Anonymous asked in Science & MathematicsChemistry · 8 months ago

Give the electron configurations for the Li2, Li2^+1, and Li2^-1 in molecular orbital terms. ?

Compare the Li-Li bond order in the three species; which has the shortest bond length?

1 Answer

  • 8 months ago

    Use simple LCAO (linear combination of atomic orbitals) MO theory. Li(0) 2s^1 Overlap of the two 2s AOs results in a σ bonding MO that is lower in energy than the constituent 2s AOs and an antibonding σ* MO that is at a higher energy than the 2s AOs. Each MO can hold two e⁻s and hence for Li2 the MO scheme is σ(↑↓) σ*(0) Bond Order = ½[Σ (bonding e⁻) - Σ (antibonding e⁻)] = ½[(σ(2e⁻)] = 1.0.

    Li2 is present to the extent of ~1% in Li(g).

    Removal of one e⁻ from Li2 to give Li2+ results in an MO scheme of σ(↑)σ*(0)

    Bond Order = ½[Σ (bonding e⁻) - Σ (antibonding e⁻)] = ½[(σ(1e⁻)] = 0.5. It is bound and should be observed in the gas phase.

    Likewise Li2^- has an MO scheme of σ(↑↓) σ*(↑) and also a bond order of 0.5 and again should be observed in the gas phase. For reasons I won’t go into Li2^- should be less stable than Li2^+.

    The higher the bond order the shorter the bond length so Li2bl>Li^+~Li^-

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