As previously stated, the elements having both HFSO and HFDOs achieved higher catalytic activity than those containing merely two or more HFDOs even if they exhibited identical electronegativity. For example, Pt, Ru and Ru experimentally outperform Os, Ir and Re (Table ). It is known that the orbital on a sub-shell has multiple spatial orientations; hence, we proposed this angular dependence on the suborbital has a significant impact on the catalytic behaviour of solid-state catalysts. Part of an atom's outermost orbitals may be blocked by neighbouring atoms in the crystal, and such entirely blocked orbitals cannot interact with reactant species via half-filled orbitals. However, when considering atoms on the crystal surface, the portion of the valence sub-orbitals on an atom is still accessible with the part of electron density, which is available for surface interactions. When an atom is lying on the inner basal plane, one-third of the outermost sub-orbitals have the part region of electron density exposed. Two-thirds and all sub-orbitals have the part region of electron density exposed when an atom is situated on the edge and corner sites, respectively. The d-orbital contains five sub-orbitals, indicating that atoms on the corner and edge sites have at least two sub-orbitals with a visible zone of electron density, but atoms in the centre of the crystal surface only expose one suborbital with a visible zone of electron density. Hence, for those elements that only possess HFDOs, the atom lying in the edge and corner sites could have a chance to interact with two atomic hydrogens via half-filled orbitals and as catalytic active sites for driving HER. Due to just one HFDO being accessible for those atoms located in the centre of the crystal surface, it is impossible to link two atomic hydrogens on their surface via half-filled orbitals interaction. As a result, the elements (such as Co, Ni, Ir and Re) with just HFDOs accessible require high surface exposure when applied as HER catalysts. On the other hand, this orbitals' orientation effect could be avoided between HFSO and HFDO. Therefore, the elements, possessed HFSO and HFDOs simultaneously, can link two atomic hydrogens via half-filled orbitals' interaction without any difficulty and superior catalytic performance for HER is delivered by such elements (Pt, Ru and Rh).