- Chlorine (Cl) is a highly reactive, greenish-yellow halogen with atomic number 17, positioned in Group 17 of the periodic table.
- It has seven valence electrons in its outermost shell, giving it a strong tendency to gain one electron to achieve a stable noble gas configuration. Its atomic structure consists of seventeen protons, typically eighteen neutrons, and seventeen electrons arranged in three shells (two in the first shell, eight in the second, and seven in the third).
- Chlorine has two stable isotopes: chlorine-35 (³⁵Cl), making up about 75.78% of natural chlorine, and chlorine-37 (³⁷Cl), about 24.22%.
- Under standard conditions, chlorine is a diatomic gas (Cl₂) with a sharp, suffocating odor, and it is about 2.5 times heavier than air.
- Chlorine is the 21st most abundant element in Earth’s crust and is widely distributed in nature, primarily in the form of chloride salts such as sodium chloride (NaCl) in seawater and evaporite minerals like halite. Large deposits of rock salt and brines are key commercial sources of chlorine. Industrially, chlorine is produced mainly by the electrolysis of concentrated brine solutions in the chlor-alkali process, which also yields hydrogen and sodium hydroxide.
- The element was first recognized as a distinct substance in 1774 by Swedish chemist Carl Wilhelm Scheele, who obtained it by reacting manganese dioxide with hydrochloric acid, though he believed it to be an oxide of an unknown element. It was named “chlorine” in 1810 by Sir Humphry Davy, from the Greek chloros, meaning “greenish-yellow,” after its color.
- Chlorine is an essential industrial chemical with numerous applications. It is used extensively as a disinfectant in water treatment and swimming pools, where it kills bacteria and other pathogens. It is also a critical raw material in the production of polyvinyl chloride (PVC) plastics, solvents, pesticides, and a variety of other organic and inorganic compounds. Chlorine compounds, such as sodium hypochlorite (NaOCl), are used in bleaching and cleaning products.
- Chemically, chlorine is a powerful oxidizing agent and reacts readily with most elements. It forms chlorides with metals and hydrogen chloride (HCl) with hydrogen, which dissolves in water to form hydrochloric acid. Organic chemistry of chlorine includes the production of chlorinated hydrocarbons, though many such compounds (e.g., CFCs, PCBs) are now regulated due to environmental and health concerns.
- Biologically, chlorine in the form of chloride ions (Cl⁻) is essential for life. Chloride maintains osmotic balance, aids nerve function, and contributes to the production of stomach acid (HCl) for digestion. However, elemental chlorine gas is highly toxic, irritating the respiratory tract, eyes, and skin, and at high concentrations, it can be fatal. Chlorine gas was infamously used as a chemical weapon during World War I.
- Environmental concerns about chlorine stem from its role in forming harmful by-products during disinfection, such as trihalomethanes (THMs), and from the persistence of certain chlorinated organic pollutants in ecosystems. Regulations and safer practices aim to minimize these effects while retaining chlorine’s benefits in sanitation and industry.
