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Chlorine (Cl), element 17
Chlorine (Cl) is the chemical element with atomic number 17: a halogen in group 17, period 3. At 25 °C it is a gas, with an atomic mass of 35.45 u. Electronegativity of 3.16 on the Pauling scale: few atoms pull electrons harder.
Physical and chemical data
| Atomic number | 17 |
|---|---|
| Symbol | Cl |
| Atomic mass | 35.45 u |
| Density | 0.003214 g/cm³ |
| Melting point | -101.5 °C (171.65 K) |
| Boiling point | -34.04 °C (239.11 K) |
| Electronegativity (Pauling) | 3.16 |
| Electron configuration | [Ne]3s2 3p5 |
| Atomic radius (van der Waals) | 175 pm |
| Oxidation states | +7, +5, +1, -1 |
| Physical state at 25 °C | gas |
| Family | halogen |
| Group and period | 17 / 3 |
| Discoverer and year | Carl Wilhelm Scheele (1774) |
The discovery record belongs to Carl Wilhelm Scheele, in 1774.
Neighbours on the table
Horizontally, one proton less lands on Sulfur (Z=16); one more lands on Argon (Z=18). Vertically, group 17 gathers relatives with similar behaviour: Fluorine, Bromine, Iodine, Astatine.
The story behind the name
Scheele produced the yellow-green gas in 1774 but thought it was a compound; it fell to Humphry Davy, in 1810, to prove it was an element and name it after the Greek chloros, pale green, the same root as chlorophyll. Chlorine has a double biography: on the bright side, drinking water chlorination ranks among the greatest life savers in public health history; on the dark side, it was the first mass chemical weapon, released over the trenches of Ypres in 1915. Day to day it sits chained to sodium in table salt and gives swimming pools their signature smell.
Physical and chemical data: PubChem Periodic Table (NCBI, compiling NIST and IUPAC), public domain, snapshot of 2026-07-10. Temperatures converted from kelvin; historical notes cross checked against classic chemistry references.
Last updated: · Methodology and sources