Suppose the charge of a proton and an electron differ slightly. One of them is -e, the other is (e+Δe). If the net of electrostatic force and gravitational force between two hydrogen atoms placed at a distance d (much greater than atomic size) apart is zero, then Δe is of the order of Given mass of hydrogen mh=1.67×10-27 kg

A hydrogen atom consists of an electron and a proton.∴ Charge on one hydrogen atom =qe+qp=-e+(e+Δe)=Δe Since a hydrogen atom carry a net charge Δe , ∴ Electrostatic force, Fe=14πεo(Δe)2d2            . . . .(i)will act between two hydrogen atoms. The gravitational force between two hydrogen atoms is given asFg=Gmhmhd2                      . . . .(ii)Since, the net force on the system is zero, Fe=Fg Using eqns.(i) and (ii), we get(Δe)24πεod2=Gmh2d2(Δe)2=4πεoGmh2=6.67×10-11×1.67×10-272/9×109Δe≈10-37C