On the molecular orbital theory, a molecule is defined as a group of nuclei, each of which has an electron configuration that is very similar to that of a free atom in an external field. In the present paper, therefore, each atom in a solid-state material is considered to have a comparable electron configuration to its free atom in an external field. In this context, we hypothesized an atomic hydrogen link on the surface of a catalytic atom via a special half-filled orbital interaction during the catalytic reaction process. Namely, a HFSO on a hydrogen atom interacts with a HFSO or HFDO on the catalytic active atom. Based on this proposal, we can obtain an illuminating explanation of the difference of various catalysts for HER. As Zn, Cd, and Hg metal catalysts lack accessible HFSO or HFDO on their atom surfaces, it is difficult to link atomic hydrogen on the atom surfaces and impossible to demonstrate active catalytic behaviour for HER. Since a hydrogen molecule requires two atomic hydrogens, the elements with just one HFSO exposed are unable to link two atomic hydrogens on an atom surface via orbital interaction and an active performance on their metal catalysts should be hard to meet. For instance, gold with only one HFSO accessible, although exhibiting a medium electronegativity (2.4), still delivered a lower catalytic activity for HER experimentally (i0,H > 6.0 A cm -2 ). The elements containing HFSO and HFDOs, or simply at least two HFDOs, however, have the potential to interact with two atomic hydrogens via half-filled orbitals. Such as Pt, Rh, Ru, Ir, Os and Re, these elements with medium electronegativity achieved significant catalytic behaviour for HER. Meanwhile, Co and Ni with at least two HFDOs, although having a lower electronegativity (1.9), still experimentally exhibited a reduced exchange current density in contrast with one HFSO's elements (Au (2.4), Ag (1.9), Cu (1.9) and Hg (1.9)). This indicates that two available half-filled orbitals on a catalytically active atom are necessary for HER catalysts.