Chlorine, a gaseous halogen has atomic number 17 and belongs in group 17, period 3 and the p-block of the periodic table. Needing only one electron to fill its outer electron shell, chlorine readily forms compounds with most other elements. Because it is so reactive, chlorine does not occur in a pure state in nature, which is fortunate, as it is poisonous.
Within compounds, chlorine is a common element with an estimated crustal abundance of 145 milligrams per kilogram and an estimated oceanic abundance of 19,400 milligrams per liter. The most common compound of chlorine is sodium chloride (common table salt) and this explains it high concentration in the salty oceans of Earth. Minerals within crustal rocks that contain the element include carnallite, and sylvite.
The gas is greenish-yellow in color and its name derives from the Greek word “chloros” meaning greenish-yellow! Although the chemist Carl Wilhelm Scheele discovered the gas in 1774, Sir Humphry Davy named it after he proved it to be an element and not a compound in 1810.
The element has a melting point of minus 101.5°C (171.65 K, minus 150.7°F) and a boiling point of minus 34.04°C (239.11 K, minus 29.27°F). The density of chlorine is 0.003214 grams per cubic centimeter.
Chlorine gas dissolves better in cold water than warm water. At 10°C (283 K, 50°F), one volume of water will dissolve 3.10 volumes of chlorine; at 30°C (303 K, 86°F), the same volume of water dissolves only 1.77 volumes of chlorine.
Chlorine is a powerful oxidizing agent as such it proves valuable in the production of many organic chemicals. It is also of use as a bleaching agent and a disinfectant. Its microbe killing action is utilized in the production of safe potable water as well as disinfecting swimming pools.
As a poisonous gas, chlorine was one of the first chemical weapons deployed in a war. During April 1915, the Germans deployed chlorine gas against the French Army in the trenches at Ypres and the true horror of a weapon of mass destruction became apparent. The immediate effect of the gas was a burning sensation in the throat and chest pains. The gas destroyed the respiratory tract leading to death by slow asphyxiation. Breathing chlorine concentrations of 1000 parts per million proves lethal. Humans can detect chlorine gas at concentrations as low as 3.5 parts per million by smell.
Despite being poisonous in its pure form, chlorine is an essential element to animal and plant life. Humans contain 1,200,000 parts per billion by weight. Chlorine is essential for our digestion as human stomachs contain hydrochloric acid.
Los Alamos National Laboratory Chemistry Division
Jefferson Laboratories Science Education website