14.2 Oxide Tetrahedron Bond Formation
269
towards a compact model of chemical bond, energy band, and SPB for metal surfaces
with chemisorbed oxygen.
14.2.2 Rules for Surface Bond Formation
Three principles govern the nature, number, and geometry (angle and length) of the
oxygen chemisorption bond formation [23, 24]:
14.2.2.1 Electronegativity Specificity
The difference of the electronegativity (χ) between two elements describes the
ability of the atom of higher χ capturing electron from the other partner. Comparatively, electro-affinity being separation between the vacuum level and the bottom
edge of the conduction band, describes the ability of the element holding the electrons caught from other elements. The electronegativity is elementally intrinsic but
the electroaffinity refers to bulk solid and it is an adjustable quantity. Pauling [25]
pointed out that if atoms differ sufficiently (by about two units) in electronegativity,
they would form bonds that are mainly ionic. If the χ is much less than this, the
bonds are mainly covalent, or polar-covalent.
The χ of oxygen is 3.5. The χ is around 1.8 for transition metals. For noble metals,
the χ is about 2.2. The high χ value of oxygen indicates a great tendency for oxygen to
form compounds with ionic or polar-covalent bonds to metals by catching electrons.
Reactions with elemental oxygen give oxide products in which the oxidation state of
oxygen is −2. The net charge transportation from metal to oxygen can be estimated
by introducing a coefficient [26]:
ε = ε c + (ε i − ε c )/2, for χ ≤ 2
This means that the lower χ element has a net charge loss:
q = εe ≤ e
where ε c = 0.5 (for χ = 0) and ε i = 1.0 (for χ = 2) correspond to ideally covalent
and ionic states, respectively. According to Pauling, if χ ≥ 2 the bond is ideally
ionic, the net charge contribution of a metal to oxygen is q = e. For H 2 O, the χ for
H is 2.2, and the H–O bond is polar-covalent (χ = 1.3) and the charge transfer is
about 0.62–0.65 e [27].
269
towards a compact model of chemical bond, energy band, and SPB for metal surfaces
with chemisorbed oxygen.
14.2.2 Rules for Surface Bond Formation
Three principles govern the nature, number, and geometry (angle and length) of the
oxygen chemisorption bond formation [23, 24]:
14.2.2.1 Electronegativity Specificity
The difference of the electronegativity (χ) between two elements describes the
ability of the atom of higher χ capturing electron from the other partner. Comparatively, electro-affinity being separation between the vacuum level and the bottom
edge of the conduction band, describes the ability of the element holding the electrons caught from other elements. The electronegativity is elementally intrinsic but
the electroaffinity refers to bulk solid and it is an adjustable quantity. Pauling [25]
pointed out that if atoms differ sufficiently (by about two units) in electronegativity,
they would form bonds that are mainly ionic. If the χ is much less than this, the
bonds are mainly covalent, or polar-covalent.
The χ of oxygen is 3.5. The χ is around 1.8 for transition metals. For noble metals,
the χ is about 2.2. The high χ value of oxygen indicates a great tendency for oxygen to
form compounds with ionic or polar-covalent bonds to metals by catching electrons.
Reactions with elemental oxygen give oxide products in which the oxidation state of
oxygen is −2. The net charge transportation from metal to oxygen can be estimated
by introducing a coefficient [26]:
ε = ε c + (ε i − ε c )/2, for χ ≤ 2
This means that the lower χ element has a net charge loss:
q = εe ≤ e
where ε c = 0.5 (for χ = 0) and ε i = 1.0 (for χ = 2) correspond to ideally covalent
and ionic states, respectively. According to Pauling, if χ ≥ 2 the bond is ideally
ionic, the net charge contribution of a metal to oxygen is q = e. For H 2 O, the χ for
H is 2.2, and the H–O bond is polar-covalent (χ = 1.3) and the charge transfer is
about 0.62–0.65 e [27].