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 substance that retains its chemical composition and properties.[2] A molecule may be composed of atoms of a single element or two or more elements joined in a fixed ratio.[3] Each molecule is a relatively stable, electrically neutral entity.[4] The atoms within each molecule are joined together by shared pairs of electrons, forming chemical bonds.
If a molecule consists of two atoms, it is described as diatomic, and if it contains more than two atoms, it is called polyatomic. Examples of diatomic molecules are the common, gaseous forms of oxygen (O2), nitrogen (N2), and hydrogen (H2). Other familiar diatomic molecules are the halogens—such as fluorine (F2), chlorine (Cl2), and bromine (Br2)—which lie in group 17 of the periodic table. Familiar examples of polyatomic molecules are water (H2O), ammonia (NH3), and carbon dioxide (CO2).
The noble gases (such as helium, neon, and argon) exist as single atoms. In the kinetic theory of gases, these atoms are referred to as "monatomic molecules."[5]
- If a diatomic molecule consists of atoms of two different elements, there is an uneven distribution of electrical charge within the molecule, making it electrically polar. This phenomenon is attributed to a property called electronegativity. In the bond between the two atoms, the one with higher electronegativity pulls negatively charged electrons closer to itself, creating a negative charge around itself and leaving a positive charge on the other atom. By contrast, if both atoms are of the same element, they have the same electronegativity and the molecule is nonpolar.
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 (or molecular formula) of a compound reflects the exact number and types of atoms that make up each molecule of the compound, using the chemical symbols for the elements. The subscript after the symbol of an element indicates the number of atoms of that element in the molecule. For example, the chemical formula for water is H2O, indicating that each molecule contains two hydrogen atoms and one oxygen atom.
The chemical formula of a substance can be used to calculate the molecular mass—that is, the mass of each molecule of the substance. By convention, one unit of molecular mass is equal to 1/12 of the mass of a carbon-12 isotope.
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 chemistry and physics
The science of molecules is called molecular chemistry or molecular physics, depending on the focus. In theory, molecular chemistry deals with the laws governing interactions between molecules, resulting in the formation and breakage of chemical bonds; and molecular physics deals with the laws governing the structures and physical properties of molecules. In practice, however, this distinction is vague.
In the molecular sciences, a molecule consists of a stable system of atoms bound together by shared pairs of electrons to form what are called "covalent bonds." The term unstable molecule is used for a very reactive, short-lived species, such as a "radical" or "molecular ion."
A peculiar use of the term molecular is as a synonym to covalent, because the bonds between the atoms in each molecule are covalent bonds. Unlike molecular covalent compounds, ionic compounds do not yield well-defined "smallest particles" that would be consistent with the above definition of a molecule. In addition, no typical "smallest particle" can be defined for covalent crystals, which consist of repeating "unit cells" that extend indefinitely. For instance, in graphite, the unit cells extend to form planar sheets, and in diamond, the unit cells extend in three dimensions.
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
The concept of molecules was first introduced in 1811 by Amadeo Avogadro and was accepted by many chemists based on Dalton's laws of Definite and Multiple Proportions (1803-1808). On the other hand, most members of the physics community, with some notable exceptions (Ludwig Boltzmann, James Clerk Maxwell, and Willard Gibbs), thought of molecules as no more than convenient mathematical constructs, until the work of Jean Perrin in 1911. Philosophers such as Ernst Mach in the school of logical positivism also strenuously resisted the idea that molecules really exist.
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 the technique of 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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