Chlorine (Cl)
“Poison in a cylinder, lifesaver in a water plant — this yellow-green gas silently disinfects most of the drinking water on Earth.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
Characteristics
- A yellow-green, poisonous gas of Cl₂ molecules about 2.5 times denser than air — it was indeed used as a chemical weapon in the trenches of the First World War.
- Electron configuration [Ne] 3s² 3p⁵ — just one electron short of an octet, so it snatches electrons fiercely and forms chloride (Cl⁻) ions with nearly every element.
- A powerful oxidising and bleaching agent: damp red litmus turns white in its presence, and it destroys bacteria and viruses by the same electron-stealing chemistry.
- So reactive that free chlorine never survives in nature — it occurs as chloride ions in seawater (about 1.9 % by mass) and in gigantic rock-salt deposits.
A yellow-green gas with a sharp, suffocating smell, about two and a half times heavier than air, so it sinks and pools in low corners. It travels as diatomic Cl₂ molecules and turns to liquid at a chilly −34 °C — which is how it is pumped into cylinders and tanks for transport.
Where you meet it
✦ Uses
- Disinfecting drinking water — a few parts per million of chlorine kill the germs of cholera and typhoid; city waterworks in Dhaka and worldwide dose it every day.
- Bleaching powder [Ca(OCl)₂] for cotton yarn and fabric — the workhorse of textile and garment-washing factories — and for disinfecting drains and ponds.
- PVC plastic (polyvinyl chloride) — water pipes, electric-cable insulation, blood bags, raincoats and footwear are all children of chlorine chemistry.
- The chlor-alkali industry splits brine into chlorine, hydrogen and caustic soda (NaOH), the feedstock for soap, bleach, hydrochloric acid and countless other chemicals.
◍ Everyday examples
- Every grain of table salt is sodium chloride — half chlorine by mass; a sachet of ORS saline carries potassium chloride as well.
- The sharp "swimming-pool smell" is chlorine chemistry attacking germs and sweat in the water.
- The faint whiff from a freshly opened tap, or from cloth washed with bleaching powder, is dilute chlorine doing its job.
History & name
In 1774 the Swedish apothecary Carl Wilhelm Scheele heated hydrochloric acid with the black mineral pyrolusite (MnO₂) and collected a greenish, pungent gas — but he believed he had made "dephlogisticated muriatic acid", not a new element. For thirty-six years chemists puzzled over the gas, until in 1810 Humphry Davy showed it could not be broken down any further and declared it an element, naming it chlorine from the Greek chlōros, "yellow-green". The green gas became the founding member of a whole family of reactive elements: the halogens.