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molecule Smallest identifiable unit into which a pure substance can be divided and retain its composition and chemical properties. Division into still smaller parts, eventually atoms, involves destroying the bonding that holds the molecule together. For noble gases, the molecule is a single atom; all other substances have two (diatomic) or more (polyatomic) atoms in a molecule.The atoms are the same in elements, such as hydrogen (H2), and different in compounds, such as glucose (C6H12O6). Atoms always combine into molecules in fixed proportions. Molecules of different substances can have the same constituent atoms, either in different proportions, as in carbon monoxide (CO) and carbon dioxide (CO2), or bonded in different ways (see isomer). The covalent bonds in molecules give them their shapes and most of their properties. (The concept of molecules has no significance in solids with ionic bonds.) Analysis with modern techniques and computers can determine and display the size, shape,and configuration of molecules, the positions of their nuclei and electron clouds, the lengths and angles of their bonds, and other details. Electron microscopy can even produce images of individual molecules and atoms. See also molecular weight. water Inorganic compound composed of hydrogen and oxygen (H2O), existing in liquid, gas (steam, water vapor), and solid (ice) states. At room temperature, water is a colorless, odorless, tasteless liquid. One of the most abundant compounds, water covers about 75% of the earth's surface. Life depends on water for virtually every process, its ability to dissolve many other substances being perhaps its most essential quality. Life is believed to have originated in the world's oceans, and living organisms use aqueous solutions (including blood and digestive juices) as mediums for carrying out biological processes. Because water molecules are asymmetric and therefore electric dipoles, hydrogen bonding between molecules in liquid water and in ice is important in holding them together. Many of water's complex physical and chemical properties (high melting and boiling points, viscosity, surface tension, greater density in liquid than in solid form) arise from this extensive hydgrogen bonding. Water undergoes dissociation to the ions H+ (or H3O+) and OH−?, particularly in the presence of salts and other solutes; it may act as an acid or as a base. Water occurs bound (water of hydration) in many salts and minerals. It has myriad industrial uses, including as a suspending agent (papermaking, coal slurrying), solvent, diluting agent, coolant, and source of hydrogen; it is used in filtration, washing, steam generation, hydration of lime and cement, textile processing, sulfurmining, hydrolysis, hydraulics, as well as in beverages and foods. See also hardwater, heavy water. hydrogen bonding Interactions between pairs of atoms in adjoining molecules, weaker than ionic orcovalent bonds but stronger than van der Waals forces, aligning them and keeping them together. One member of the pair (the donor) has hydrogen atoms covalently bonded to nitrogen or oxygen atoms (-NH or -OH); the other member of the pair (acceptor) has northern or O atoms or negatively charged particles. The donor molecule effectively shares its hydrogen with the acceptor by sharing its electrons with the acceptor nitrogen or oxygen atom. Water is a good solvent for ionic compounds and many others because it forms hydrogen bonds between the solute and the solvent readily. Hydrogen bonds between amino acids in proteins determine their tertiary structures. Hydrogen bonds between nitrogenous bases in nucleotides on the two chains of DNA (guanine with cytosine, adenine with thymine) hold the key to the transmission of genetic information. hydrogen Lightest chemical element, chemical symbol H, atomic number 1. A colorless, odorless, tasteless, flammable gas, it occurs as the diatomic molecule H2. Its atom consists of one proton (the nucleus) and one electron; the isotopes deuterium and tritium have an additional one and two neutrons, respectively. Though only the ninth most abundant element on earth, it represents about 75% of all matter in the universe. Hydrogen was formerly used to fill airships; nonflammable helium has replaced it. It is used to synthesize ammonia, ethanol, aniline, and methanol; to treat petroleum fuels; as a reducing agent (see reduction) and to supply a reducing atmosphere; to make hydrogen chloride(see hydrochloric acid) and hydrogen bromide; and in hydrogenation. Liquid hydrogen (boiling point −?487F, or −?252.8C) is used in the laboratory to produce extremely low temperatures, in bubble chambers, and as a rocket fuel. Combustion of hydrogen with oxygen produces water. The properties of most acids, especially in water solutions, arise from the hydrogen ion (H+, also referredto as the hydronium ion, H3O+, the form in which H+ is found in a water environment) See also hydride, hydrocarbon. hydride Inorganic compound of hydrogen with another element. Three common types are differentiated by their bonding. In saline (ionic) hydrides(see ionic bond), the hydrogen is an anion, H−?, and behaves like a halogen. Saline hydrides such as sodium hydride (NaH) and calcium hydride (CaH2) react vigorously with water, giving off hydrogen gas (H2), and are used as portable sources of it. Metallic hydrides, such as titanium hydride (TiH2), are alloylike materials (see alloy) with some properties of metals, such as luster and electrical conductivity. Covalent hydrides (see covalent bond) are mostly compounds of hydrogen and nonmetallic elements; they include water, ammonia, hydrogen sulfide (H2S), and methane. In polymeric hydrides, the hydrogen forms bridges between other atoms (e.g., hydrides of boron and aluminum). Those hydrides give off large amounts of energy when burned and may be useful as rocket fuels. Halogens Any of the five nonmetallic elements with similar chemical properties: fluorine, chlorine, bromine, iodine (I), and astatine (At). They occur next to the rightmost column of the periodic table as usually arranged. All are highly reactive oxidizing agents (see oxidation-reduction) with valence 1 (for fluorine, the only valence). They combine readily with most metals and nonmetals to form a variety of compounds and never occur uncombined in nature. Astatine occurs naturally in minute amounts as an intermediate decay product; it has no stable nonradioactive isotope.
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