CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 419
which is analogous to the Debye equation (12.9). In (12.14),αmis themagnetizabilityof the
molecules andmmis their permanentmagnetic moment. The magnetic moment of molecules
is a consequence of the quantum behaviour of electrons in the electric field of atomic nuclei. It is
strong for molecules with unpaired electrons (see12.2.6). Magnetizability of molecules reflects
their ability to generate an induced magnetic moment by the influence of the surrounding
magnetic field. If we substitute these quantities for the given substance into relation (12.14),
we obtain the temperature dependence of molar susceptibility
χM=A+B
T
, (12.15)
which was discovered empirically as theCurie lawearlier.
Permanent moments strengthen the magnetic field in a substance (paramagnetic behaviour)
while induced moments usually weaken it (diamagnetic behaviour). Consequently, the diamag-
netic or paramagnetic behaviour [see12.1.6] of a substance is determined by the relative
magnitudes of both terms in equation (12.15).
12.1.9 System interaction with light
The last type of a substance interaction with the external field to be discussed is the absorption
of light passing through the substance. In order to be able to measure the light attenuation, we
first have to define itsintensityIas the amount of energy transferred by light in one second
across an area of 1 m^2 , perpendicularly to the direction of the radiation.
Let us observe the behaviour of light passing through a solution of the absorbing substance
in a solvent which does not absorb light in the given wavelength range. For a solution of
the concentrationc(see1.6.4), with the light passing through a layer of the thickness, the Lambert-Beerlaw applies I′= 10−ε cI , (12.16)
whereI′andIare the light intensities after and before passing through the solution, andεis the
molar absorption coefficient. Its value depends on temperature, the type of the dissolved
substance and the solvent, and on the wavelength of the passing light (12.42). In addition to
the absorption coefficient, also used is the quantity
A=ε c ` (12.17)