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A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. The science of molecules is called molecular chemistry or molecular physics, depending on the particular aspect of focus. Molecular chemistry is concerned with the laws governing the interaction between molecules that results in the formation and breakage of chemical bonds, while molecular physics is concerned with the laws governing their structure and properties. In practice, however, this distinction is rather vague.
According to the strict definition, molecules can consist of one atom (as in noble gases) or more atoms bonded together. The concept of monatomic (single-atom) molecule is used almost exclusively in the kinetic theory of gases. In molecular sciences a molecule consists of a stable system (bound state) comprising two or more atoms. 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 (see Rydberg atoms), 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 covalent compounds, ionic compounds do not yield well-defined smallest particles that would be consistent with the definition above.
Although the concept of molecule was first introduced in 1811 by Avogadro, the existence of molecules was still an open debate in the chemistry community till the work of Perrin (1911). The modern theory of molecules makes great use of the many numerical techniques offered by computational chemistry.
For a list of molecules see the List of compounds.
Figure 1. 3D (left and center) and 2D (right) representations of the terpenoid, atisane. In the 3D model on the left, carbon atoms are represented by gray spheres, white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) electric charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single-bonds.
Chemical bond
- see main article chemical bond
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).
Size
Most molecules are much too small to be seen with the naked eye, but there are exceptions. DNA, a macromolecule, can reach macroscopic sizes.
The smallest molecule is the hydrogen molecule. The interatomic distance is 1.5 Å. But the size of its electron cloud is difficult to define precisely. Under normal conditions molecules have a dimension of a few to a few dozen Å.
Empirical formula
- see main article empirical formula
The empirical formula of a molecule is the integer ratio of the 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.
Chemical formula
- see main article chemical formula
The chemical 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 units equal to 1/12 from the mass of a 12C isotope atom.
Molecular geometry
- see main article molecular geometry
Molecules have fixed equilibrium geometries—bond lengths and angles—. A pure substance is composed of molecules with the same 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
- see main article spectroscopy
Molecular spectroscopy is the study of the response (spectrum) of a molecule to a signal characterized by its tunable frequency (or, according to Planck formula, its energy). This signal is usually an electromagnetic wave or a beam of electrons, but new molecular spectroscopies are under development like the positron spectroscopy. The molecular response can be the absorption of the signal (absorption spectroscopy), the emission of another signal (emission spectroscopy), its fragmentation or a change of its chemical nature.
Spectroscopy is recognized as the most powerful tool in the investigation of the microscopic properties of molecules, and, in particular, their energy levels. Nowadays, in order to extract the maximum microscopic information from the experimental results, spectroscopical studies are very often coupled with computational chemical investigations. The theoretical background of spectroscopy is the scattering theory.
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