Mean molecular weight per ion
= Mean molecular weight per ion
{title2=$\mu_i^{-1}=\sum_jX_j/A_j$}
= Ion mean molecular weight
{synonym}
The <mean molecular weight> per ion counts nuclei rather than nuclei plus free electrons: $\rho/(\mu_i m_u)$ is the ion number density. For species with mass fractions $X_j$ and mass numbers $A_j$, $\mu_i^{-1}=\sum_jX_j/A_j$. It is useful when ions provide a thermal <ideal gas> pressure but <electron degeneracy pressure> supplies a separate contribution.