In a Rutherford scattering experiment when a projectile of charge ${\mathrm{Z}}_1$ and mass ${\mathrm{M}}_1$ approaches a fixed target nucleus of charge ${\mathrm{Z}}_2$ and mass ${\mathrm{M}}_2$, the distance of closest approach is ${\mathrm{r}}_{\mathrm{o}}$. The energy of the projectile is

A particle of mass ${\mathrm{M}}_1$ and charge ${\mathrm{Z}}_1$ possesses initial velocity u, when it is at a large distance from the nucleus of an atom having atomic number ${\mathrm{Z}}_2$. At the distance of closest approach, the kinetic energy of particle is completely converted to potential energy.
Mathematically,

$\frac{1}{2}{\mathrm{M}}_1{\mathrm{u}}^2=\frac{1}{4{\mathrm{\pi \epsilon }}_{\mathrm{o}}}\frac{{\mathrm{Z}}_1{\mathrm{Z}}_2}{{\mathrm{r}}_{\mathrm{o}}}$

So the energy of the particle is directly proportional to ${\mathrm{Z}}_1$${\mathrm{Z}}_2$