For a pure
monoatomic gas at low density, which has no rotational or vibrational degrees of freedom, thermal conductivity was calculated through a rigorous analysis by Brokaw [13]:
As radiative heat transfer is no longer efficient in the solar convective zone where heat transfer occurs by convection, we assumed the adiabatic gradient of a
monoatomic gas for the solar photosphere.
Here [S.sup.0] (comp) is the absolute standard entropy and [summation] [S.sup.0] (at.gas.) is the sum of the absolute entropies of elemental constituents of the considered compound in a monoatomic gas state.
The difference between the entropy of a substance (with the same formula) in a crystal state and the entropy of a monoatomic gas mixture is nearly two orders smaller than the value of the atomization entropy and therefore does not affect the magnitude of the latter.