The chemical element Chlorine has the symbol Cl and atomic number 17.

On the periodic table Chlorine is in the halogen group and it is the 2nd lightest halogen gas after fluorine.

In its standard form chlorine is a yellow-green gas, but its common compounds are usually colorless. Chlorine has a strong distinctive odor such as the smell of household bleach.

The name Chlorine is from the Greek word chloros which means greenish yellow.

Chlorine has a melting point of -150.7 °F (-101.5 °C) and a boiling point of -29.27 °F (-34.04 °C).

Free chlorine is rare on Earth. Chlorine combines with nearly all elements to create chlorine compounds called chlorides, which are much more common. 

There are over 2000 naturally occurring organic chlorine compounds.

The most common compound of chlorine has been known about since ancient times, it is sodium chloride we know it better as 'common salt'.

Swedish chemist, Carl Wilhelm Scheele, discovered Chlorine in 1774 believing it contained oxygen. In 1810, Sir Humphry Davy tried the same experiment and concluded that Chlorine was in fact an element, and not a compound.

Chlorine is the 3rd most abundant element in Earth's oceans (about 1.9% of the mass of seawater is chloride ions) and the 21st most common chemical element in the Earth's crust.

Chlorine's high oxidizing properties saw it used to purify water in the U.S as early as 1918. Today chlorine and its various compounds are used in most swimming pools throughout the world to keep them clean and in many household cleaning products such as disinfectants and bleaches.

Chlorine is also used in a range of other industrial and consumer products such as the making of plastics, whitening of textiles, pharmaceuticals, in chloroform, insecticides, paper products, solvents, dyestuffs and paints.

In high concentrations chlorine is extremely dangerous and poisonous. It is also heavier than air, so can fill up enclosed spaces. Because of these facts chlorine was the first gaseous chemical used as a weapon in warfare when both sides at times dispersed it in low-lying foxholes and trenches of World War I.