Physical chemistry is the part of chemistry that seeks to account for the properties and transformations of matter in terms of concepts, principles, and laws drawn from physics. This glossary is a compilation of definitions, descriptions, formulae, and illustrations of concepts that are encountered throughout the subject. This section describes the concepts that begin with the letter J; where appropriate, the entries also describe subsidiary but related concepts. Refer to the Directory for a full list of all the concepts treated.
A Jablonski diagram is a simple schematic portrayal of the relative energies of the electronic states of molecules and atoms and the vibrational levels of molecules (Figure J.1). Each stack of levels is portrayed as a ladder but the relative horizontal positions of the stacks bear no relation to the actual location of the molecular potential energy curve.
In a
jj-coupling scheme, first the spin and orbital angular momenta of each electron are coupled, using the Clebsch–Gordan series
Then the total angular momentum of each electron is coupled into an overall coupled angular momentum by using the series again in the form
See
Figure J.2. It is most relevant to atoms in which the spin–orbit coupling of an atom is so strong that Russell–Saunders coupling is inappropriate, which is for atoms of high atomic number.
The
Joule–
Thomson effect is the change in temperature that occurs when a gas expands isenthalpically as it passes through a throttle from high to low pressure (
Figure J.3). The isenthalpic (constant enthalpy) character of the expansion is ensured by arranging for it to be adiabatic. The magnitude of the effect is expressed by the
Joule–
Thomson coefficient, μ, which is defined as
The temperature falls on expansion if
μ > 0 and rises if
μ < 0. The change in the sign of
μ occurs at the
inversion temperature of the gas.
The Joule–Thomson coefficient is measured indirectly by determining the
isothermal Joule–
Thomson coefficient,
μ_{T}, where
The two coefficients are related by
For a perfect gas
μ = 0 and
μ_{T} = 0. For a real gas
μ and
μ_{T} are nonzero even as
p→0 and the limiting value of
μ_{T} can be expressed in terms of the second virial coefficient,
B, as