Chlorine
Reactive nonmetal · Period 3 · Group 17 · p-block
Poison gas turned guardian of clean water
- Atomic mass
- 35.45 u
- State at 25 °C
- Gas
- Electronegativity
- 3.16
- Melting point
- -101.5 °C
- Boiling point
- -34 °C
- Configuration
- [Ne] 3s2 3p5
Chlorine, explained
Overview
Chlorine is one electron short of argon's configuration ([Ne]3s2 3p5), and it captures an extra electron more eagerly than any other element: its electron affinity, 349 kJ/mol, is even higher than fluorine's, because electrons in fluorine's tiny outer shell repel each other. Chlorine therefore readily forms the chloride ion, Cl-, and the greenish-yellow Cl2 gas is a strong oxidizing agent. Chlorine can also adopt positive oxidation states up to +7 when combined with oxygen, as in bleach (+1) and perchlorates (+7).
Industrially, chlorine is made by electrolysing brine in the chlor-alkali process, which also produces sodium hydroxide and hydrogen. A large share goes into PVC plastic, and much of the rest into making pharmaceuticals, solvents and titanium dioxide pigment. Adding chlorine to drinking water, starting in the early 1900s, dramatically reduced deaths from typhoid and cholera, because the hypochlorous acid it forms kills microbes. Chlorofluorocarbons, however, released chlorine atoms that destroyed stratospheric ozone, leading to the 1987 Montreal Protocol.
Discovery
In 1774 Carl Wilhelm Scheele produced chlorine by reacting the mineral pyrolusite (manganese dioxide) with hydrochloric acid, noting its suffocating smell and bleaching power. He and others believed it was a compound containing oxygen. In 1810 Humphry Davy showed that it was an element and named it from the Greek chloros, meaning pale green or yellow-green, after its colour.
- Where it’s found
- Chlorine occurs mainly as chloride in seawater and rock salt deposits; the gas is produced by electrolysis of sodium chloride brine.
- What it’s used for
- Water disinfection
- PVC plastics
- Household bleach
- Pharmaceutical synthesis
- Titanium dioxide pigment
- Solvents and refrigerants
- In living things
- Chloride is an essential electrolyte that helps regulate fluid balance and nerve signals, and stomach cells use it to make hydrochloric acid for digestion. Chlorine gas itself is highly toxic.
- Safety
- Chlorine gas is toxic and corrosive, damaging the lungs even at low concentrations; its sharp odour is detectable below 1 ppm. Household bleach must never be mixed with acids or ammonia, which release chlorine or toxic chloramines.
How it compares
Where Chlorine sits among all 118 elements on the trends that shape the table.
- Electronegativity3.16Pauling
#3 of 100 elements with data
- Ionization energy1,250kJ/mol
#9 of 104 elements with data
- Atomic radius100pm
#85 of 95 elements with data
- Electron affinity349kJ/mol
#1 of 109 elements with data
- Density0.003g/cm³
#107 of 114 elements with data
- Melting point172K
#101 of 110 elements with data
- Density
- 3.2 g/L
- Atomic radius
- 100 pm
- Covalent radius
- 99 pm
- Molar heat
- —
- Appearance
- pale yellow-green gas
- Discovered by
- Carl Wilhelm Scheele
Inside the atom
17 electrons arranged in 3 shells: 2 · 8 · 7.
Orbital diagram
1s2 2s2 2p6 3s2 3p5 · shorthand [Ne] 3s2 3p5
- 1s2
- 2s2
- 2p6
- 3s2
- 3p5
Each box is an orbital holding up to two electrons of opposite spin. Highlighted subshells hold the valence electrons that decide how Chlorine bonds.
Ionization energies
Signature reactions
The chemistry Chlorine is known for.
Redox · Electrolysis
Chlor-alkali electrolysis
Electric current passed through concentrated brine releases chlorine at the anode and hydrogen at the cathode, leaving sodium hydroxide in solution. A membrane keeps the products apart so they do not react. This single process supplies most of the world's chlorine and caustic soda.
Conditions: electrolysis, membrane cell
Redox · Disproportionation
Chlorine dissolving in water
Chlorine reacts with water to form hypochlorous acid and hydrochloric acid, with chlorine going from oxidation state 0 to both +1 and -1. Hypochlorous acid is the actual disinfectant, passing through microbial cell walls and destroying proteins. This is the chemistry at work in chlorinated drinking water and swimming pools.
Conditions: room temperature
Redox
Scheele's preparation of chlorine
Manganese dioxide oxidizes chloride ions to chlorine gas, which bubbles off with a yellow-green colour and pungent smell. Scheele first made chlorine this way in 1774. It remains a classic laboratory preparation.
Conditions: warm concentrated hydrochloric acid
Where you’ll meet it
Swimming pool
Pool water is kept safe by chlorine compounds that form hypochlorous acid.
Photo: Wikimedia Commons
Chemical weapons in World War I
Clouds of chlorine released from cylinders were used as a weapon on the Western Front from 1915.
Photo: Wikimedia Commons
Bleach
Household bleach is a solution of sodium hypochlorite, made by reacting chlorine with sodium hydroxide.
Photo: Wikimedia Commons
Polyvinyl chloride
PVC, used in pipes, flooring and cables, is about 57% chlorine by mass.
Photo: Wikimedia Commons
Research
No research on Chlorine yet
When Bhavya publishes a project that studies Chlorine, it will be linked here.