Chlorine (CL)

Part of the halogen group in the periodic table of the elements, chlorine (Cl) is a gas that can combine with almost any other element. It has an atomic number of 17 and an atomic mass of 35.453 atomic mass units (u). It melts at −103 degrees Celsius (−153 degrees Fahrenheit) and boils at −34 degrees Celsius (−29 degrees Fahrenheit). Its density at one atmosphere and 0 degrees Celsius (32 degrees Fahrenheit) is 3.214 grams per liter. Its oxidation states are −1, +1, +3, +5, and +7. Its electron configuration is 1s²2s²2p⁶3s²3p⁵.

Pure chlorine is yellowish green. It is considered dangerous to inhale, as it can irritate the mucus membranes of the lungs. An airborne dose of one part chlorine in one thousand can be fatal within minutes if inhaled. Chlorine gas was used as a chemical weapon during World War I.

Most chlorine found in nature is combined with something else. For instance, chlorine readily bonds with sodium to produce sodium chloride, or table salt. Scientists typically extract chlorine from a chloride substance through a highly specialized separation technique called electrolysis. Around the world, chlorine serves a variety of purposes and is found in products that most people use every day. It is used to clean swimming pools and even helps produce safe drinking water.

Brief History

Swedish chemist Carl Wilhelm Scheele (1742–86) discovered chlorine in 1774. After extracting and isolating the gas, Scheele noted that it was quite oppressive to the lungs and carried a suffocating smell, produced a yellowish-green color in the air, and made water taste acidic. After further research, Scheele discovered that this newly discovered gas was very reactive with other elements.

Despite his detailed observations of the gas, Scheele did not think that chlorine was its own element. This confusion was brought on by the phlogiston theory, which was accepted by nearly every chemist during the 1700s. By the early 1800s, the general consensus was that chlorine was an oxygen-containing compound. In 1810, English chemist Humphry Davy (1778–1829) concluded that the gas was, in fact, an element. He gave it the name chlorine, derived from the Greek term choloros, meaning “yellow-green.”

Overview

Chlorine is a relatively dense gas that smells similar to bleach. It is capable of bonding with almost all other elements and, as a liquid, can be used as a powerful oxidizing agent for disinfecting and cleaning purposes. Chlorine was part of the first chain reaction ever observed. It occurred in 1913 when German physical chemist Max Bodenstein (1871–1942) observed a mixture of hydrogen gas and chlorine; the two elements exploded when exposed to light. German physicist Walther Nernst (1864–1941) explained this phenomenon in detail in 1918.

Oceans contain a large amount of chlorine—so much that if all of the chlorine on Earth were simultaneously released as a gas, it would measure approximately five times larger than Earth’s current atmosphere. Chlorine gas is much heavier than air, and even low concentrations have been known to cause fatalities. During World War I, the Germans used chlorine for chemical warfare, although it ended up being less effective than they had originally hoped; chlorine had such a strong smell that soldiers from the opposing side were able to detect it and protect themselves by placing damp cloths over their faces.

Chlorine is reactive because it is unstable. Atoms of different elements all have a certain degree of stability, depending on the configuration of their electrons. In simple terms, an atom is said to be stable if its outermost, or valence, electron shell is filled to capacity with electrons, and it is said to be unstable if its valence electron shell is lacking electrons. Chlorine lacks a single electron in its valence shell. It therefore readily combines with elements that need to give up a single electron in order to become stable.

The most common chlorine-containing compound is sodium chloride, commonly known as table salt. Sodium and chlorine readily bond because sodium has only one electron in its valence electron shell and must give up that electron in order to become stable. When the two elements combine, a stable ionic compound is formed. Chlorine also bonds efficiently with aluminum to form a compound called aluminum trichloride (AlCl₃). A single aluminum atom, missing three electrons in its outermost orbital, combines with three chlorine atoms to make a single molecule of this compound.

Depending on the elements involved, chlorine can form either a covalent bond, in which two atoms share electrons, or an ionic bond, in which one element gives up one or more electrons to another. The type of bond formed dramatically affects the behavior of the final molecule. For instance, when chlorine forms an ionic compound, as in the case of sodium chloride, the ions that make up the compound are held together by the attraction of their opposing electrical charges. This is why water becomes much more electrically conductive as the concentration of salt increases.

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