Chlorine (Cl)
Element 17 · Halogen · atomic mass 35.45 u · group 17, period 3
Electron shells: 2, 8, 7
Chlorine is a choking, greenish gas that is violently reactive in its elemental form. As the chloride ion it is completely benign and essential — half of table salt.
Chlorinating drinking water is one of the largest single contributors to human life expectancy in history.
- Atomic number
- 17
- Atomic mass
- 35.45 u
- Electron configuration
- [Ne] 3s² 3p⁵
- Group · period · block
- 17 · 3 · p
- Oxidation states
- −1, +1, +3, +5, +7
- Electronegativity
- 3.16 (Pauling)
- Melting point
- -101 °C
- Boiling point
- -34.04 °C
- Density
- 0.003214 g/cm³
- State at 20 °C
- Gas
- Appearance
- Pale yellow-green gas
- Discovered
- Carl Wilhelm Scheele, 1774
17 protons, and 17 electrons when neutral
IUPAC interval [35.446, 35.457]
Halogens
What Chlorine is used for
- Drinking-water and pool disinfection
- PVC plastic manufacture
- Bleaches and disinfectants
- Pharmaceutical synthesis
Atomic structure of Chlorine
A neutral chlorine atom has 17 protons and 17 electrons. Its most common isotope carries about 18 neutrons, which is where the mass number 35 comes from. Change the neutron count and you have a different isotope of chlorine; change the proton count and you have a different element entirely.
Those electrons fill shells from the inside out: 2, 8, 7. In orbital notation that is [Ne] 3s² 3p⁵, leaving 7 valence electrons in the outer shell — the ones that do the chemistry.
3D electron orbitals
See where Chlorine's electrons actually are — a cloud of 26,000 points sampled from the probability density |ψ|², not a cartoon of a lobe.
Where Chlorine sits in the periodic table
Group 17, one electron short of a full shell, which makes them aggressive oxidisers. They form salts directly with metals — "halogen" literally means salt-former.
Discovery and naming
Chlorine was discovered by Carl Wilhelm Scheele in 1774. Its name comes from Greek chloros, "pale green", which is also why its symbol (Cl) does not match its English name.
Keep exploring
Atomic weight from IUPAC/CIAAW; physical properties from PubChem (US National Library of Medicine). Values that have never been measured are shown as “not measured” rather than estimated.
Related Calculators
Frequently Asked Questions
What is the atomic mass of Chlorine?
The standard atomic weight of Chlorine is 35.45 u. Because the natural isotope mix varies between sources, IUPAC publishes it as the interval [35.446, 35.457], and 35.45 is the conventional single value to use in calculations.
How many protons, neutrons and electrons does Chlorine have?
Chlorine has 17 protons, because the atomic number is the proton count and that is what makes an atom Chlorine at all. A neutral Chlorine atom has the same number of electrons, 17. The neutron count varies by isotope; the most common one has about 18, since 35 − 17 = 18.
What is the electron configuration of Chlorine?
The electron configuration of Chlorine is [Ne] 3s² 3p⁵. Its electrons are arranged 2, 8, 7 across 3 shells.
Is Chlorine a metal?
No. Chlorine is classified as a halogen, on the right-hand side of the table where elements gain or share electrons rather than losing them.
What group and period is Chlorine in?
Chlorine is in group 17 (the halogens), period 3, in the p-block. The group is its column and the period is its row.
What is the charge of Chlorine?
Chlorine most commonly forms a 1− ion. It is 1 electron short of a full outer shell, so it gains one rather than losing 7. Across all its compounds it shows oxidation states of −1, +1, +3, +5, +7.
What is the melting point of Chlorine?
Chlorine melts at -101 °C and boils at -34.04 °C. That puts it in the gases at everyday temperatures.
What is Chlorine used for?
Drinking-water and pool disinfection, PVC plastic manufacture, Bleaches and disinfectants, Pharmaceutical synthesis. Chlorine is a choking, greenish gas that is violently reactive in its elemental form.
Who discovered Chlorine, and where does its name come from?
Chlorine (Cl) was discovered by Carl Wilhelm Scheele in 1774. The name comes from Greek chloros, "pale green".