Chlorine · element 17
A poison gas that keeps swimming pools clean and sits in every grain of table salt
Key facts
Chlorine has two faces. As a free gas it is greenish-yellow, heavier than air and dangerous to breathe, and it was used as a weapon in the First World War. Combined with sodium it is the harmless white crystal you sprinkle on chips. The difference between the two is a single electron, and that electron is the whole story of chlorine.
It is the second lightest of the halogens, the column of reactive non-metals in group 17. Chemists produce tens of millions of tonnes of it each year, not mainly for bleach or pools, but as a building block for plastics, medicines and solvents.
Where it is found
Chlorine is far too reactive to exist as a free element in nature. Instead it is present as chloride ions, mostly dissolved in seawater, where every litre contains about 19 grams of chloride. Ancient evaporated seas have left thick beds of rock salt underground, mined in places such as Cheshire in England and Wieliczka in Poland. Sylvite and carnallite, potassium chloride minerals, are another source.
Your body holds chloride too. It is the main negative ion in blood plasma, it balances the sodium and potassium in and around your cells, and your stomach makes hydrochloric acid to break down food and kill microbes.
How it was discovered
Carl Wilhelm Scheele, a Swedish pharmacist, prepared chlorine gas in 1774 by warming hydrochloric acid with the mineral pyrolusite, which is manganese dioxide. He described its colour, its suffocating smell, its bleaching effect on flowers and leaves, and the way it attacked metals. But he believed it was a compound containing oxygen. It took until 1810 for Humphry Davy in London to argue that it could not be broken down further and must be an element. He named it from the Greek chloros, meaning pale green.
What we use it for
About a third of chlorine production goes into PVC, the plastic used for window frames, pipes, cable insulation and flooring. Much of the rest is used as a chemical stepping stone: chlorine is added to a molecule during manufacture and later removed, leaving products such as polyurethane foams, silicones and titanium dioxide pigment. A large share of modern medicines and crop protection chemicals need chlorine somewhere in their synthesis.
Its most visible job is disinfection. Adding small amounts of chlorine or its compounds to drinking water kills bacteria and viruses, and this is one of the most important public health measures of the last century. Swimming pools use the same chemistry. Household bleach is sodium hypochlorite, made by bubbling chlorine into sodium hydroxide solution.
Reading the numbers
Chlorine is a gas at room temperature. It condenses to a liquid at about -34 °C and freezes at about -101 °C, so it can be liquefied under modest pressure and shipped in steel tanks. Its density as a gas is 3.2 grams per litre, roughly two and a half times that of air, which is why it collects in low ground and trenches. The configuration [Ne] 3s2 3p5 has seven outer electrons, one short of the stable argon arrangement. That is why chlorine grabs an electron so eagerly, forming the chloride ion Cl-, and why its electronegativity of 3.16 is among the highest in the table. Its first ionisation energy, about 1251 kJ per mole, is high, showing it holds its own electrons tightly. It sits between sulfur and argon in period 3. Below it in group 17, bromine is a liquid and iodine a solid: the halogens get heavier and less reactive as you go down.
Safety and the environment
Chlorine gas damages the lungs even at low concentrations because it reacts with moisture to make hydrochloric and hypochlorous acids in the airways. It is one of the most common causes of accidental chemical exposure at home, usually when someone mixes bleach with an acidic cleaner or with ammonia-based products. Never mix cleaning chemicals. Environmentally, chlorine's difficult legacy is the chlorofluorocarbons (CFCs) that thinned the ozone layer, and persistent chlorinated pollutants such as DDT and PCBs, both now banned in most countries.
Did you know?
- Chlorine was the first chemical agent used on a large scale in war, released from cylinders at Ypres in April 1915.
- The swimming pool smell most people call chlorine is mostly chloramines, formed when chlorine reacts with sweat and urine; a well-kept pool smells of very little.
- A single chlorine atom released from a CFC in the stratosphere can destroy many thousands of ozone molecules before it is finally removed.
- Natural chlorine is a mix of chlorine-35 and chlorine-37 in roughly a three to one ratio, which is why its atomic mass of 35.45 is nowhere near a whole number.
- Chlorination of public water supplies began in the early twentieth century and is credited with a large drop in deaths from typhoid and cholera.
More in the halogens
These share a column with chlorine, so their outer electrons are arranged in a similar way and they tend to behave alike.
See all 6 halogens on one page, or browse all 118 elements.
Compare Chlorine with another element
Each comparison has a side-by-side data table and a plain-English summary of the differences.
Frequently asked questions
Is chlorine gas poisonous?
Yes. Chlorine reacts with the moisture in your airways to form acids, causing coughing, chest pain and fluid in the lungs. Even the small amounts released by mixing household bleach with other cleaners can be harmful in a closed room.
Why is chlorine added to drinking water?
Tiny doses of chlorine kill the bacteria and viruses that cause diseases such as cholera and typhoid, and a small residual amount keeps the water safe as it travels through pipes. The amount used is far too low to harm people.
What happens if you mix bleach and vinegar?
Bleach contains sodium hypochlorite, and adding an acid such as vinegar releases chlorine gas. This can cause coughing and lung irritation quickly in a small space, so cleaning products should never be mixed.
Why is chlorine so reactive?
A chlorine atom has seven electrons in its outer shell and needs just one more to reach the stable arrangement of argon. It pulls that electron from almost any metal or shares one with almost any non-metal, so it reacts with nearly everything.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Chlorine reference article. Spotted an error? Tell us.