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Chlorine facts for kids

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pale yellow-green gas
General properties
Name, symbol, number chlorine, Cl, 17
Group, period, block 173, p
Standard atomic weight {{{atomic mass}}} g/mol
Electron configuration [Ne] 3s2 3p5
Electrons per shell 2, 8, 7 (Image)
Physical properties
Phase gas
Density (0 °C, 101.325 kPa)
3.2 g/L
Liquid density at b.p. 1.5625 g/cm3
Melting point 171.6 K, −101.5 °C, −150.7 °F
Boiling point 239.11 K, −34.04 °C, −29.27 °F
Critical point 416.9 K, 7.991 MPa
Heat of fusion (Cl2) 6.406 kJ/mol
Heat of vaporization (Cl2) 20.41 kJ/mol
Specific heat capacity (25 °C) (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 7, 6, 5, 4, 3, 2, 1, −1
((a strongly acidic oxide))
Electronegativity 3.16 (Pauling scale)
Ionization energies
1st: {{{1st ionization energy}}} kJ/mol
2nd: {{{2nd ionization energy}}} kJ/mol
3rd: {{{3rd ionization energy}}} kJ/mol
Covalent radius 102±4 pm
Van der Waals radius 175 pm
Crystal structure orthorhombic
Magnetic ordering diamagnetic
Electrical resistivity (20 °C) >10Ω·m
Thermal conductivity (300 K) 8.9×10−3 W/(m·K)
Speed of sound 206 m/s
CAS registry number (Cl2) 7782-50-5
Most stable isotopes
Main article: Isotopes of chlorine
iso NA half-life DM DE (MeV) DP
35Cl 76% stable
36Cl trace 3.01×105 y β 36Ar
ε 36S
37Cl 24% stable
Chlorine gas in a tube

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.


Physical properties

Chlorine is a very irritating and greenish-yellow gas. It has a strong smell like bleach. It is toxic. 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

See also: Category: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.


Chlorine is widely used to purify water (usually in a swimming pool), as a disinfectant and bleach, and in the making of many important compounds including chloroform and carbon tetrachloride. It was used as a poison gas in some wars.


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.

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