Table presents the selection of previous experimental values of exchange current density for the electrolytic evolution of hydrogen on pure metal catalysts. It is obvious that reduced exchange current density for catalytic HER was achieved by elements with medium electronegativity (EN) compared with elements that had lower electronegativity. This suggests that the catalytic activity of the pure metal catalysts has a close correlation with their atom's electronegativity. In addition, under identical electronegativity, the elements with half-filled s-orbitals (HFSO) and half-filled dsuborbitals (HFDOs) delivered higher catalytic activity than those elements only having HFDOs. For instance, Pt, Ru and Rh achieved values of i0,H lower than Os, Re and Ir, meanwhile, Mo and Cr also deliver a reduced value of i0,H compared to Fe and Mn. Intriguingly, the elements with only one halffilled s-orbital or with all orbitals fully-filled exhibit the lowest catalytic activity for hydrogen evolution. Such as Cu, Zn, Ag, Cd, Au, and Hg; all of them achieved a high exchange current density experimentally. These experimental results imply that the accessible HFSO and/or HFDOs on the outermost of an atom are the major factors behind the catalytic activity of the pure metal catalysts, and medium electronegativity could contribute to improving catalytic activity for HER. As a result, knowing the relationships between those HFSO and/or HFDO on the catalytic atom and atomic hydrogen (having a HFSO) is critical for completely understanding the HER catalytic process.