  We have used the generalized two-atom ideal gas model in Tsallis statistics
for the statistical description of a real gas. By comparing the heat capacity
with the experimental results for the two-atom molecule gases such as N2, O2
and CO, we find that these gases appear extensive at normal temperature, but
they may be nonextensive at the lower temperature. Furthermore, we study the
heat capacity of the generalized many-atom gas model. We conclude that, for the
many-atom gas with a high degree of freedom, a weak nonextensivity of 1-q<0 can
lead to the instability.
