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Molecular orbitals

Molecular orbital diagram of Be₂

Bond order 0, diamagnetic. 8 electrons: Nb = 4, Na = 4.

BeBe
  • Be

Be₂Beryllium (two atoms)

Bond order 0

8 electrons, all paired

i

Bond order zero: Be2 is not a stable molecule.

  • Bond order0
  • Magnetismdiamagnetic
  • Electrons8

Molecular orbitals

8 electrons
0bond order
diamagnetic

(Nb − Na) / 2 = (4 − 4) / 2

1s1s2s2sσ1sσ*1sσ2sσ*2sBe atomMoleculeBe atomEnergy

σ1s² σ*1s² σ2s² σ*2s²

Working

Filling the diagram

  1. Be₂ has 8 electrons: 4 from each Be.
  2. Up to N₂ (and its ions), the π2p pair lies below σ2pz and fills first.
  3. Configuration: σ1s² σ*1s² σ2s² σ*2s²
  4. Bonding electrons Nb = 4, antibonding electrons Na = 4.
  5. Bond order = (Nb − Na) / 2 = (4 − 4) / 2 = 0.
  6. Every electron is paired, so Be₂ is diamagnetic.
  7. A bond order of zero means there is no bond: Be₂ is not a stable molecule.

Class 11, Chapter 4 Chemical Bonding and Molecular Structure.

Questions

MO diagram of Be2: quick answers

What is the molecular orbital configuration of Be2?

σ1s² σ*1s² σ2s² σ*2s². Up to N₂ (and its ions), the π2p pair lies below σ2pz and fills first.

What is the bond order of Be2 from its MO diagram?

0: Nb = 4 and Na = 4, so (4 - 4) / 2 = 0. It is diamagnetic.

Why is σ2pz above π2p in Be2?

Up to N2 the 2s and 2p orbitals are close in energy and mix, which pushes σ2pz above the π2p pair.

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