There are two steps in Horiuti-Polanyi mechanism. The first step is the adsorption (including physisorption and chemisorption) of the hydrogen molecule, forming atomic hydrogen at the surface of the metal. The second step is the addition of unsaturated bond with atomic hydrogen. Throughout the adsorption process, the energy of the reacting system varies with the distance from the metal surface to the H2 molecule (Figure ). Due to the asymmetry of the force applied to the atoms on the metal surface, a force field is formed to adsorb the liquid and gaseous molecules. When the H2 molecule is approaching the metal surface, dispersion forces are applied to the H2 molecule and the energy of the reaction system is reduced during the physisorption process getting into the precursor state for the chemisorption. When the distance gets further closer, the H2 molecule begins to be chemisorbed and dissociated. During the chemisorption process, electrons in the s orbital of the H2 molecule are donated to the lowest unoccupied d orbital of metal and the s* orbital of the H2 molecule also accepts back-donated electrons from the occupied d orbital of metal (Figure ). Activation energy is required for the chemisorption. In some cases, the potential energy of the transition state can be higher than the potential energy of the distant particle.