Molecule

In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces. Generally, a molecule is considered the smallest [[list of particles#Molecules|particle]] of a pure chemical substance that still retains its composition and chemical properties. A molecule may be composed of the same element or atoms of two or more elements joined in a fixed ratio. In the molecular sciences, a molecule is a sufficiently stable, electrically neutral entity composed of two or more atoms.

Molecules can be monatomic, diatomic or polyatomic. The concept of "monatomic molecule", i.e. a single-atom as found in noble gases, is used almost exclusively in the kinetic theory of gases.

Exactly two atoms make up diatomic molecules. Examples include O₂, N₂, Cl₂. When diatomic The elements of Group 7A in the periodic table, the halogens, exist as diatomic elements. Diatomic molecules of two different elements are polar molecules. This is due to the difference in electronegativity - the atom with the higher electronegativity pulls the more negatively charged electrons closer to it. The higher concentration of electrons towards one side of the molecule creates a negative charge on that side and a positive charge on the other end. On the other hand, diatomic molecules of like elements have the same electronegativity and are thus nonpolar.

Polyatomic molecules contain two or more atoms. H₂O and NH₃ are some examples.

2D]] (right) representations of the terpenoid molecule atisane.

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. The actual molecule may not have the exact number of atoms stated in the empirical formula but may have a multiple of them. For example the molecule acetylene has molecular formula C₂H₂, but it has the empirical formula of CH - its simplest integer ratio of elements. The subscripts of the molecular formula, or chemical formula, are reduced to the smallest whole numbers to derive the empirical formula. In some cases, the empirical formula is the same as the molecular formula. Examples include water (H₂O) and methane (CH₄).

Chemical formula

See main article chemical formula

The chemical formula reflects the exact number of atoms that compose a molecule.

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 several dozen Å.

Molecular mass

See main article molecular mass

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 Models

Molecular models are useful in visualizing how molecules look like three-dimensionally. This is achieved in two types of models: ball-and-stick models and space-filling models. In the former method, atoms are represented by balls; each type of atom with a unique color. The sizes of the balls are the same for all atoms except for the Hydrogen atom which is smaller. Chemical bonds between the atoms are represented by sticks which are bent to show the bond angles in the actual molecules. However, the sticks are often show chemical bonds with exagerrated lengths. Ball-and-stick model kits use plastic and wooden balls and sticks. In space-filling models, truncated balls are used to represent atoms; no chemical bonds are visible. The balls are attached to each other with snap fasteners. These balls are proportional to the size of the atoms.

Molecular geometry

See main article 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 physics

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 bonding|Van der Waals complex]]es, or systems of colliding atoms as in [[Bose-Einstein condensate]]s. 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) three-dimensionally (such as in diamond).

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.

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 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.

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

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