Chlorine is a chemical element with symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is a yellow-green gas at room temperature. It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity, behind only oxygen and fluorine. 

The most common compound of chlorine, sodium chloride (common salt), has been known since ancient times. Around 1630, chlorine gas was first synthesised in a chemical reaction, but not recognised as a fundamentally important substance. Carl Wilhelm Scheele wrote a description of chlorine gas in 1774, supposing it to be an oxide of a new element. In 1809, chemists suggested that the gas might be a pure element, and this was confirmed by Sir Humphry Davy in 1810, who named it from based on its colour. 

Because of its great reactivity, all chlorine in the Earth's crust is in the form of ionic chloride compounds, which includes table salt. It is the second-most abundant halogen (after fluorine) and twenty-first most abundant chemical element in Earth's crust. These crustal deposits are nevertheless dwarfed by the huge reserves of chloride in seawater.
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Chlorine
Chlorine
a chemical element
Chlorine is a chemical element
symbol Cl
symbol Cl
yes
atomic number 17
17
number 17
yellow-green
Chlorine is a yellow-green
gas
Chlorine is a yellow-green gas
no
It is an extremely reactive
electron affinity
the highest electron affinity
electronegativity
 third-highest electronegativity
unknown
unknown
in 1774
Carl Wilhelm Scheele wrote a description of chlorine gas in 1774
Carl Wilhelm Scheele
Carl Wilhelm Scheele
an oxide of a new element
supposing it to be an oxide of a new element
Sir Humphry Davy
Sir Humphry Davy in 1810
1810
n 1810
yes
the huge reserves of chloride in seawater
no
huge reserves
unknown
unknown
yes
common salt