A molecule
is defined as a sufficiently stable, electrically neutral group of at least two atoms in a definite arrangement held together by very strong (covalent) chemical bonds. [1] [2] Molecules are distinguished from polyatomic ions in this strict sense. In organic chemistry and biochemistry, the term molecule
is used less strictly and also is applied to charged organic molecules and biomolecules.
In the kinetic theory of gases the term molecule
is often used for any gaseous particle regardless of its composition. [3] According to this definition noble gas atoms are considered molecules despite the fact that they are composed of a single non-bonded atom. [4]
A molecule may consist of atoms of a single chemical element, as with oxygen (O2), or of different elements, as with water (H2O). Atoms and complexes connected by non-covalent bonds such as hydrogen bonds or ionic bonds are generally not considered single molecules.
No typical molecule can be defined for ionic crystals (salts) and covalent crystals (network solids), although these are often composed of repeating unit cells that extend either in a plane (such as in graphene) or three-dimensionally (such as in diamond or sodium chloride). The theme of repeated unit-cellular-structure also holds for most condensed phases with metallic bonding. In glasses (solids that exist in a vitreous disordered state), atoms may also be held together by chemical bonds without any definable molecule, but also without any of the regularity of repeating units that characterises crystals.
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MOLECULE TICKETS
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Molecular science
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 condensate
History and etymology
According to
Merriam-Webster and the
Online Etymology Dictionary, the word "molecule" derives from the
Latin "
moles" or small unit of mass.
- Molecule
(1794) - "extremely minute particle," from Fr. molécule
(1678), from Mod.L. molecula
, dim. of L. moles "mass, barrier". A vague meaning at first; the vogue for the word (used until late 18th century only in Latin form) can be traced to the philosophy of Descartes.
Although the existence of molecules has been accepted by many chemists since the early 19th century as a result of
Dalton's laws of
Definite and
Multiple Proportions (1803-1808) and
Avogadro's law (1811), there was some resistance among
positivists and physicists such as
Mach,
Boltzmann,
Maxwell, and
Gibbs, who saw molecules merely as convenient mathematical constructs. The work of
Perrin on Brownian motion (1911) is considered to be the final proof of the existence of molecules.
The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise, defining molecules as the smallest
particles of pure
chemical substances that still retain their
composition and chemical properties.
[5] This definition often breaks down since many substances in ordinary experience, such as
rocks,
salts, and
metals, are composed of large networks of
chemically bonded atoms or
ions, but are not made of discrete molecules.
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 molecule is the
diatomic hydrogen (H
2), with an overall length of roughly twice the 74
picometres (0.74
Å) bond length. 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
macromolecules or
supermolecules.
Radius
Effective molecular radius
is the size a molecule displays in solution.
[6] [7]
The
table of permselectivity for different substances contains examples.
Molecular formula
A compound's
empirical formula is the
simplest
integer ratio of the
chemical elements that constitute it. For example,
water is always composed of a 2:1 ratio of
hydrogen to
oxygen atoms, 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 ratios as ethanol, for instance. Molecules with the same
atoms in different arrangements are called
isomers. Also carbohydrates, for example, have the same ratio (carbon:hydrogen:oxygen= 1:2:1) (and thus the same empirical formula) but different total numbers of atoms in the molecule.
The
molecular formula reflects the exact number of atoms that compose the molecule and so characterizes different isomers.
The empirical formula is often the same as the molecular formula but not always. For example the molecule
acetylene has molecular formula C
2H
2, but the simplest integer ratio of elements is CH.
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 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). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through
absorbance or
emission.
[8]
Spectroscopy does not generally refer to
diffraction studies where particles such as
neutrons,
electrons, or high energy
X-rays interact with a regular arrangement of molecules (as in a crystal).
Theoretical aspects
The study of molecules by
molecular physics and
theoretical chemistry is largely based on
quantum mechanics and is essential for the understanding of the
chemical bond. The simplest of molecules is the
hydrogen molecule-ion, H
2+, and the simplest of all the chemical bonds is the
one-electron bond. H
2+ is composed of two positively-charged
protons and one negatively-charged
electron, which means that the
Schrödinger equation for the system can be solved more easily due to the lack of electron–electron repulsion. With the development of fast digital computers, approximate solutions for more complicated molecules became possible and are one of the main aspects of
computational chemistry.
When trying to define rigorously whether an arrangement of atoms is "sufficiently stable" to be considered a molecule, IUPAC suggests that it "must correspond to a depression on the
potential energy surface that is deep enough to confine at least one vibrational state".
This definition does not depend on the nature of the interaction between the atoms, but only on the strength of the interaction. In fact, it includes weakly-bound species that would not traditionally be considered molecules, such as the
helium dimer, He
2, which has one vibrational
bound state [9] and is so loosely bound that it is only likely to be observed at very low temperatures.
See also
Notes