Why does bond order skip the 1s core for Li2 and heavier?

The 1s bonding and antibonding pair is a closed “KK” shell that cancels in the usual valence bond order; chemists quote BO from valence σ/π interactions to match triple/double/single language.

Are the energy spacings quantitative?

No. Only the qualitative ordering and electron count are modeled; real term splittings need multi-electron configuration interaction.

The same late ordering as O2 applies; this page stops at O₂ as in the curriculum note but F₂ would be BO = 1 and diamagnetic with a filled π* set.

MO Diagram: Homonuclear Diatomics (H₂–O₂)

σ/π ladder with Li–N vs O ordering; bond order from valence MOs; O₂ π* unpaired → paramagnetism.

σ₁s(1s)

σ*₁s(1s)

σ₂sbonding

σ*₂santibonding

π₂pbonding

π′

σ₂pbonding

π*₂pantibonding

π′

(empty)

σ*₂pantibonding

(empty)

Energy increases upward. π shells show two degenerate components.

About this model

Molecular orbital theory builds delocalized one-electron states from linear combinations of atomic orbitals (LCAO). For homonuclear diatomics of second-row elements, the ordering of σ2p versus π2p is not fixed: s–p mixing stabilizes π2p relative to σ2p from roughly Li2 through N2, whereas for O2 and F2 the σ2p level drops below π2p, matching photoelectron spectroscopy. This page draws a schematic ladder (σ1s, σ*1s, σ2s, σ*2s, then the valence 2p-derived set in the appropriate order), fills electrons from the bottom with Hund’s rule within degenerate π shells, and reports bond order as ½(N_b − N_a) using valence (2s–2p) contributions for Li2 onward while counting the 1s-derived MOs for H2 and He2. O2 receives two unpaired electrons in π* orbitals, predicting paramagnetism that a classical Lewis structure cannot explain.

Who it's for: General chemistry after atomic orbitals; bridges to crystal-field and diatomic electronic structure in physical chemistry.

Key terms

LCAO
bond order
σ and π MOs
s–p mixing
paramagnetism
HOMO
antibonding

How it works

Homonuclear diatomics X₂ combine atomic orbitals into σ and π molecular orbitals. For second-row species, s–p mixing reverses the π₂p / σ₂p ordering from Li₂ through N₂; for O₂ and heavier, σ₂p lies lower, matching photoelectron spectra. Bond order here uses the valence (2s–2p) MOs (KK 1s core omitted from the BO count). O₂ gets two unpaired electrons in π* — paramagnetic (Lewis structure alone misses this).

Key equations

BO = ½ (N_b − N_a) (valence 2s–2p MOs only)

Li₂–N₂: π₂p below σ₂p · O₂–F₂: σ₂p below π₂p

Frequently asked questions

Why does bond order skip the 1s core for Li2 and heavier?: The 1s bonding and antibonding pair is a closed “KK” shell that cancels in the usual valence bond order; chemists quote BO from valence σ/π interactions to match triple/double/single language.
Are the energy spacings quantitative?: No. Only the qualitative ordering and electron count are modeled; real term splittings need multi-electron configuration interaction.
Where is F2?: The same late ordering as O2 applies; this page stops at O₂ as in the curriculum note but F₂ would be BO = 1 and diamagnetic with a filled π* set.

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