Suppose the charge of a proton and an electron differ slightly. One of them is –e, the other is (e + $\Delta$e). 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 $\Delta$e is of the order of [Given mass of hydrogen mh = 1.67 × $10^{–27}$ kg]