Quick answerChlorine (Cl) is a halogen with atomic number 17 and an atomic mass of 35.45 (2023), in group 17, period 3. It was discovered by Carl Wilhelm Scheele in 1774. It is used for water disinfection and swimming pool sanitation. Its electron configuration is [Ne] 3s2 3p5.
Chlorine is the greenish gas that keeps swimming pools clear and drinking water safe, killing microbes by the tonne. It also makes PVC plastic, bleach, and the solvents that dry-clean clothes. The same reactivity that disinfects water makes chlorine gas dangerous, it was used as a chemical weapon in the First World War.
Key facts
| Symbol | Cl |
|---|---|
| Atomic number | 17 |
| Atomic mass | 35.45 (IUPAC standard atomic weight, 2023) |
| Category | halogen |
| Group | 17 |
| Period | 3 |
| Block | p-block |
| Electron configuration | [Ne] 3s2 3p5 |
| Discovered | Carl Wilhelm Scheele, 1774 |
| Uses | water disinfection, swimming pool sanitation, PVC plastic production, bleach and disinfectants, paper bleaching |
Overview
Chlorine is the greenish gas that keeps swimming pools clear and drinking water safe, killing microbes by the tonne. It has atomic number 17, an atomic mass of 35.45, and the electron configuration [Ne] 3s2 3p5, and it sits in group 17, the halogens, in period 3. Carl Wilhelm Scheele isolated it in 1774, and Humphry Davy later recognised it as an element and named it from the Greek for greenish-yellow. Chlorine is far too reactive to occur free in nature; it is found as chloride, above all in common salt and seawater, from which it is made by passing an electric current through brine. Its reactivity is both its value and its danger: it disinfects drinking water and swimming pools, bleaches paper and cloth, and makes PVC plastic, solvents and medicines, yet chlorine gas itself is poisonous and was used as a chemical weapon in the First World War. Your body needs chloride ions for digestion and fluid balance, and the hydrochloric acid in your stomach is a chlorine compound. Chlorine compounds also keep hospitals clean and treat sewage.
Frequently asked questions
What is chlorine used for?
Chlorine is mainly used to disinfect drinking water and swimming pools, killing microbes that cause disease. It also makes PVC plastic, bleach, paper and solvents, and its compounds clean hospitals and treat sewage. Your body needs chloride for digestion and fluid balance, and stomach acid is a chlorine compound.
What is the symbol for chlorine?
The chemical symbol for chlorine is Cl, from its name, which comes from the Greek word chloros, meaning greenish-yellow, the colour of chlorine gas. It has atomic number 17 and a standard atomic mass of 35.45, and it sits in group 17 and period 3. Carl Wilhelm Scheele isolated it in 1774.
What is the atomic number of chlorine?
Chlorine has atomic number 17, so it is the seventeenth element in the periodic table. Its atom holds seventeen protons and seventeen electrons, and its standard atomic mass is 35.45. It is a halogen in group 17 and period 3, with the electron configuration [Ne] 3s2 3p5, one electron short of a full shell.
Is chlorine a metal?
No, chlorine is a nonmetal. It is a halogen in group 17 of the periodic table, and at room temperature it is a greenish-yellow gas with a sharp, choking smell. It never occurs as a free gas in nature, but only as chloride in salt and seawater, because it reacts so readily with other elements.
Is chlorine in drinking water safe?
Yes, chlorine in drinking water is safe at the low levels used to disinfect it. Water companies add just enough chlorine to kill bacteria and viruses, and the amount is strictly regulated and monitored. The faint smell of chlorine in tap water is a sign that the water has been treated, and it fades if the water stands.
Quick quiz
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Keep exploring
- Periodic table — all 118 elements
- Fluorine (F) — another halogen, atomic number 9
- Bromine (Br) — another halogen, atomic number 35
- Iodine (I) — another halogen, atomic number 53
- Astatine (At) — another halogen, atomic number 85
- Tennessine (Ts) — another halogen, atomic number 117
Sources & last updated
- CIAAW — Standard Atomic Weights
- PubChem Periodic Table
- Wikipedia — Timeline of chemical element discoveries
Last updated: . Figures are shown with their year; where sources disagree, the value is left out rather than estimated.