  The concept of basic chemical models is introduced, which is new from the
standpoint of the physics of nonideal atomic plasma. This concept is based on
the requirement of full conformity of the expression for free energy in the
chemical model of plasma to exact asymptotic expansions obtained in the grand
canonical ensemble within the physical model of plasma. The thermodynamic
functions and equations of state and ionization equilibrium are obtained for
three basic chemical models differing from one another by the choice of the
atomic partition function. Comparison is made with the experimental results for
nonideal plasma of cesium and inert gases. It is demonstrated that the best fit
to experiment is shown by the results obtained using a basic chemical model
with atomic partition function in the nearest neighbor approximation with
classical determination of the size of excited atom.
