= Solution
<Thermal bremsstrahlung> is radiation from the acceleration of <electrons> in the <Coulomb force> of ions. At fixed <temperature>, the encounter rate per <electron> is proportional to ion <number density> $n_i$, and the number of radiating <electrons> per unit <volume> is $n_e$. Therefore $j_\nu\propto n_en_i$ in a dilute, <optically thin> <plasma>. This is a binary-encounter rate, not a one-particle emission process.
For a fixed encounter geometry, the ionic <electric field> and hence the <electron> acceleration are proportional to ionic charge $Ze$. The <Larmor formula> is quadratic in acceleration, giving $j_\nu\propto Z^2$ at fixed species <number densities>. The dimensionless <Gaunt factor> $g(\nu,T)$ corrects the simple classical result for the detailed quantum scattering and radiation calculation, after averaging over the thermal <electron> distribution. It depends on <frequency> and <temperature> and, in a more complete notation, ionic charge.
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