We continue to review some of the most important materials in heat treatment and metallurgy.  

Chlorine (chemical symbol: Cl)

Chlorine is a greenish-yellow dense gas, which is toxic and about 2.5 times heavier than air. Chlorine has a pungent odor often associated with the smell of bleach. Chlorine gas is a respiratory (lung) irritant, and inhaling it can cause fluid buildup in the lungs and difficulty breathing. In addition, contact of the eyes and skin to chlorine gas can be extremely irritating and will cause severe burns and ulcerations.

Chlorine belongs to the group of halogen elements, which are named for their salt-forming properties (Fig. 1) and include fluorine, bromine, iodine and astatine. It does not appear in its pure form naturally due to its ability to readily combine with nearly all other elements. Therefore, chlorine in nature is commonly found within salts located in mineral deposits such as nickel chloride (Fig. 2).

Chlorine was first produced in 1774 by Swedish chemist Carl Wilhelm Scheele when he collected the gas released by the chemical reaction between manganese dioxide and hydrochloric acid. He thought that the new gas was a vaporous form of hydrochloric acid since he was otherwise unable to explain what exactly it was through the technology of his time. In 1810, Sir Humphry Davy did further analysis on the mystery gas and determined that he could not further reduce it and that it was a new element. Davy named the new element chlorine from the Greek word “chloros” meaning pale green.

Chlorine is used in a wide variety of applications, most commonly as an antiseptic to make drinking water safe from contaminants. Its most common commercial use is to treat water in swimming pools.

Chlorine also has many uses for industrial purposes and sees use in the production of paper goods, plastics, dyes, textiles, medicine, insecticides, solvents and paints. Furthermore, chlorine has seen use in the manufacturing of new materials for the automotive industry for the intent of making vehicles lighter, ranging from seat cushions and seat covers to tire cords and bumpers. In the chemical-products industry, chlorine is used as an oxidizing agent that has strong disinfecting and whitening (bleaching) qualities.

Here are a few important facts about chlorine (Fig. 3).[2,5]

  • Atomic number: 17
  • Atomic weight: 35.4527
  • Melting point: 171.65 K (-101.5°C or -150.7°F)
  • Boiling point: 239.11 K (-34.04°C or -29.27°F)
  • Density: 0.003214 grams per cubic centimeter
  • Phase at room temperature: Gas
  • Element classification: Non-metal
  • Period number: 3    
  • Group number: 17    
  • Group name: Halogen
  • Electron configuration: [Ne] 3s23d5



  1. KnowledgeDoor (www.knowledgedoor.com)
  2. Jefferson Lab (https://www.jlab.org)
  3. Chemicool (www.chemicool.com/)
  4. Wikipedia (www.wikipedia.org)
  5. Royal Society of Chemistry (rsc.org)