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Polarizability is the ability for a molecule to be polarized. It is a property of matter. Polarizabilities determine the dynamical response of a bound system to external fields, and provide insight into a molecule's internal structure.
Electric polarizability is the relative tendency of a charge distribution, like the electron cloud of an atom or molecule, to be distorted from its normal shape by an external electric field, which is applied typically by inserting the molecule in a charged parallel-plate capacitor, but may also be caused by the presence of a nearby ion or dipole.
The electronic polarizability is defined as the ratio of the induced dipole moment of an atom to the electric field that produces this dipole moment.
Polarizability has the SI units of C·m2·V−1 = A2·s4·kg−1 while its cgs unit is cm^3. Usually it is expressed in cgs units as a so-called polarizability volume, instead of cm^3 it is sometimes expressed in Å3 = 10−24 cm3. One can convert from SI units to cgs units as follows:
where is the vacuum permittivity.
Note that the polarizability as defined above is a scalar quantity. This implies that the applied electric fields can only produce polarization components parallel to the field. For example, an electric field in the -direction can only produce an component in . However, it can happen that an electric field in the -direction, produces a or component in the vector . In this case is described as a tensor of rank 2, which is represented with respect to a given system of axes (frame of reference) by a matrix.
Generally, polarizability increases as volume occupied by electrons increases. In atoms, this occurs because larger atoms have more loosely held electrons in contrast to smaller atoms with tightly bound electrons. On rows of the periodic table, polarizability therefore increases from right to left. Polarizability increases down on columns of the periodic table. Likewise, larger molecules are generally more polarizable than smaller ones.
Though water is a very polar molecule, alkanes and other hydrophobic molecules are more polarizable. Alkanes are the most polarizable molecules. Although alkenes and arenes are expected to have larger polarizability than alkanes because of their higher reactivity compared to alkanes, alkanes are in fact more polarizable. This results because of alkene's and arene's more electronegative sp2 carbons to the alkane's less electronegative sp3 carbons.
It is important to note that ground state electron configuration models are often inadequate in studying the polarizability of bonds because dramatic changes in molecular structure occur in a reaction.
Magnetic polarizability defined by spin interactions of nucleons is an important parameter of deuterons and hadrons. In particular, measurement of tensor polarizabilities of nucleons yields important information about spin-dependent nuclear forces.
The method of spin amplitudes uses quantum mechanics formalism to more easily describe spin dynamics. Vector and tensor polarization of particle/nuclei with spin S ≥ 1 are specified by the unit polarization vector and the polarization tensor P`. Additional tensors composed of products of three or more spin matrices are needed only for the exhaustive description of polarization of particles/nuclei with spin S ≥ 3⁄2 .
- Polarization density
- MOSCED, an estimation method for activity coefficients; uses polarizability as parameter
- L. Zhou; F. X. Lee, W. Wilcox, J. Christensen (2002). "Magnetic polarizability of hadrons from lattice QCD" (PDF). European Organization for Nuclear Research (CERN). Retrieved 25 May 2010.
- Anslyn, Eric; Dougherty, Dennis (2006). Modern Physical Organic Chemistry. University Science. ISBN 978-1-891389-31-3.
- Schwerdtfeger, Peter (2006). G. Maroulis, ed. Atomic Static Dipole Polarizabilities. IOS Press. Unknown parameter
- A. J. Silenko (18 Nov 2008). "Manifestation of tensor magnetic polarizability of the deuteron in storage ring experiments". Springer Berlin / Heidelberg. doi:10.1140/epjst/e2008-00776-9. Retrieved 25 May 2010.
- The theory of the electromagnetic field by David M. Cook
- Consistent Calculation of the Nucleon Electromagnetic Polarizabilities in Chiral Perturbation Theory Beyond Next-to-Leading Order
- Hadron polarizabilities and magnetic moments with background field methods (PDF)