Molecules and Compounds

Generally, a molecule is the smallest particle of a pure chemical substance that still retains its composition and chemical properties.[1] In chemistry and molecular sciences, a molecule is a sufficiently stable, electrically neutral entity composed of two or more atoms.[2] The concept of monatomic molecule (single-atom, as in noble gases) is used almost exclusively in the kinetic theory of gases. [3] [4] [5]

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. 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. 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).

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.

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 positvists such as Mach. 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.

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 0.15 nanometres (1.5 Å). But the size of its electron cloud is difficult to define precisely. Under standard 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 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.

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. For network solids, the term formula unit is used in stoichiometric calculations.

Molecular geometry

See main article molecular geometry

Molecules have fixed equilibrium geometries—bond lengths and angles— about which they continously 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

Main article: Spectroscopy

Molecular spectroscopy deals with the spectrum of a molecule arising due to a signal of known energy (or frequency, according to Planck's formula). [[Scattering theory provides the theoretical background for spectroscopy.

This signal used in spectroscopy is usually an electromagnetic wave or beam of electrons; new molecular spectroscopies, such as the positron spectroscopy, however, are under development. The molecular response can consist of signal absorption (absorption spectroscopy), the emission of another signal (emission spectroscopy), fragmentation, and chemical changes.

Spectroscopy is recognized as a powerful tool in investigating the microscopic properties of molecules, in particular their energy levels. At present (as of 2005), 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 orbital
• Nonpolar molecule
• Polar molecule

Related lists

• For a list of molecules see the List of compounds
• List of molecules in interstellar space

• A compound is an area of land that is surrounded by fences, walls, or barbed wire and is used for a particular purpose, especially an area containing buildings and where the entry and exit of people is controlled. (For example, the sprawling estate located in southern Maine which houses the Bush Family residence is known as the Bush Compound.)
• In chemistry, a compound (chemical compound) is a chemical combination of two or more elements. See list of compounds.
• In linguistic morphology, a compound is a word that consists of more than one radical element, for example summertime. See also English compound. This is not to be confused with a complex phrase.
• In botany, compound is a quality of leaves. Leaves that are compound are in an array of small, symmetrically-arranged leaflets on each stem. In contrast, a plant with simple leaves has one leaf per stem.
• In economics, Compound Annual Growth Rate (CAGR) is the average annual growth rate of a value over a given number of years.
• In finance, compound Interest is interest that is paid on both the principal and interest earned.
• In music, a compound is an attribute of an interval or time signature. An interval that is compound is an interval which exceeds or is wider than one octave, whereas a simple interval lies within one octave. A time signature that is compound is one based on groups of three notes (most often quavers or eighth notes) whereas a simple time signature is one based on groups of two notes (most often crotchets or quarter notes).
• In steam locomotive engineering, a compound locomotive has steam that is passed that has already passed through one cylinder is then passed through another; i.e. the cylinders are in series as opposed to the normal arrangement of a simple locomotive in which the cylinders are in parallel.
• In the art world, Compound is an international exhibition space which was part of the Portland millennial art renaissance see Compound gallery for more information.
• In geometry, a polyhedral compound is a polyhedron which is itself composed of several other polyhedra sharing a common centre, the three-dimensional analogs of polygonal compounds.