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In the CGS system of units the Clausius–Mossotti relation is typically rewritten to show the molecular polarizability volume ′ = which has units of volume [m 3]. [2] Confusion may arise from the practice of using the shorter name "molecular polarizability" for both α {\displaystyle \alpha } and α ′ {\displaystyle \alpha '} within ...
Polarizability increases down on columns of the periodic table. [9] 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.
In water, this is where the absorptive index starts to drop sharply, and the minimum of the imaginary permittivity is at the frequency of blue light (optical regime). At high frequencies (such as UV and above), molecules cannot relax, and the energy is purely absorbed by atoms, exciting electron energy levels.
Molar refractivity, [1] [2], is a measure of the total polarizability of a mole of a substance. For a perfect dielectric which is made of one type of molecule, the molar refractivity is proportional to the polarizability of a single molecule of the substance. For real materials, intermolecular interactions (the effect of the induced dipole ...
Yet another definition exists [5] where and are expressed in the cgs system and is still defined as =. Using the cgs units gives α {\displaystyle \alpha } the dimension of a volume, as in the first definition, but with a value that is 4 π {\displaystyle 4\pi } lower.
For instance, dichloromethane has a value of ε r of 9.08 (20 °C) and is rather poorly soluble in water (13 g/L or 9.8 mL/L at 20 °C); at the same time, tetrahydrofuran has its ε r = 7.52 at 22 °C, but it is completely miscible with water.
One difference between the Gaussian and SI systems is in the factor 4π in various formulas that relate the quantities that they define. With SI electromagnetic units, called rationalized, [3] [4] Maxwell's equations have no explicit factors of 4π in the formulae, whereas the inverse-square force laws – Coulomb's law and the Biot–Savart law – do have a factor of 4π attached to the r 2.
The respective CGS susceptibilities are multiplied by 4 π to give the corresponding ISQ quantities (often referred to as SI quantities) with the same units: [7] = = = For example, the CGS volume magnetic susceptibility of water at 20 °C is 7.19 × 10 −7 , which is 9.04 × 10 −6 using the SI convention, both quantities being dimensionless.