Question

Assertion: Bond order is half the difference between number of bonding and anti bonding orbitals.
Reason: Bond order is $L{i_2}$ which means the molecule is highly unstable.
A.Both assertion and reason are true and reason is the correct explanation for assertion.
B.Both assertion and reason are true but reason is not the correct explanation for assertion.
C.Assertion is true but the reason is false.
D.Both assertion and reason are false statements.

Answer 1

According to the Pauli principle and Hund’s rule: each orbital can accommodate a maximum of two electrons with opposite spins, and the orbitals are filled in order of increasing energy. Because each H atom contributes one valence electron, the resulting two electrons are exactly enough to fill the ${\sigma _{1s}}$ bonding molecular orbital.

Complete answer:
In molecular orbital theory, bond order is also defined as half the difference between the number of bonding electrons and the number of antibonding electrons. The molecule dilithium ($L{i_2}$). The 1s electrons do not take part in the bonding, but the 2s electrons fill the bonding orbital. The molecule $L{i_2}$​ is a stable molecule in the gas phase, with a bond order of one.
The bonding in a homonuclear diatomic molecule (a molecule that consists of two atoms of the same element) such as ${H_2}$, we use molecular orbitals; that is, for a molecule in which two identical atoms interact, we insert the total number of valence electrons into the energy-level diagram. We fill the orbitals according to the Pauli principle and Hund’s rule: each orbital can accommodate a maximum of two electrons with opposite spins, and the orbitals are filled in order of increasing energy. Because each H atom contributes one valence electron, the resulting two electrons are exactly enough to fill the σ1s bonding molecular orbital.

Note:
We need to know that the two electrons enter an orbital whose energy is lower than that of the parent atomic orbitals, so the ${H_2}$ molecule is more stable than the two isolated hydrogen atoms. Thus molecular orbital theory correctly predicts that ${H_2}$ is a stable molecule. Because bonds form when electrons are concentrated in the space between nuclei, this approach is also consistent with our earlier discussion of electron-pair bonds.