Molecule
From Encyclopædia
A molecule is the smallest particle of a substance that exhibits the chemical properties of that substance. Molecules are groups of
atoms (see
atom) held together by chemical bonds (see CHEMICAL BOND). Each molecule of a given substance contains the same
number and kinds of
atoms. In a chemical REACTION the bonds are broken, and rearrangements of
atoms takes place to form new substances.The
number of
atoms in molecules ranges from two to hundreds or thousands. Common two-
atom (diatomic) molecules include
oxygen and
carbon monoxide. Ozone and
carbon dioxide are examples of three-
atom (triatomic) molecules. Examples thereafter range up to huge PROTEIN molecules with many thousands of
atoms.IsomersMolecules may have the same
number and kinds of
atoms but different chemical properties, because the
atoms are bonded or arranged differently within the molecules. Such molecules are called isomers (see ISOMER) of one another.Isomers that differ in bonding are called structural isomers. For example, molecules of the chemicals called ethyl
alcohol and methyl ether both contain one
oxygen atom, two
carbon atoms, and six hydrogen
atoms, but their properties are different because the
atoms are bonded in a different way.Stereoisomers, on the other
hand, are molecules in which the same kinds of bonds exist between
atoms. The molecules differ only in the arrangement of the
atoms relative to one another. Two kinds of stereoisomer exist: geometric and optical. The chemical PtNH(3)2Cl(2) is an example of a geometric isomer. One isomer with this formula has the two
chlorine (Cl)
atoms next to each other. It is effective in treating some forms of cancer. The other isomer, in which the
chlorine atoms are not neighbors, has virtually no effect on cancer.Optical isomers occur when two molecules are mirror images of but not superimposable on each other. (In the same way, the left and right
hands are mirror images of but not superimposable on each other; see
stereochemistry.) Usually such isomers differ only in the way they rotate plane-polarized
light (see POLARIZED
light). In living organisms, however, some optical isomers--such as among the
sugars--differ greatly in their acceptability by the body.Intermolecular ForcesThe attractive forces between similar molecules (see INTERMOLECULAR FORCES) are responsible for whether a substance is a solid, liquid, or gas. Gases have weak attractive forces between molecules, whereas liquids and solids have stronger forces. All intermolecular forces are weak compared to the chemical bonds in a molecule. Covalent chemical bonds arise because of the attractive forces between the positive nuclei and the negative
electrons (see
electron). For ionic bonds the attractive forces arise from the attraction between oppositely charged ions (see
ION AND IONIZATION).Laboratory TechniquesIn order to characterize a chemical compound, a scientist may use a
number of techniques. Thus, the
molecular weight is determined by
Mass SPECTROMETRY and by solution-boiling-point and freezing-point data.
quantitative chemical analysis of molecules is accomplished by standard methods or by more advanced methods such as atomic absorption spectrometry. Molecular structure is usually determined by X-ray or
electron diffraction; by infrared, ultraviolet, or visible
spectroscopy; by NUCLEAR MAGNETIC RESONANCE techniques; or by
electron paramagnetic resonance or
Mass spectroscopy. Microscopes such as the atomic-force microscope, an advanced version of the scanning tunneling
electron MICROSCOPE, can map and even cleave organic molecules. Advanced optical microscopes such as the near-field scanning microscope can also probe living or other matter at the molecular level.Gerald C. RoperBibliography: Alcock, N. W., Bonding and Structure (1990); Avery, John, et al., eds., Understanding Molecular Properties (1987); Coulson, C. A., The Shape and Structure of Molecules, 2d ed. (1982); Maruani, Jean, ed., Molecules in
physics, Chemistry, and Biology, 3 vols. (1988).
