The gravitational force between a H-atom and another particle of mass m will be given by Newton's law  $\mathrm{F}=\mathrm{G}\frac{\mathrm{M}\cdot \mathrm{m}}{{\mathrm{r}}^2}$ :where r is in km and 

During formation of H-atom, some mass of nucleons convert into energy by the equation
 E=mc²
This energy is used to bind the nucleons along with nucleus. So mass of atom becomes slightly less than sum of actual masses of nucleons and electrons.
Actual mass of H-atom 
$={\mathrm{M}}_{\mathrm{p}}+{\mathrm{M}}_{\mathrm{e}}-\frac{\mathrm{B}\cdot \mathrm{E}}{{\mathrm{c}}^2}\left(\frac{\mathrm{B}}{{\mathrm{c}}^2}= \mathrm{Bindingenergy} \right)$
where, the binding energy of H atoms is 13.6 eV per atom.