A family of elements
The Halogens are a family of five naturally-occurring chemical elements represented in one of the vertical columns of the Periodic Table of the Elements. Chemical elements are the basic building blocks of matter: substances that cannot be broken down into simpler forms by ordinary chemical means. The Halogen family consists of the elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At).
Each of nature’s chemical elements is made of atoms comprising a positively charged nucleus surrounded by concentric shells of negatively charged electrons. In the Periodic Table of the Elements, vertical columns contain elements whose atoms have the same number of electrons in their outermost electron shells. All of the members of the Halogen family, for example, contain seven outermost electrons. In the world of chemistry, seven outermost electrons is a highly unstable configuration, while eight is extremely stable. As a result, the Halogen elements are always “swiping” electrons from other atoms—a process known as oxidation—to achieve a “stable octet.” This electron swiping forms a chemical bond between Halogens and other elements, resulting in chemical compounds. Because of their reactive nature—a consequence of their “pursuit” of a stable outer electron shell—the Halogens are always found in nature chemically bonded.
Not one chemistry
The members of the Halogen family are known for their tendency to form salts. Table salt, sodium chloride, is an example of a familiar Halogen salt. Sodium chloride is found in natural geologic deposits of salt minerals left over from the slow evaporation of ancient seawater (see photo to the right). The mineral name for table salt is halite. Calcium chloride, applied to roads in winter to help melt ice and snow for public safety, is another example of a common, useful salt.
Due to their enormous reactivity, the Halogen elements can form many types of compounds in addition to salts. One of the tenets of chemistry is that the properties of compounds are entirely independent of the properties of their constituent elements (for example, water, H2O, is very different from hydrogen or oxygen). While the Halogen elements share similar properties (their chemical tendency to swipe electrons, for example, makes them all good oxidizers), the physical, chemical and even biological properties and the toxicities of halogen-containing compounds can vary greatly. It is not possible, therefore, to make general statements about the characteristics of all halogen-containing compounds.
With their wide diversity, many halogen-containing compounds are used to make thousands of everyday products. Halogen compounds also may be useful in intermediate steps in manufacturing processes, and absent in the final product.
Q&A:
Are all halogens the same?
No, although they have some similar properties. The Halogen family of chemical elements, including fluorine, chlorine, bromine, iodine and astatine, are grouped together in the Periodic Table of the Elements based upon their electronic structure. Because of this electron configuration, all of the Halogen elements are quite reactive and form compounds by gaining an electron from other chemical elements. However, there are significant differences among the Halogen elements. For example, at room temperature fluorine and chlorine are gases, bromine is a liquid and iodine and astatine are solids. Astatine, naturally radioactive, is much rarer than the other Halogens.
The properties of chemical elements are unrelated to the properties of the compounds in which they occur. It is not possible to make broad statements about the chemical, physical or biological properties of the wide variety of halogen-containing compounds.
Do halogens occur in nature?
Yes. Halogens occur naturally combined with other elements in compounds in plants, animals, rocks, minerals and soils of all types. Fluoride is bound with other elements in bone; iodide, an essential component of human thyroid hormones, is found in seawater and marine life. Bromide is also present in seawater, and in the human body. The U.S. Geologic Survey estimates the Dead Sea, actually a lake more than eight times saltier than ocean water, contains one billion tons of bromine. The long-prized brilliant Tyrian purple of ancient times was available because a little snail in the Mediterranean had the ability to take bromide from the sea and bind it to indigo, forming dibromoindigo—Tyrian purple.
More than 2,000 naturally occurring “organochlorine” compounds—chlorine-containing organic compounds—have been identified in living organisms. Sodium chloride literally keeps our bodies from drying up, moves our muscles, makes our meals matter, and attacks germs to help keep us healthy.
What are some common products made from halogens?
From Fluorine: Many man-made fluorochemicals have been developed over the years. At the top of a very long list: the inorganic fluorides used in drinking water and dental products; one out of every five active pharmaceutical products is fluorinated, and the synthetic blood substitutes and inhalation drug delivery systems use fluorocarbons. Industrial and household refrigeration and air conditioning industries use low-toxicity, nonflammable, and energy-efficient fluorocarbon fluids. Fluoropolymers and fluoroelastomers are used widely in homes, buildings, automobiles, aerospace applications, and wherever high-quality thermal, flame, electrical, chemical, and solvent resistance and low oxygen and moisture permeability are needed. Other low-molecular-weight perfluoroalkyl-based materials provide oil-, water-, and soil-repellent surface properties for textile, fiber, and paper coatings; and similar materials are used as surfactants to stabilize aqueous fire-fighting foams. Fluorocarbons are used as fire extinguishants in aerospace and other critical areas. Modern high-energy-density lithium-ion batteries used in handheld electronic devices rely on LiPF6. The manufacture of silicon chips relies on the wet and dry etch processes utilizing materials such as ultra-high-purity HF and NF3.1
From Chlorine: Public health relies on chlorine-based disinfectants to help purify drinking water, sanitize swimming pools, and kill pathogens in our homes using household bleach. From helping provide one of the most basic human needs—clean drinking water—to contributing to the production of high-tech first-responder equipment, sustainable building materials, food protection chemicals, computer microprocessor chips and 93 percent of prescription pharmaceuticals, chlorine chemistry is essential to everyday life in America. Among its many uses, chlorine chemistry is instrumental in producing high-purity silicon which is needed to make the tiny integrated circuit “chips” that are at the heart of modern electronics. Solar panels, which generate greenhouse gas-free electricity directly from sunlight, are also made using high-purity silicon. And fiber optic cable, also manufactured with high-purity silicon, is currently in great demand as the communications industry competes to deliver integrated voice, data and Internet services to consumers. The on-line Chlorine Tree demonstrates the many products of chlorine chemistry.
From Bromine: Bromine’s main uses are in producing flame retardants, drilling fluids, agrochemicals, water purification compounds, dyes, medicines and photography chemicals. Brominated flame retardants, such as tetrabromobisphenol A, decabromodiphenyl ether, and vinyl bromide, represent retardants of growing importance. The bromide salts of calcium, sodium, and zinc form dense solutions in water that are used as drilling fluids.
From Iodine: Iodine is an essential micronutrient for humans. Table salt is iodized to help prevent goiter and mental retardation. Potassium iodide is used to help reduce the dose of radiation to the thyroid gland in treating victims of radiation poisoning. Iodine is used in halogen lamps and in ink pigments. Tincture of iodine is used as a topical antiseptic to kill bacteria. Silver iodide is used in the preparation of some photographic films.
From Astatine: Astatine, a rare radioactive chemical element with an extremely short half-life, decays to isotopes of lead. Researchers are investigating the use of astatine for the treatment of human tumors.
No comments:
Post a Comment