Editing Period 3 element (section)

=== Chlorine ===
{{Main|Chlorine}}
''Chlorine'' (symbol '''Cl''') is the second-lightest [[halogen]]. The element forms diatomic molecules under [[standard conditions]], called dichlorine. It has the highest [[electron affinity]] and the one of highest [[electronegativity]] of all the elements; thus chlorine is a strong [[oxidizing agent]].

The most common compound of chlorine, sodium chloride ([[table salt]]), has been known since ancient times; however, around 1630, chlorine gas was obtained by the Belgian chemist and physician Jan Baptist van Helmont. The synthesis and characterization of elemental chlorine occurred in 1774 by Swedish chemist Carl Wilhelm Scheele, who called it "dephlogisticated muriatic acid air", as he thought he synthesized the oxide obtained from the [[hydrochloric acid]], because acids were thought at the time to necessarily contain oxygen. A number of chemists, including Claude Berthollet, suggested that Scheele's "dephlogisticated muriatic acid air" must be a combination of oxygen and the yet undiscovered element, and Scheele named the supposed new element within this oxide as ''muriaticum''. The suggestion that this newly discovered gas was a simple element was made in 1809 by Joseph Louis Gay-Lussac and Louis-Jacques. This was confirmed in 1810 by [[Sir Humphry Davy]], who named it chlorine, from the Greek word ''χλωρός'' (chlōros), meaning "green-yellow".

Chlorine is a component of many other compounds. It is the [[Abundance of elements in Earth's crust|second most abundant halogen and 21st most abundant element]] in Earth's crust. The great oxidizing power of chlorine led it to its [[Bleach (chemical)|bleaching]] and disinfectant uses, as well as being an essential reagent in the chemical industry. As a common disinfectant, chlorine compounds are used in [[swimming pool]]s to keep them clean and [[swimming pool sanitation|sanitary]]. In the [[upper atmosphere]], chlorine-containing molecules such as [[chlorofluorocarbons]] have been implicated in [[ozone depletion]].