidation states: -1, +1, +3, +4, +5, and +7. The -1 state is most often in chloride. Chlorides are not reactive. Compounds containing chlorine in its +1 oxidation state are hypochlorites. Only one is common. They are a strong oxidizing agent, as are all + oxidation state compounds. +3 is in chlorites. +4 is in chlorine dioxide, a common chlorine compound that is not a chloride. +5 is in chlorates. +7 is in perchlorates. Hypochlorites are most reactive, while perchlorates are the least reactive.

Many organic compounds have chlorine in them. Freon has chlorine in it. PVC (poly-vinyl chloride), a common plastic, has chlorine in it.

Chlorine oxides can be made, but most of them are very reactive and unstable.



Chlorine is not found as an element. Sodium chloride is the most common chlorine ore. It is in the ocean (sea salt) and in the ground (rock salt). There are some organic compounds that have chlorine in them, too.



It is made by electrolysis (the passing of electricity through a solution to make chemical reactions happen) of sodium chloride. This is known as the chloralkali process. It can also be made by reacting hydrogen chloride with oxygen and a catalyst. It can be made in the laboratory by reacting manganese dioxide with hydrochloric acid. It is made when sodium hypochlorite reacts with hydrochloric acid. This is a dangerous reaction that can happen without anyone knowing.

For water purification


As of 2021, the main use of chlorine is for bleach. It is also added to water, as a way of purifying it. Chlorine is both very reactive, and very poisonous. It will act as a disinfectant: if it is added to water, it will kill off bacteria and other organisms. Swimming pools are often filled with water that has been treated that way.

As a chemical weapon


Germany used chlorine as a chemical weapon in the First World War. They used it at the Second Battle of Ypres in 1915.[6][7] According to soldiers, which were present at the battle, the chlorine smelled like a mixture of pepper and pineapple. It also tasted metallic and stung the back of the throat and chest. Chlorine reacts with water in the mucosa of the lungs to form hydrochloric acid. Hydrochloric acid will destroy living tissue. It often kills. Gas masks with activated charcoal or other filters can protect the respiratory system. This makes chlorine gas much less deadly than other chemical weapons. German scientist Fritz Haber of the Kaiser Wilhelm Institute in Berlin was the first to use it. Together with IG Farben, he developed methods of how to use chlorine gas against an entrenched enemy.[8] Chlorine is heavier than air, so it will stay in the trenches. On 22nd April, 1915, German forces attacked the French army. With 150 tonnes (150 long tons; 170 short tons) of chlorine, they only killed about 1.200 French soldiers.[9] For this reason, chlorine was soon replaced with the more deadly phosgene and mustard gas.[10]

Because it is easily available, chlorine is still used as a chemical weapon in war. That way, it has been used in the Syrian civil war.[11] [12][13] It has also been used in an improvised explosive device in Iraq, in 2015.[14]

Other uses


Chlorine is used to make many compounds that are important: both chloroform and carbon tetrachloride contain chlorine.



It was discovered in 1774 by Carl Wilhelm Scheele who thought it had oxygen in it. Chlorine was named in 1810 by Humphry Davy who insisted it was an element. The US made all water chlorinated (added chlorine to water) by 1918.



It is poisonous in large amounts and can damage skin. When it is inhaled (breathed in), it irritates the lungs, eyes, and skin badly. It can cause fire with some things because it is very reactive. It is heavier than air, so it can fill up enclosed spaces.



  1. "Standard Atomic Weights: Chlorine". CIAAW. 2009.
  2. Chlorine, Gas Encyclopaedia, Air Liquide
  3. Magnetic susceptibility of the elements and inorganic compounds, in Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
  4. Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4.
  5. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  6. "Battle of Ypres" The Canadian Encyclopedia
  7. Everts, Sarah (February 23, 2015). "When Chemicals Became Weapons of War". Chemical & Engineering News. 93 (8). Archived from the original on March 30, 2016.
  8. Smil, Vaclav (2004-04-01). Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production. p. 226. ISBN 978-0-262-69313-4. Archived from the original on 2015-12-31.
  9. Gerhard Hirschfeld, Gerd Krumeich, Irina Renz: Enzyklopädie Erster Weltkrieg. 2. Auflage. Paderborn 2004, ISBN 978-3-506-73913-1, S. 520.
  10. "Weapons of War: Poison Gas". First World Archived from the original on 2007-08-21. Retrieved 2007-08-12.
  11. Gladstone, Rick (2017-02-13). "Syria Used Chlorine Bombs Systematically in Aleppo, Report Says". The New York Times. Archived from the original on 2017-05-15. Retrieved 2017-05-10.
  12. "Syrian forces 'drop chlorine' on Aleppo". BBC News. 2016-09-07. Archived from the original on 2017-05-13. Retrieved 2017-05-10.
  13. "Ignoring UN, Russia and Assad continue Syrian chemical weapons and bombing attacks labeled war crimes". Fox News. 2017-03-06. Archived from the original on 2017-04-25. Retrieved 2017-05-11.
  14. "Lab report on chlorine gas usage" (PDF). Kurdistan Region Security Council. March 14, 2015.