Periodic Table > Chlorine


Some elements exist in several different structural forms, called allotropes. Each allotrope has different physical properties.

For more information on the Visual Elements image see the Uses and properties section below.



A vertical column in the periodic table. Members of a group typically have similar properties and electron configurations in their outer shell.

A horizontal row in the periodic table. The atomic number of each element increases by one, reading from left to right.

Elements are organised into blocks by the orbital type in which the outer electrons are found. These blocks are named for the characteristic spectra they produce: sharp (s), principal (p), diffuse (d), and fundamental (f).

Atomic number
The number of protons in an atom.

Electron configuration
The arrangements of electrons above the last (closed shell) noble gas.

Melting point
The temperature at which the solid–liquid phase change occurs.

Boiling point
The temperature at which the liquid–gas phase change occurs.

The transition of a substance directly from the solid to the gas phase without passing through a liquid phase.

Density (g cm−3)
Density is the mass of a substance that would fill 1 cm3 at room temperature.

Relative atomic mass
The mass of an atom relative to that of carbon-12. This is approximately the sum of the number of protons and neutrons in the nucleus. Where more than one isotope exists, the value given is the abundance weighted average.

Atoms of the same element with different numbers of neutrons.

CAS number
The Chemical Abstracts Service registry number is a unique identifier of a particular chemical, designed to prevent confusion arising from different languages and naming systems.

Fact box

Group 17  Melting point −101.5°C, −150.7°F, 171.7 K 
Period Boiling point −34.04°C, −29.27°F, 239.11 K 
Block Density (g cm−3) 0.002898 
Atomic number 17  Relative atomic mass 35.45  
State at 20°C Gas  Key isotopes 35Cl, 37Cl 
Electron configuration [Ne] 3s23p5  CAS number 7782-50-5 
ChemSpider ID 4514529 ChemSpider is a free chemical structure database


Image explanation

Murray Robertson is the artist behind the images which make up Visual Elements. This is where the artist explains his interpretation of the element and the science behind the picture.


The description of the element in its natural form.

Biological role

The role of the element in humans, animals and plants.

Natural abundance

Where the element is most commonly found in nature, and how it is sourced commercially.

Uses and properties

Image explanation
The symbol shows a gas mask. This is because chlorine is a toxic gas, and has been used as a chemical weapon. Chlorine is yellowy-green in colour, as is the image.
A yellowy-green dense gas with a choking smell.
Chlorine kills bacteria – it is a disinfectant. It is used to treat drinking water and swimming pool water. It is also used to make hundreds of consumer products from paper to paints, and from textiles to insecticides.

About 20% of chlorine produced is used to make PVC. This is a very versatile plastic used in window frames, car interiors, electrical wiring insulation, water pipes, blood bags and vinyl flooring.

Another major use for chlorine is in organic chemistry. It is used as an oxidising agent and in substitution reactions. 85% of pharmaceuticals use chlorine or its compounds at some stage in their manufacture.

In the past chlorine was commonly used to make chloroform (an anaesthetic) and carbon tetrachloride (a dry-cleaning solvent). However, both of these chemicals are now strictly controlled as they can cause liver damage.

Chlorine gas is itself very poisonous, and was used as a chemical weapon during the First World War.
Biological role
The chloride ion is essential to life. It is mostly present in cell fluid as a negative ion to balance the positive (mainly potassium) ions. It is also present in extra-cellular fluid (eg blood) to balance the positive (mainly sodium) ions.

We get most of the chloride we need from salt. Typical daily salt intake is about 6 grams, but we could manage with half this amount.
Natural abundance
Chlorine is not found uncombined in nature. Halite (sodium chloride or ‘common salt’) is the main mineral that is mined for chlorine. Sodium chloride is a very soluble salt that has been leached into the oceans over the lifetime of the Earth. Several salt beds, or ‘lakes’ are found where ancient seas have evaporated, and these can be mined for chloride.

Chlorine is also found in the minerals carnallite (magnesium potassium chloride) and sylvite (potassium chloride).

40 million tonnes of chlorine gas are made each year from the electrolysis of brine (sodium chloride solution). This process also produces useful sodium hydroxide.

Atomic radius, non-bonded
Half of the distance between two unbonded atoms of the same element when the electrostatic forces are balanced. These values were determined using several different methods.

Covalent radius
Half of the distance between two atoms within a single covalent bond. Values are given for typical oxidation number and coordination.

Electron affinity
The energy released when an electron is added to the neutral atom and a negative ion is formed.

Electronegativity (Pauling scale)
The tendency of an atom to attract electrons towards itself, expressed on a relative scale.

First ionisation energy
The minimum energy required to remove an electron from a neutral atom in its ground state.

Atomic data

Atomic radius, non-bonded (Å) 1.75 Covalent radius (Å) 1.00
Electron affinity (kJ mol−1) 348.575 Electronegativity
(Pauling scale)
Ionisation energies
(kJ mol−1)

Bond enthalpy (kJ mol−1)
A measure of how much energy is needed to break all of the bonds of the same type in one mole of gaseous molecules.

Bond enthalpies

Covalent bond Enthalpy (kJ mol−1) Found in
Cl–Cl 242.1 Cl2
C–Cl 339 general
C–Cl 327.2 CCl4
Cl–H 431.4 HCl


Data for this section been provided by the British Geological Survey.

Relative supply risk

An integrated supply risk index from 1 (very low risk) to 10 (very high risk). This is calculated by combining the scores for crustal abundance, reserve distribution, production concentration, substitutability, recycling rate and political stability scores.

Crustal abundance (ppm)

The number of atoms of the element per 1 million atoms of the Earth’s crust.

Recycling rate

The percentage of a commodity which is recycled. A higher recycling rate may reduce risk to supply.


The availability of suitable substitutes for a given commodity.
High = substitution not possible or very difficult.
Medium = substitution is possible but there may be an economic and/or performance impact
Low = substitution is possible with little or no economic and/or performance impact

Production concentration

The percentage of an element produced in the top producing country. The higher the value, the larger risk there is to supply.

Reserve distribution

The percentage of the world reserves located in the country with the largest reserves. The higher the value, the larger risk there is to supply.

Political stability of top producer

A percentile rank for the political stability of the top producing country, derived from World Bank governance indicators.

Political stability of top reserve holder

A percentile rank for the political stability of the country with the largest reserves, derived from World Bank governance indicators.

Supply risk

Relative supply risk 4
Crustal abundance (ppm) 145
Recycling rate (%) Unknown
Substitutability Unknown
Production concentration (%) 24.3
Reserve distribution (%) Unknown
Top 3 producers
  • 1) China
  • 2) India
  • 3) USA
Top 3 reserve holders
  • Unknown
Political stability of top producer 24.1
Political stability of top reserve holder Unknown


Common oxidation states

The oxidation state of an atom is a measure of the degree of oxidation of an atom. It is defined as being the charge that an atom would have if all bonds were ionic. Uncombined elements have an oxidation state of 0. The sum of the oxidation states within a compound or ion must equal the overall charge.


Atoms of the same element with different numbers of neutrons.

Key for isotopes

Half life
  y years
  d days
  h hours
  m minutes
  s seconds
Mode of decay
  α alpha particle emission
  β negative beta (electron) emission
  β+ positron emission
  EC orbital electron capture
  sf spontaneous fission
  ββ double beta emission
  ECEC double orbital electron capture

Oxidation states and isotopes

Common oxidation states 7, 5, 3, 1, -1
Isotopes Isotope Atomic mass Natural abundance (%) Half life Mode of decay
  35Cl 34.969 75.76
  37Cl 36.966 24.24


Specific heat capacity (J kg−1 K−1)

Specific heat capacity is the amount of energy needed to change the temperature of a kilogram of a substance by 1 K.

Young's modulus

A measure of the stiffness of a substance. It provides a measure of how difficult it is to extend a material, with a value given by the ratio of tensile strength to tensile strain.

Shear modulus

A measure of how difficult it is to deform a material. It is given by the ratio of the shear stress to the shear strain.

Bulk modulus

A measure of how difficult it is to compress a substance. It is given by the ratio of the pressure on a body to the fractional decrease in volume.

Vapour pressure

A measure of the propensity of a substance to evaporate. It is defined as the equilibrium pressure exerted by the gas produced above a substance in a closed system.

Pressure and temperature data – advanced

Specific heat capacity
(J kg−1 K−1)
479 Young's modulus (GPa) Unknown
Shear modulus (GPa) Unknown Bulk modulus (GPa) 1.1 (liquid)
Vapour pressure  
Temperature (K)
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
Pressure (Pa)
- - - - - - - - - - -
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Hydrochloric acid (HCl) was known to the alchemists. The gaseous element itself was first produced in 1774 by Carl Wilhelm Scheele at Uppsala, Sweden, by heating hydrochloric acid with the mineral pyrolusite which is naturally occuring manganese dioxide, MnO2. A dense, greenish-yellow gas was evolved which he recorded as having a choking smell and which dissolved in water to give an acid solution. He noted that it bleached litmus paper, and decolourised leaves and flowers.

Humphry Davy investigated it in 1807 and eventually concluded not only that it was a simple substance, but that it was truly an element. He announced this in 1810 and yet it took another ten years for some chemists finally to accept that chlorine really was an element.
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Listen to Chlorine Podcast
Transcript :

Chemistry in its element: chlorine


You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry.

(End promo)

Chris Smith

Hello. What's got three isotopes, keeps swimming pools clean, damages the ozone layer and is used in more chemical synthesis reactions than you can shake a benzene ring at. Well the man with the answer is Tim Harrison.

Tim Harrison

Chlorine is what you might describe as a Jekyll and Hyde element; it is the friend of the synthetic chemist and has found a use in a number of 'nice' applications such as the disinfecting of drinking water and keeping our swimming pools clean. It also has an unpleasant side, being the first chemical warfare agent and taking some of the blame in the depletion of the Earth's ozone layer.

Elemental chlorine is a pale, yellowy green gas at room temperature. It was the Greek word khlôros meaning 'yellowish-green' that was used as inspiration by Sir Humphrey Davy when he named this element in the 19th century.

This element was first isolated in 1774 by the Swiss-German chemist Carl Wilhelm Scheele, by reacting hydrochloric acid with manganese (IV) oxide. But he failed to realise his achievement, mistakenly believing it also contained oxygen. It was Davy in 1810 who finally concluded that Scheele had made elemental chlorine.

Chlorine is in group 17 of periodic table, also called the halogens, and is not found as the element in nature - only as a compound. The most common of these being salt, or sodium chloride, and the potassium compounds sylvite (or potassium chloride) and carnallite (potassium magnesium chloride hexahydrate). It is also estimated that there are around two thousand organic chlorine compounds.

Chlorine has two stable isotopes chlorine-35 and chlorine-37with Chlorine-35 accounting for roughly 3 out of every 4 naturally occurring chlorine atoms. Chlorine-36 is also known naturally and is a radioactive isotope with a half life of about 30,000 years.

Chlorine has a major role to play in synthetic organic chemistry, taking part in three of the most common reaction mechanisms. In the first of these, the photochemical substitution reaction, chlorine reacts with an alkane by replacing one of the hydrogen atoms attached to a carbon forming a chloroalkane. This radical reaction is initiated by the use of sunlight or ultraviolet light to split diatomic chlorine into two radicals.

Chlorine can also react with alkenes via the electrophilic addition mechanism. This time two chlorine atoms add to a molecule across the electron-rich carbon-carbon double bond. This reaction has to be carried out in the dark to avoid complications with competing free radical substitutions.

A third common mechanism is electrophilic substitution, which occurs when chlorine reacts with a benzene ring by replacing a hydrogen atom forming chlorobenzene and hydrogen chloride. This reaction is most commonly known as the Friedal-Crafts reaction.

Chlorine also has a multitude of industrial uses. Including making bulk materials like bleached paper products, plastics such as PVC and the solvents tetrachloromethane, chloroform and dichloromethane. It is also used to make dyes, textiles, medicines, antiseptics, insecticides and paints.

It's best known uses however are probably in making bleaches such as 'Domestos' and in treating drinking and swimming pool waters to make them safe to use and of course its role as a chemical warfare agent.

The treatment of water with chlorine began in London after a cholera outbreak in 1850 when the physician and pioneering hygienist John Snow identified a well in Soho as the source of the outbreak. Chlorine is still used in most sewage treatment works today.

Snow also used a compound of chlorine - chloroform with the formula CHCl3 - as an anesthetic to aid the childbirth of two of Queen Victoria's children.

The use of chlorine gas as a chemical weapon was pioneered by German chemist Fritz Haber, who is better known for his work with ammonia. It was first used against the Allied soldiers in the battle of Ypres during the first world war. While it was quickly replaced by the more deadly phosgene and mustard gases, chlorine gas has been used as a weapon as recently as 2007 in Iraq during the second gulf war.

Chlorine was also once used to make a series of aerosol solvents and refrigerants called chlorofluorocarbons or CFCs. However their use was stopped once it became apparent that when in the atmosphere these compounds absorb ultraviolet light and cause homolytic bond fission producing a chlorine free radical which in turn reacts with ozone.

This has led to a reduction in the concentration of ozone in the so-called ozone layer, and therefore a reduction in the protection for those of us on the surface of the planet making us more susceptible to skin cancers. So, that's chlorine - a Jeckll and Hyde element with an extremely wide range of applications.

Chris Smith

So slap on your sun screen. Tim Harrison was telling the tale of Element number 17, and that's chlorine. Tim's based at the University of Bristol's ChemLabs. Next week, the stuff that gives itself an x-ray.

Brian Clegg

This grey metallic element gives off beta particles as it decays. These can cause radioactive damage in their own right, but prometheum is probably most dangerous because those beta particles generate X-rays when they hit heavy nuclei, making a sample of promethium bathe its surroundings in a constant low dosage x-ray beam. It was initially used to replace radium in luminous dials. Promethium chloride was mixed with phosphors that glow yellowy-green or blue when radiation hits them. However, as the dangers of the element's radioactive properties became apparent, this too was dropped from the domestic glow-in-the-dark market, only employed now in specialist applications.

Chris Smith

And you can hear what some of those applications are when Brian Clegg looks at the story of promethium in next week's Chemistry in its Element. In the meantime more elements are available from the Chemistry in its Element podcast, that's on iTunes or on the web at I'm Chris Smith, thank you very much for listening and goodbye.


Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by There's more information and other episodes of Chemistry in its element on our website at

(End promo)
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Description :
Studying the physical characteristics of the group 7 non-metals known as the halogens
Description :
How does the reaction compare with other halogens?
Description :
Demonstrating how the more reactive fluorine displaces the less reactive halogens
Description :
This experiment involves some reactions of the halogens.
Description :
This is a safe and reliable way of demonstrating the photosensitive, free radical reaction between hydrogen and chlorine, which can be difficult to do safely in the lab.
Description :
In this experiment, the similarities and differences in the solubilities of the halides of some Group 1 & 2 metals and silver are investigated using a microscale technique.

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Visual Elements images and videos
© Murray Robertson 2011.



W. M. Haynes, ed., CRC Handbook of Chemistry and Physics, CRC Press/Taylor and Francis, Boca Raton, FL, 95th Edition, Internet Version 2015, accessed December 2014.
Tables of Physical & Chemical Constants, Kaye & Laby Online, 16th edition, 1995. Version 1.0 (2005), accessed December 2014.
J. S. Coursey, D. J. Schwab, J.J. Tsai, and R. A. Dragoset, Atomic Weights and Isotopic Compositions(version 3.0), 2010, National Institute of Standards and Technology, Gaithersburg, MD, accessed December 2014.
T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 1954.


Uses and properties

John Emsley, Nature’s Building Blocks: An A-Z Guide to the Elements, Oxford University Press, New York, 2nd Edition, 2011.
Thomas Jefferson National Accelerator Facility - Office of Science Education, It’s Elemental - The Periodic Table of Elements, accessed December 2014.
Periodic Table of Videos, accessed December 2014.


Supply risk data

Derived in part from material provided by the British Geological Survey © NERC.


History text

© John Emsley 2012.



Produced by The Naked Scientists.


Periodic Table of Videos

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