12. molecules of hydrogen escape from earth, but molecules of oxygen and nitrogen are held to the surface. why?

            Because Hydrogen gas is lighter than Oxygen and Nitrogen Gases.

                        Those molecules will escape from earth which can overcome the earth's gravitational pull. Therefore, molecules small in size, lighter in weight and with high velocity can escape from earth's surface.

The masses of given molecules are as follow,

                      M.Mass of Hydrogen  =  H₂  =  2.02 g.mol⁻¹

                      M.Mass of Nitrogen  =  N₂  =  28.02 g.mol⁻¹

                      M.Mass of Oxygen  =  O₂  =  32.00 g.mol⁻¹

Hence, Hydrogen has far less molecular mass than oOxygen and Nitrogen gases. Also, according to Graham's law of Effusion, Hydeogen will move faster than the remaining two.