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
= Definition = 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 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 cm3. Usually it is expressed in cgs
units as a so-called polarizability volume, instead of cm3 it is sometimes
expressed in Å3 = 10−24 cm3. One can convert from SI units to cgs units as
follows: ≃ 8.988×1015 ×
where , the vacuum permittivity, is ~8.854 × 10−12. If the polarizability
volume is denoted the relation can also be expressed generally as .
The polarizability of individual particles is related to the average
electric susceptibility of the medium by the Clausius-Mossotti relation.
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 by a matrix. = Tendencies =
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
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 . See also
Polarization density MOSCED, an estimation method for
activity coefficients; uses polarizability as parameter
References External links
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