Question

Assertion ${{\text{B}}_{\text{2}}}$ molecules is diamagnetic
Reason: The highest occupied molecular orbital is of $\sigma$ type
A.If both assertion and reason are true and the reason is the correct explanation of the assertion
B.If both assertion and reason are true but the reason is not the correct explanation of the assertion
C.if the assertion is true but the reason is false
D.if the assertion and reason both are false
E.if the assertion is false but the reason is true

Answer 1

To answer this question, you should recall the concept of molecular orbital theory. After the combination of atomic orbitals, fill each molecular orbital in order of their energies. Now check whether the last electron is filled in the sigma or pi orbital and whether it is paired or unpaired.

Complete step by step answer:
The filling of these orbitals takes place in the order of increasing energy and is governed by the following rules:
The molecular orbitals are arranged in the increasing order of their energies. The molecular orbital with the lowest energy is filled first as per the Aufbau principle.
Each molecular orbital can accommodate a maximum of two electrons with opposite spins as per Pauli's exclusion principle.
If there are two or more molecular orbitals of the same energy, these are the first singly filled and then pairing starts as per the Hund's rule.
Using the above rules ${{\text{B}}_{\text{2}}}{\text{}}$ molecule has no. of electrons = 10.
$\therefore$Molecular orbital configuration: $\sigma {\left( {1s} \right)^2}\sigma * {\left( {1s} \right)^2}\sigma {\left( {2s} \right)^2}\sigma * {\left( {2s} \right)^2}\pi 2{p_x}^1\pi 2{p_y}^1$.
Due to the unpaired electron, it is paramagnetic and the highest occupied MO is of $\pi$ type.
Hence, both the assertion and reason are false.

Thus, the correct answer is option D.

Note:
Important characteristics of Bonding Molecular Orbitals
The probability of finding the electron in the internuclear region of the bonding molecular orbital is more than that of combining atomic orbitals.
The presence of electrons present in the bonding molecular orbital is the cause of attraction between the two atoms.
The bonding molecular orbital has lower energy as a result of attraction and hence has greater stability than that of the combining atomic orbitals.
They are formed by the additive effect of the atomic orbitals so that the amplitude of the new wave is given by $\Phi = {\Psi _A} + {\Psi _B}$
They are represented by $\sigma ,{\text{ }}\pi {\text{ and }}\delta$