Chlorine Facts 1

Chlorine is the 17th element of the periodic table. These chlorine facts contain chemical and physical data along with general information and history.

Element cell for Chlorine

Chlorine Periodic Table Cell

Basic Chlorine Facts

Name: Chlorine

Atomic Number: 17

Element Symbol: Cl

Group: 17

Period: 3

Block: p

Element Family: Halogen

Atomic Mass: [35.446; 35.457]
IUPAC guidelines to reflect the physical and chemical history of the magnesium sample. If a single value of the atomic mass is needed, use 35.4527.

Electron Configuration: [Ne]3s23p5 (shorthand) or 1s22s22p63s23p(full)

Discovery: Carl Wilhelm Scheele in 1774

Scheele isolated chlorine from hydrochloric acid. He thought the yellow gas he collected was an oxide of another compound, not a new element. This belief continued until 1811 when Sir Humphry Davy showed there was no oxygen in chlorine gas. Chlorine was its own element.

Name Origin: Davy named the new element chlorine from the Greek word chloros meaning yellow-green.


Natural chlorine is comprised of two stable isotopes: 35Cl, and 37Cl. Twenty two radioactive isotopes exist ranging from 28Cl to 51Cl.

Chlorine-35 is a stable isotope containing 18 neutrons. 75.77% of natural chlorine is chlorine-35.

Chlorine-37 is a stable isotope containing 20 neutrons. 24.23% of natural chlorine is chlorine-37.

Chlorine-36 is a radioactive isotope containing 19 neutrons. It is formed when cosmic radiation interacts with atmospheric chlorine. Chlorine-36 decays by β- decay into 36Ar 98.1% of the time or 36S by electron capture 1.9% of the time. The half-life is 301,200 years and can be found naturally in trace quantities.

Liquid Chlorine

Liquid chlorine in a flask. Credit: Workingclass91/Creative Commons

Physical Data

(gas) 0.002898 g/cm3
(liquid at boiling point) 1.5625 g/cm3

Melting Point: 171.6 K ​(−101.5 °C, ​−150.7 °F)

Boiling Point: 239.11 K ​(−34.04 °C, ​−29.27 °F)

Critical Point: 416.9 K at 7.991 MPa

State at 20ºC: Gas

Heat of Fusion: 6.406 kJ/mol for Cl2

Heat of Vaporization: 20.41 kJ/mol for Cl2

Molar Heat Capacity: 33.949 J/mol·K for Cl2

Chlorine Atom

Electron shell configuration of a chlorine atom.

Atomic Data

Atomic Radius: 1.75 Å

Covalent Radius: 1.02 Å

Van der Waals Radius:  1.75 Å

Electron Affinity: 348.575 kJ/mol

Electronegativity: 3.16

1st Ionization Energy: 1251.186 kJ/mol

2nd Ionization Energy: 2297.663 kJ/mol

3rd Ionization Energy: 3821.78 kJ/mol

4th Ionization Energy: 5158.608 kJ/mol

5th Ionization Energy: 6541.7 kJ/mol

6th Ionization Energy: 9361.97 kJ/mol

7th Ionization Energy: 11018.221 kJ/mol

8th Ionization Energy: 33603.91 kJ/mol

Oxidation States: +7, +5, +1, -1 (common), +6, +6, +2 (uncommon)

Fun Chlorine Facts

  • Chlorine rarely occurs freely in nature. You are much more likely to find chlorine combined with other elements (such as table salt NaCl) or itself the form Cl2.
  • Chlorine is the third most abundant element in the Earth’s oceans.
  • Chlorine is the 21st most abundant element in the Earth’s crust.
  • Chlorine is the ninth most abundant element in the human body.
  • Chlorine gas is has a yellow-green color and a strong smell.
  • The smell of chlorine is an irritant. Humans can detect the smell at concentrations as low as 3.5 parts per million.
  • Chlorine was an early chemical weapon. It reacts with the mucous membranes and can burn the skin. Concentrations around 1000 parts per million are typically fatal after a few breaths. Chlorine gas is also denser than air and would settle in low lying areas such as trenches and foxholes.
  • Leaking chlorine containers are hazardous. Leaks in containers are detected using ammonia. The ammonia reacts with the chlorine and forms a white mist near the leak.
  • Chlorine appears often in organic chemistry reactions where it replaces hydrogen atoms in molecules.
  • Chlorine is used in the manufacture of a multitude of products such as plastics, paper, medicine, insecticides, disinfectants, textiles, dyes and petroleum products. It is commercially extracted by electrolysis from brine solutions.
  • Scheele’s name for chlorine was muriaticum. The old name for hydrochloric acid was muriatic acid.

Learn more about elements on the periodic table.