Question

Which one of the following species does not exist under
normal conditions?
(a) Be₂⁺ (b) Be₂ (c) B₂ (d) Li₂

Answer 1

answer : option (b) Be₂

concept : if bond order of a molecule is zero, that molecule is imaginary. means, molecule doesn't exist if bond order is zero.

Let's find bond order of each molecule.

for Be₂⁺ , number of electrons = 7

so, configuration = σ1s², σ*1s², σ2s², σ*2s¹

bond order = 1/2 [4 - 3 ] = 0.5

for Be₂, number of electrons = 8

so, configuration = σ1s², σ*1s², σ2s², σ*2s²

bond order = 1/2 [4 - 4] = 0

for B₂ , number of electrons = 10

so, configuration = σ1s², σ*1s², σ2s², σ*2s², π2px¹ ≈ π2py¹

bond order = 1/2 [6 - 4] = 1

for Li₂, number of electrons = 6

so, configuration = σ1s², σ*1s², σ2s²,

bond order = 1/2 [4 - 2] = 1

here it is clear bond orde of only Be₂ has zero. so, it doesn't exist under normal conditions.

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