Question

Which of the following statement is not correct from the view point of molecular orbital theory?

Options

• \[\ce{Be2}\] is not a stable molecule.

• \[\ce{He2}\] is not stable but \[\ce{He^{+}2}\] is expected to exist.

• Bond strength of \[\ce{N2}\] is maximum amongst the homonuclear diatomic molecules belonging to the second period.

• The order of energies of molecular orbitals in \[\ce{N2}\] molecule is σ2s < σ^*2s < σ2p_z < (π2p_x = π2p_y) < (π^*2p_x = π^*2p_y) < σ^*2p_z 

Answer 1

The order of energies of molecular orbitals in \[\ce{N2}\] molecule is σ2s < σ^*2s < σ2p_z < (π2p_x = π2p_y) < (π^*2p_x = π^*2p_y) < σ^*2p_z 

Explanation:

(i) \[\ce{Be2}\] (4 + 4 = 8) = σ1s², σ*1s², σ1s², σ*2s²

Bond order \[\ce{(BO) = 1/2}\] [Number of bonding electron (N_b) – Number of anti-bonding electrons N_a]

= `(4 - 4)/2` = 0

Here, bond order of \[\ce{He2}\] is zero. THus. it does not exist.

(ii) \[\ce{NHe2}\] (2 + 2 = 4) = σ1s², σ*1s² 

BO = `(2 - 2)/2` = 0

Here, bond order of \[\ce{He2}\] is zero. Thus, it does not exist.

\[\ce{He^{+}2}\] (2 + 2 – 1 = 3) = σ1s², σ*1s¹  

BO = `(2 - 1)/2` = 0.5

Since, the bond order is not zero, this molecule is expected to exist.

(iii) \[\ce{N2}\] (7 + 7 = 14) = \[\ce{σ1s^2, σ^* 1s^2, σ2s^2, σ^* 2s^2, π2p^2_x ≈ π2p^2_y, π2p^2_z}\]

BO = `(10 - 4)/2` = 3   

Thus, dinitrogen \[\ce{(N2)}\] molecule contains triple bond and no other molecule of second period have more than double bond. Hence, bond strength of \[\ce{N2}\] is maximum amongst the homonuclear diatomic molecules belonging to the second period.

(iv) It is incorrect. The correct order of energies of molecular orbitals in \[\ce{N2}\] molecule is

σ2s < σ^*2s < σ2p_z < (π2p_x = π2p_y) < π^*2p_x = π^*2p_y < σ^*2p_z