Molecule
In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces.[1] Generally, a molecule is considered the smallest particle of a pure chemical substance that still retains its composition and chemical properties.[2] A molecule may be composed of the same element or atoms of two or more elements joined in a fixed ratio.[3] In the molecular sciences, a molecule is a sufficiently stable, electrically neutral entity composed of two or more atoms.[4]
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.[5] Exactly two atoms make up diatomic molecules. Examples include O2, N2, Cl2. The elements of Group 7A in the periodic table, the halogens, exist as diatomic molecules. Diatomic molecules of two different elements are polar molecules. This is due to the difference in electronegativity - the atom with the higher electronegativity pulls 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. The atoms of molecules are joined by shared pairs of electrons and form chemical bonds. See main article chemical bond
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 empirical formula of CH - its simplest integer ratio of elements - but exists as C2H2, which is its molecular formula. 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. The subscript after each element indicates the number of atoms of that element is present. For example, H2O shows there are two Hydrogen atoms and one Oxygen atom.
The chemical formula can be used to calculate the molecular mass. Molecular mass is the weight of one molecule of a substance. It is expressed in conventional units equal to 1/12 from the mass of a 12 Carbon isotope atom.
Molecular geometry
- See main article molecular geometry
Molecular geometry or molecular structure is the three dimensional arrangement of the atoms that constitute a molecule. Molecules have fixed equilibrium geometries—bond lengths and angles— about which they continuously oscillate through vibrational and rotational motions. The geometry can be inferred through spectroscopic studies of the compound, or predicted using the Valence Bond Theory. The molecular geometry depends on several factors: how the atoms bond together, which is predicted using the Lewis Structure based on its chemical formula; the type of chemical bond; and the orbital hybridisation, or mixing of atomic orbitals. The molecule's properties, particularly its reactivity, is greatly determined by its molecular geometry.
Isomers share a chemical formula but normally have very different properties because of their different molecular geometries. For example, n-Butane and Isobutane are structural isomers. They contain the same ratio of Carbon and Hydrogen atoms. However, n-Butane (n means normal) is a straight chain while Isobutane is a branched chain. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.
An allotrope not only varies in structure but is one of two or more distinct forms of an element.[6] The element carbon has two allotropes: diamond and graphite. Both are made of carbon atoms.
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.[7]
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 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.
ReferencesISBN links support NWE through referral fees
- ↑ Chang, Raymond (1998). Chemistry, 6th Ed.. New York: McGraw Hill. ISBN 0071152210.
- ↑ Molecule Definition
- ↑ Chang, Raymond (1998). Chemistry, 6th Ed.. New York: McGraw Hill. ISBN 0071152210.
- ↑ IUPAC Defintion of Molecule
- ↑ [1] [2] [3]
- ↑ Chang, Raymond (1998). Chemistry, 6th Ed.. New York: McGraw Hill. ISBN 0071152210.
- ↑ Chang, Raymond (1998). Chemistry, 6th Ed.. New York: McGraw Hill. ISBN 0071152210.
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
| Particles in physics - composite particles |
| Hadrons: Baryons (list) | Mesons (list)
Baryons: Nucleons | Hyperons | Exotic baryons | Pentaquarks
|
| Particles in physics - composite particles |
| Hadrons: Baryons (list) | Mesons (list)
Baryons: Nucleons | Hyperons | Exotic baryons | Pentaquarks
|
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