Polarizability

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.

The polarizability ${\displaystyle \alpha }$ in isotropic media is defined as the ratio of the induced dipole moment ${\displaystyle {\boldsymbol {p}}}$ of an atom to the electric field ${\displaystyle {\boldsymbol {E}}}$ that produces this dipole moment.[3]

${\displaystyle \alpha ={\frac {||{\boldsymbol {p}}||}{||{\boldsymbol {E}}||}}}$

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, sometimes expressed in ų = 10⁻²⁴ cm³. One can convert from SI units (${\displaystyle \alpha }$) to cgs units (${\displaystyle \alpha '}$) as follows:

${\displaystyle \alpha '(\mathrm {cm} ^{3})={\frac {10^{6}}{4\pi \varepsilon _{0}}}\alpha (\mathrm {C{\cdot }m^{2}{\cdot }V^{-1}} )={\frac {10^{6}}{4\pi \varepsilon _{0}}}\alpha (\mathrm {F{\cdot }m^{2}} )}$ ≃ 8.988×10¹⁵ × ${\displaystyle \alpha (\mathrm {F{\cdot }m^{2}} )}$

where ${\displaystyle \varepsilon _{0}}$, the vacuum permittivity, is ~8.854 × 10⁻¹² (F/m). If the polarizability volume in cgs units is denoted ${\displaystyle \alpha '}$ the relation can be expressed generally[4] (in SI) as ${\displaystyle \alpha =4\pi \varepsilon _{0}\alpha '}$.

The polarizability of individual particles is related to the average electric susceptibility of the medium by the Clausius–Mossotti relation:

${\displaystyle R={\displaystyle \left({\frac {4\pi }{3}}\right)N_{\text{A}}\alpha _{c}=\left({\frac {M}{p}}\right)\left({\frac {\varepsilon _{\mathrm {r} }-1}{\varepsilon _{\mathrm {r} }+2}}\right)}}$

where R = molar refractivity , ${\displaystyle N_{\text{A}}}$ = Avogadro constant, ${\displaystyle \alpha _{c}}$ = electronic polarizability, p = density of molecules, M = molar mass, and ${\displaystyle \varepsilon _{r}=\epsilon /\epsilon _{0}}$ is the material's relative permittivity or dielectric constant (or in optics, the square of the refractive index).

Polarizability for anisotropic or non-spherical media cannot in general be represented as a scalar quantity. Defining ${\displaystyle \alpha }$ as a scalar implies both that applied electric fields can only induce polarization components parallel to the field and that the ${\displaystyle x,y}$ and ${\displaystyle z}$ directions respond in the same way to the applied electric field. For example, an electric field in the ${\displaystyle x}$-direction can only produce an ${\displaystyle x}$ component in ${\displaystyle {\boldsymbol {p}}}$ and if that same electric field were applied in the ${\displaystyle y}$-direction the induced polarization would be the same in magnitude but appear in the ${\displaystyle y}$ component of ${\displaystyle {\boldsymbol {p}}}$. Many crystalline materials have directions that are easier to polarize than others and some even become polarized in directions perpendicular to the applied electric field, and the same thing happens with non-spherical bodies. Some molecules and materials with this sort of anisotropy are optically active, or exhibit linear birefringence of light.

To describe anisotropic media a polarizability rank two tensor or ${\displaystyle 3\times 3}$ matrix ${\displaystyle \alpha }$ is defined,

${\displaystyle \mathbb {\alpha } ={\begin{bmatrix}\alpha _{xx}&\alpha _{xy}&\alpha _{xz}\\\alpha _{yx}&\alpha _{yy}&\alpha _{yz}\\\alpha _{zx}&\alpha _{zy}&\alpha _{zz}\\\end{bmatrix}}}$

so that:

${\displaystyle {\boldsymbol {p}}=\mathbb {\alpha } {\boldsymbol {E}}}$

The elements describing the response parallel to the applied electric field are those along the diagonal. A large value of ${\displaystyle \alpha _{yx}}$ here means that an electric-field applied in the ${\displaystyle x}$-direction would strongly polarize the material in the ${\displaystyle y}$-direction. Explicit expressions for ${\displaystyle \alpha }$ have been given for homogeneous anisotropic ellipsoidal bodies.[5][6]

The matrix above can be used with the molar refractivity equation and other data to produce density data for crystallography. Each polarizability measurement along with the refractive index associated with its direction will yield a direction specific density that can be used to develop an accurate three dimensional assessment of molecular stacking in the crystal. This relationship was first observed by Linus Pauling.[1]

Generally, polarizability increases as the volume occupied by electrons increases.[7] In atoms, this occurs because larger atoms have more loosely held electrons in contrast to smaller atoms with tightly bound electrons.[7][8] On rows of the periodic table, polarizability therefore decreases from left to right.[7] Polarizability increases down on columns of the periodic table.[7] Likewise, larger molecules are generally more polarizable than smaller ones.

Water is a very polar molecule, but alkanes and other hydrophobic molecules are more polarizable. Water with its permanent dipole is less likely to change shape due to an external electric field. Alkanes are the most polarizable molecules.[7] 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.[7] This results because of alkene's and arene's more electronegative sp² carbons to the alkane's less electronegative sp³ carbons.[7]

Ground state electron configuration models are often inadequate in studying the polarizability of bonds because dramatic changes in molecular structure occur in a reaction.[7]