Abstract In the present work, the H-H bond enthalpy in hydrogen molecule explained/calculated by using the Bohr atom approach, as well as Slater rules and taking into account relativistic effects. It is shown that in the hydrogen molecule, the electron moves at 0.7155 c, with a relativistic mass of 13.04 x 10-31 kg and that the most part of the H-H bond enthalpy are related with the change in potential energy in individual H atoms.