F = gravitational force between electron and proton = 1.26 x 10⁻⁴⁷ N
m = mass of proton = 1.67 x 10⁻²⁷ kg
M = mass of the electron = 9.11 x 10⁻³¹ kg
r = distance between electron and proton = ?
G = universal gravitational constant = 6.67 x 10⁻¹¹
gravitational force between electron and proton is given as
F = G M m/r²
inserting the values
(1.26 x 10⁻⁴⁷) = (6.67 x 10⁻¹¹) (1.67 x 10⁻²⁷) (9.11 x 10⁻³¹)/r²
r = 8.97 x 10⁻¹¹ m