MO diagram: H₂ and He₂ template

Molecular orbital energy diagrams of H₂ (bond order 1) and He₂ (bond order 0) with electrons as spin arrows, σ1s and σ*1s, configuration and bond order.

What the template shows

• Two 1s atomic orbitals give one bonding and one antibonding molecular orbital
• Two 1s orbitals combine in phase (σ1s, bonding) and out of phase (σ*1s, antibonding)
• Bond order = ½ (2 − 0) = 1
• diamagnetic (all electrons paired)
• Dihelium (not bound)
• Four electrons fill σ1s and σ*1s: the antibonding pair cancels the bond, so He₂ does not form
• Bond order = ½ (2 − 2) = 0
• n atomic orbitals give n molecular orbitals: here σ1s (lower than 1s) and σ*1s (raised a little more than σ1s is lowered).
• Bond order = ½ (bonding − antibonding electrons): H₂ 1, H₂⁺ ½, He₂ 0.
• Electrons fill from the lowest level, at most two per orbital with opposite spins (Pauli).
• H₂: bond length 74 pm, bond enthalpy 436 kJ mol⁻¹. Energies are drawn schematically, not to scale.
• Every part of this figure is editable · sources on the template card

Drawn with 253 shapes – every one editable in MolDraw.

Sources

• Atkins, de Paula & Keeler, Atkins' Physical Chemistry, 11th ed., OUP 2018, ch. 9 Textbook
• Miessler, Fischer & Tarr, Inorganic Chemistry, 5th ed., Pearson 2014, ch. 5 Textbook