Chlorine is a greenish-yellow, highly reactive halogen. At standard conditions it forms a toxic, corrosive diatomic gas (Cl₂) used widely for disinfection and as an oxidizing agent.
Chlorine has the ground-state configuration \([Ne]3s^2\,3p^5\), leaving one vacancy to complete an octet. It has a high electron affinity and electronegativity, so it readily gains an electron to form \(\mathrm{Cl^-}\) or shares electrons in covalent bonds, making Cl2 a strong oxidizing agent.
Chlorine is produced by electrolysis of brine (NaCl solution). At the anode: \(\mathrm{2\,Cl^- \rightarrow Cl_2(g) + 2\,e^-}\). The cathodic half-reaction generates hydrogen and hydroxide: \(\mathrm{2\,H_2O + 2\,e^- \rightarrow H_2(g) + 2\,OH^-}\). Overall, NaOH, H2, and Cl2 are co-products. Historically, the Deacon process (\(\mathrm{4\,HCl + O_2 \rightarrow 2\,Cl_2 + 2\,H_2O}\)) was also used.
In water, chlorine undergoes hydrolysis/disproportionation to form hypochlorous acid (HOCl), a powerful oxidant that destroys cell membranes and enzymes:
\(\mathrm{Cl_2 + H_2O \rightleftharpoons HCl + HOCl}\)
In alkaline media: \(\mathrm{Cl_2 + 2\,OH^- \rightarrow Cl^- + OCl^- + H_2O}\). The antimicrobial species are mainly HOCl/OCl− ("free available chlorine").
Chlorine exhibits a wide range from −1 to +7:
With cold, dilute alkali (e.g., NaOH), chlorine forms chloride and hypochlorite:
\(\mathrm{Cl_2 + 2\,NaOH \xrightarrow{cold} NaCl + NaOCl + H_2O}\)
With hot, concentrated alkali, it forms chloride and chlorate:
\(\mathrm{3\,Cl_2 + 6\,NaOH \xrightarrow{hot} 5\,NaCl + NaClO_3 + 3\,H_2O}\)
Bleaching powder (commercial calcium oxychloride) is produced by passing chlorine over dry slaked lime:
\(\mathrm{Ca(OH)_2 + Cl_2 \rightarrow Ca(OCl)Cl + H_2O}\)
It releases \(\mathrm{Cl_2}\)/HOCl on contact with moisture/acids, enabling bleaching and disinfection.
Add AgNO3 to an acidified sample: a white precipitate of AgCl forms:
\(\mathrm{Ag^+ + Cl^- \rightarrow AgCl(s)}\)
AgCl dissolves in aqueous ammonia due to complex formation: \(\mathrm{AgCl + 2\,NH_3 \rightarrow [Ag(NH_3)_2]^+ + Cl^-}\).
They all generate the same active species in water—HOCl/OCl−—but differ in storage stability, concentration, and byproducts. Gaseous Cl2 requires specialized handling; NaOCl (bleach) is convenient for solutions; Ca(OCl)2 is a solid with longer shelf life.
Chlorine gas is toxic, corrosive, and irritating to eyes and lungs. Work in well-ventilated areas with gas detection; use compatible materials (PVC, PTFE); avoid mixing household bleach with acids or ammonia (dangerous chloramines/Cl2 can form). For exposure, move to fresh air and seek medical assistance.
Chlorine forms interhalogens such as \(\mathrm{ClF}\), \(\mathrm{ClF_3}\), and \(\mathrm{ICl}\). Their polar halogen–halogen bonds and electron-deficient centers make them potent fluorinating/chlorinating agents and useful in synthesis—handled with care due to reactivity.