What does chlorine mean?
Chlorine is a chemical element with the symbol Cl and atomic number 17, belonging to the halogen group of the periodic table. At room temperature it exists as a pale-green gas with a sharp, suffocating odor, and it is so chemically reactive that it is never found free in nature, occurring instead only in combination — most abundantly as sodium chloride in seawater. Its powerful oxidizing ability makes it invaluable: diluted chlorine compounds disinfect drinking water and swimming pools, bleach paper and textiles, and serve as raw materials for plastics such as PVC. Yet the same reactivity makes elemental chlorine hazardous, and it was notoriously deployed as a poison gas during the First World War. The name comes from the Greek 'chloros', meaning pale green, coined after the element's distinctive color. For all its dangers, chlorine remains one of the most consequential substances in public health and modern industry.
nounA chemical element (symbol Cl, atomic number 17): a toxic, pale-green gaseous halogen with a pungent odor, used chiefly to disinfect water and to manufacture bleaches and other chemicals.
- The chemical element chlorine (symbol Cl, atomic number 17), a highly reactive pale-green halogen gas that is toxic in concentrated form and is widely used to disinfect water, sanitize swimming pools, and produce bleaches, plastics, and other industrial chemicals.
- By extension, a preparation or solution containing chlorine compounds used as a disinfectant or bleaching agent, as in household cleaning products.
"The municipal water supply is treated with a small amount of chlorine to kill harmful bacteria."
"The pool had been over-treated, and the sharp smell of chlorine stung her eyes as she stepped outside."
"In 1774 the Swedish chemist Carl Wilhelm Scheele first produced chlorine gas while investigating manganese dioxide."
Rarely pluralized; the plural appears only in specialized contexts referring to individual atoms of the element, as in chemistry discussions.
"Each molecule of chlorine gas consists of two chlorines bonded together."
Named after its sickly yellow-green color, chlorine was unleashed on the battlefields of World War I in 1915 — yet today it keeps drinking water safe for billions of people.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chlorine
The word 'chlorine' derives from the Greek 'chloros' (χλωρός), meaning pale green or yellowish-green, chosen because of the gas's characteristic color. It was coined around 1810 by the English chemist Sir Humphry Davy, who argued that the newly isolated substance was an element; the French chemist Antoine Lavoisier's school had earlier called it 'dephlogisticated marine acid'. The same Greek root gives us related scientific terms such as chlorophyll (the green pigment in plants) and chloroform.
Related word forms
How chlorine is actually used
Strictly a scientific and technical term, 'chlorine' is uncountable in ordinary use ('add more chlorine', not 'two chlorines'). In everyday speech it often stands loosely for the disinfecting products that contain chlorine compounds rather than the pure element itself.
Easily confused with chlorine
Chlorine is the element itself, while chloride refers to the negatively charged ion (Cl⁻) or compounds formed when chlorine combines with other elements, such as sodium chloride.
Chlorine is the element, whereas a chlorate is a salt containing the polyatomic ion ClO₃⁻, such as potassium chlorate.