Covalent chemical bonds are different from ionic bonding. Covalent bonding involves the sharing of a pair of electrons by two atoms. A
molecule of water, written H^0, is held together by covalent bonds. The two elements, hydrogen and oxygen, share electrons.
Covalent bonding occurs between two elements that are nonmetals. Much of the matter of substances in your everyday life, the solids, liquids, and gases, are the results of covalent bonding. Covalent bonds occur in all sorts of organic substances, including materials associated with living things, such as plant or animal tissue, food, and the like. Covalent bonding predominates in biological systems, although ions have very important roles in these systems, too.
A small molecule that is held together by covalent bonding is called a monomer (from Greek mono "one" and meros "part"). Examples of monomers are hydrocarbons that consist only of the elements carbon (C) and hydrogen (H). Hydrocarbons are combustible when they combine with oxygen. They are the main components of fossil fuels, which include petroleum, coal, and natural gas. When the covalent bonds of these types of substances are broken they will release energy, which is what occurs when fossil fuels burn.
Monomers may become chemically bonded to several other similar monomers to form a polymer. A polymer is a term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. When monomers are linked together to each other during a chemical reaction, it is called polymerization. Often polymerization occurs as a chain reaction, which will continue until a large number of the monomers have combined (polymerized) with each other. The result of polymerization is a chain or other network of linked monomers that can be formed into fibers, sheets, fabrics, foams, or other structures, depending on the type of polymer.
Polymers can occur naturally, mainly in living organisms, or synthetically, through manufacturing. A few general examples of naturally occurring polymers include proteins, starches, and cellulose. Starches are polymers composed of sugars. Latex, from the sap of rubber trees, can be made to polymerize into latex rubber. The genetic materials DNAand RNAfound in chromosomes and cells are also types of polymers. Several natural fibers such as wool, silk, and spider web thread are polymers. Scientists and engineers study the formation, structure, and properties of such natural polymers as models for synthetic nanomaterials.
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