Chlorine – facts and properties
- Symbol
- Cl
- Atomic number
- 17
- Atomic mass
- 35.45 u
- Period
- 3
- Group
- 17
- Category
- halogen
- Electron configuration
- [Ne] 3s² 3p⁵
- Electronegativity
- 3.16
- Atomic radius
- 79 pm
- Valence electrons
- 7
- State at standard conditions
- gas
- Melting point
- -101.5 °C
- Boiling point
- -34.04 °C
- Density
- 3.21 g/L
- Discovered
- 1774
Chlorine is a yellow-green gas with a sharp smell and a member of the halogen family. Its atomic number is 17, and it sits between fluorine and bromine in the periodic table. As a gas it is toxic and attacks the lungs. Combined with other elements, however, it is part of daily life: table salt is sodium chloride, stomach acid contains hydrochloric acid, and chlorine compounds keep drinking water and swimming pools free of germs.
Position in the periodic table
Occurrence and extraction
Chlorine is far too reactive to occur free in nature and is found almost entirely as the chloride ion. The largest reservoir is the sea, where sodium chloride makes up most of the dissolved salt. Ancient seas that dried out left behind thick beds of rock salt, often together with potassium chloride.
Carl Wilhelm Scheele produced chlorine in 1774 by reacting hydrochloric acid with manganese dioxide. Neither he nor his contemporaries recognised the gas as an element; it was long thought to be a compound containing oxygen. Only in 1810 did Humphry Davy show that it was an element, naming it after the Greek word for yellowish green.
Today nearly all chlorine is made by the chlor-alkali process, in which an electric current is passed through concentrated brine. Chlorine forms at the anode, hydrogen at the cathode, and sodium hydroxide solution is left behind. In modern plants a membrane keeps the products apart. Older cells with mercury cathodes are no longer permitted in the EU because they released mercury into the environment.
Properties and reactions
Chlorine has the electron configuration [Ne] 3s² 3p⁵ and seven valence electrons. It belongs to period 3 and group 17. With an electronegativity of 3.16 it is the most electronegative element after fluorine and oxygen. A chlorine atom readily gains one electron to become a chloride ion with the same electron arrangement as argon, which is why minus one is its most common oxidation state. In oxygen compounds such as hypochlorite, chlorate and perchlorate it reaches positive oxidation states as high as plus seven.
Elemental chlorine consists of molecules made of two atoms. At room temperature it is a yellow-green gas with a density of 3.21 g/L, considerably heavier than air, so it collects at ground level and in hollows. It is easily liquefied under pressure for transport. Chlorine boils at -34.04 °C and freezes at -101.5 °C.
As a powerful oxidising agent, chlorine reacts with almost every element. Sodium burns in it with a dazzling flame to form common salt, and hot iron wool glows as it turns into iron chloride. A mixture of chlorine and hydrogen explodes when exposed to strong light, producing hydrogen chloride, which dissolves in water to give hydrochloric acid. In water, chlorine partly forms hydrochloric acid and germ-killing hypochlorous acid.
Chlorine gas is highly toxic. It irritates the eyes and airways and at higher concentrations can cause life-threatening fluid build-up in the lungs. It was used as a chemical weapon in the First World War.
Uses in everyday life and technology
Almost all chlorine goes to the chemical industry. A major outlet is PVC, polyvinyl chloride, used for window frames, pipes, cable insulation and floor coverings. Chlorine is also needed to make many other plastics, medicines and crop protection products, even though the final product often contains no chlorine at all. Hydrochloric acid is used to clean steel and as a starting material for other chemicals.
For more than a century chlorine has made drinking water safe. Small amounts of chlorine or sodium hypochlorite reliably kill bacteria and have greatly reduced diseases such as cholera and typhoid. Swimming pool water is disinfected in the same way. Sodium hypochlorite is also the active ingredient in household bleach.
Chloride ions are essential to the human body. Together with sodium ions they control the balance of water in the body, and specialised cells in the stomach lining use them to produce hydrochloric acid for digestion.
Molecules containing this element
- Chlorine Cl2
- Hydrogen chloride HCl
- Ammonium chloride NH4Cl
Quick question
How many valence electrons does chlorine (Cl) have?
FAQ
Is chlorine poisonous?
Yes, chlorine gas is highly toxic. Even small amounts irritate the eyes, nose and airways, and larger amounts can severely damage the lungs. Chlorine in compounds behaves quite differently: the chloride ion in table salt is harmless and even essential. Drinking water contains so little chlorine that it is safe to drink.
Why do swimming pools smell of chlorine?
The typical pool smell usually does not come from the chlorine added for disinfection but from chloramines. These form when chlorine reacts with nitrogen compounds from sweat, urine and skin. A strong smell is therefore more a sign of dirty water than of too much chlorine.
Why should you never mix bleach with other cleaners?
Bleach contains sodium hypochlorite. When it meets an acid, for example in vinegar or limescale removers, it releases chlorine gas, which damages the airways even in small amounts. Mixed with cleaners containing ammonia, it gives off toxic chloramines. Never mix cleaning products, and use bleach only in well-ventilated rooms.