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Chlorine, ₁₇Cl
General properties
Pronunciation	/ˈklɔːriːn, -aɪn/ (klohr-EEN-,_---eyen)
Appearance	pale yellow-green gas
Standard atomic weight (Ar, standard)	[35.446, 35.457]
Chlorine in the periodic table
	sulfur ← chlorine → argon
Atomic number (Z)	17
Group	group 17 (halogens)
Period	period 3
Block	p-block
Element category	reactive nonmetal
Electron configuration	[Ne] 3s2 3p5
Electrons per shell	2, 8, 7
Physical properties
Phase at STP	Cl: Gas
Melting point	171.6 K (−101.5 °C, −150.7 °F)
Boiling point	239.11 K (−34.04 °C, −29.27 °F)
Density (at STP)	3.2 g/L
when liquid (at b.p.)	1.5625 g/cm3
Critical point	416.9 K, 7.991 MPa
Heat of fusion	(Cl2) 6.406 kJ/mol
Heat of vaporization	(Cl2) 20.41 kJ/mol
Molar heat capacity	(Cl2); 33.949 J/(mol·K)
Atomic properties
Oxidation states	−1, +1, +2, +3, +4, +5, +6, +7 (a strongly acidic oxide)
Electronegativity	Pauling scale: 3.16
Ionization energies	1st: 1251.2 kJ/mol ; 2nd: 2298 kJ/mol ; 3rd: 3822 kJ/mol ; (more) ;
Covalent radius	102±4 pm
Van der Waals radius	175 pm
	Spectral lines of chlorine
Other properties
Natural occurrence	Cl: Primordial
Crystal structure	orthorhombic
Speed of sound	206 m/s (gas, at 0 °C)
Thermal conductivity	8.9×10−3 W/(m·K)
Electrical resistivity	>10 Ω·m (at 20 °C)
Magnetic ordering	diamagnetic
Magnetic susceptibility	−40.5·10−6 cm3/mol
CAS Number	(Cl2) 7782-50-5
History
Discovery and first isolation	Carl Wilhelm Scheele (1774)
Recognized as an element by	Humphry Davy (1808)
Main isotopes of chlorine
ε	36S
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Chlorine gas in a tube

Chlorine (chemical symbol Cl) is a chemical element. Its atomic number (which is the number of protons in it) is 17, and its atomic mass is 35.45. It is part of the 7th column (halogens) on the periodic table of elements.