Molecule
In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds.[1] Chemical substances are not infinitely divisible into smaller fractions of the same substance: a molecule is generally considered the smallest particle of a pure substance that still retains its composition and chemical properties.[2] Certain pure substances (e.g., metals, molten salts, crystals, etc.) are best understood as being composed of networks or aggregates of atoms or ions instead of molecular units.
In the molecular sciences, a "molecule" is a sufficiently stable, electrically neutral entity composed of two or more atoms.[3] The concept of a single-atom or monatomic molecule, as found in noble gases, is used almost exclusively in the kinetic theory of gases, where the fundamental gas particles are conventionally termed "molecules" regardless of their composition. [4]
History
Although the concept of molecules was first introduced in 1811 by Avogadro, and was accepted by many chemists as a result of Dalton's laws of Definite and Multiple Proportions (1803-1808), with notable exceptions (Boltzmann, Maxwell, Gibbs), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of Perrin (1911), and was strenuously resisted by early positivists such as Mach. The word molecule was coined by the Scottish botanist Robert Brown in his famous paper[5], as a derivation of Leeuwenhoek's term animalcules. The modern theory of molecules makes great use of the many numerical techniques offered by computational chemistry. Dozens of molecules have now been identified in interstellar space by microwave spectroscopy.
Overview
The science of molecules is called molecular chemistry or molecular physics, depending on the focus. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. In molecular sciences, a molecule consists of a stable system (bound state) comprising two or more atoms. Polyatomic ions may sometimes be usefully thought of as electrically charged molecules. The term unstable molecule is used for very reactive species, i.e., short-lived assemblies (resonances) of electrons and nuclei, such as radicals, molecular ions, Rydberg molecules, transition states, Van der Waals complexes, or systems of colliding atoms as in Bose-Einstein condensates.
A peculiar use of the term molecular is as a synonym to covalent, which arises from the fact that, unlike molecular covalent compounds, ionic compounds do not yield well-defined smallest particles that would be consistent with the definition above. However, the same problem also arises for some (but not all) covalent compounds. No typical "smallest particle" can be defined for covalent crystals, or network solids, which are composed of repeating unit cells that extend indefinitely either in a plane (such as in graphite) or three-dimensionally (such as in diamond).
While all gases exist as molecules by definition (as the term for gas particles), not all solids and liquids do. In fact, many of the most familiar substances in ordinary experience, such as rocks, crystals, and metals, are composed of atoms or ions, but are not made of molecules.
In a molecule, the atoms are joined by shared pairs of electrons in a chemical bond. It may consist of atoms of the same chemical element, as with oxygen (O2), or of different elements, as with water (H2O).
Molecular size
Most molecules are far too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes, as can molecules of many polymers. The smallest of all molecules is the hydrogen ion molecule H2+, comprised of two protons bonded together by the sharing of one electron.[citation needed] The next largest molecule is the hydrogen molecule H2, with a length roughly twice the 74 picometres (0.74 Å) distance between the two hydrogen nuclei; but as with all molecules, however, the exact size of its electron cloud is difficult to define precisely. Molecules commonly used as building blocks for organic synthesis have a dimension of a few Å to several dozen Å. Single molecules cannot usually be observed by light (as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope. Some of the largest molecules are supermolecules.
Molecular formula
The empirical formula of a molecule is the simplest integer ratio of the chemical elements that constitute the compound. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers. The empirical formula is often the same as the molecular formula but not always. For example the molecule acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH. The molecular formula reflects the exact number of atoms that compose a molecule.
The molecular mass can be calculated from the chemical formula and is expressed in conventional atomic mass units equal to 1/12th of the mass of a neutral carbon-12 (12C isotope) atom. For network solids, the term formula unit is used in stoichiometric calculations.
Molecular geometry
Molecules have fixed equilibrium geometries—bond lengths and angles— about which they continuously oscillate through vibrational and rotational motions. A pure substance is composed of molecules with the same average geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.
Molecular spectroscopy
Molecular spectroscopy deals with the response (spectrum) of molecules interacting with probing signals of known energy (or frequency, according to Planck's formula). Scattering theory provides the theoretical background for spectroscopy.
The probing signal used in spectroscopy can be an electromagnetic wave or a beam of particles (electrons, positrons, etc.) The molecular response can consist of signal absorption (absorption spectroscopy), the emission of another signal (emission spectroscopy), fragmentation, or chemical changes.
Spectroscopy is recognized as a powerful tool in investigating the microscopic properties of molecules, in particular their energy levels. In order to extract maximum microscopic information from experimental results, spectroscopy is often coupled with chemical computations.
See also
- Covalent bond
- Diatomic molecule
- Molecular geometry
- Molecular Hamiltonian
- Molecular orbital
- Nonpolar molecule
- Polar molecule
- For a list of molecules see the List of compounds
- List of molecules in interstellar space
References
- ^ Pauling, Linus (1970). General Chemistry. New York: Dover Publications, Inc. ISBN 0-486-65622-5.
Ebbin, Darrell, D. (1990). General Chemistry, 3th Ed. Boston: Houghton Mifflin Co. ISBN 0-395-43302-9.{{cite book}}: CS1 maint: multiple names: authors list (link)
Brown, T.L. (2003). Chemistry – the Central Science, 9th Ed. New Jersey: Prentice Hall. ISBN 0-13-066997-0.
Chang, Raymond (1998). Chemistry, 6th Ed. New York: McGraw Hill. ISBN 0-07-115221-0.
Zumdahl, Steven S. (1997). Chemistry, 4th ed. Boston: Houghton Mifflin. ISBN 0-669-41794-7. - ^ Molecule Definition - Frostburg State University (Department of Chemistry)
- ^ Compendium of Chemical Terminology, molecule
- ^ [1] [2] [3]
- ^ R. Brown, Edinb. New Philos. J. 5, 358 (1828).
External links
- Molecule of the Month - School of Chemistry, University of Bristol
- Antibody Molecule - The National Health Museum
- Data Book of Molecules - Home Page for Learning Environmental Chemistry