NEET 2023 Chemistry Molecular Orbitals of Diatomic Molecules MCQ Question

To determine the correct order of molecular orbitals for the nitrogen molecule (N₂), we need to analyze the molecular orbital (MO) theory, which describes the behavior of electrons in molecules.

1. Explanation of the Correct Answer

The molecular orbitals for diatomic molecules such as N₂ are filled according to their increasing energy levels. For N₂, which has a total of 14 electrons (7 from each nitrogen atom), the filling of molecular orbitals follows the order defined by the interaction of atomic orbitals.

The correct order of energies of molecular orbitals for N₂ is:

This order can be broken down as follows:

• 1s Orbitals: The bonding orbital $\sigma_{1s}$ is lower in energy than the antibonding orbital $\sigma^*_{1s}$.
• 2s Orbitals: Similarly, $\sigma_{2s}$ is lower than $\sigma^*_{2s}$.
• 2p Orbitals: In the case of p orbitals, for N₂, the $\sigma_{2p_z}$ orbital is lower in energy than the degenerate $\pi_{2p_x}$ and $\pi_{2p_y}$ orbitals. This is due to the difference in the overlap of p-orbitals which leads to varying energy levels.

The configuration of the electrons in N₂ is as follows:

• 2 electrons in $\sigma_{1s}$
• 2 electrons in $\sigma^*_{1s}$
• 2 electrons in $\sigma_{2s}$
• 2 electrons in $\sigma^*_{2s}$
• 2 electrons in $\sigma_{2p_z}$
• 2 electrons in $\pi_{2p_x}$ and $\pi_{2p_y}$ (each filled with 1 electron)

Thus, the correct option, as stated, is: A) $\sigma_{1s} < \sigma^*_{1s} < \sigma_{2s} < \sigma^*_{2s} < \sigma_{2p_z} < (\pi_{2p_x} = \pi_{2p_y}) < (\pi^*_{2p_x} = \pi^*_{2p_y}) < \sigma^*_{2p_z}$

2. Explanation of Why Other Options Are Incorrect

• Option B: It incorrectly suggests that the $\sigma_{2p_z}$ orbital appears before the degenerate $\pi_{2p}$ orbitals, which contradicts the established energy order in diatomic nitrogen.

• Option C: This option places $\sigma_{2p_z}$ after the degenerate $\pi_{2p}$ orbitals, which is again incorrect. The $\sigma_{2p_z}$ orbital is lower in energy compared to the $\pi_{2p}$ orbitals.

• Option D: While this option correctly orders the initial orbitals, it places the $\sigma_{2p_z}$ before the degenerate $\pi_{2p}$ orbitals, which is not appropriate according to molecular orbital theory for N₂.

3. Relevant Concepts

Molecular Orbital Theory is a fundamental concept in chemistry that helps us understand the electronic structure of molecules. The energy levels of molecular orbitals are determined by:

• The type of atomic orbitals involved in bonding (s, p, d).
• The degree of overlap between the atomic orbitals.

For diatomic molecules of elements with atomic numbers up to 14 (like nitrogen), the order of molecular orbitals is often:

1. For lighter diatomic molecules: $\sigma_{1s} < \sigma^*_{1s} < \sigma_{2s} < \sigma^*_{2s} < \sigma_{2p_z} < \pi_{2p_x} = \pi_{2p_y} < \pi^*_{2p_x} = \pi^*_{2p_y} < \sigma^*_{2p_z}$

2. For heavier diatomic molecules:

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