Question

The linear combination of atomic orbitals to form molecular orbitals takes place only when the combining atomic orbitals
A. have the same energy 
B. have the minimum overlap 
C. have same symmetry about the molecular axis 
D. have different symmetry about the molecular axis. 
Choose the most appropriate from the options given below:

• A. A, B, C only
• B. A and C only
• C. B and C only
• D. B and D only

Answer 1

The Correct Option is B

The concept in focus here is about the formation of molecular orbitals through the linear combination of atomic orbitals (LCAO). To ensure the combination occurs effectively, the atomic orbitals must satisfy certain conditions:

• Same Energy: Atomic orbitals can combine effectively if they have the same or very similar energies. This allows the wave functions to combine constructively, forming stable molecular orbitals.
• Same Symmetry: The combining atomic orbitals must have the same symmetry with respect to the molecular axis. Only orbitals that possess compatible symmetries can combine to form molecular orbitals.

Now, let's analyze each option: 

• Option A (Same Energy): Correct, as having the same energy is essential for effective overlap and combination.
• Option B (Minimum Overlap): Incorrect, as a significant overlap is necessary for the effective combination of orbitals. Minimum overlap would defeat the purpose since it doesn't provide enough interaction space.
• Option C (Same Symmetry): Correct, same symmetry is a critical requirement for the combination of atomic orbitals to form molecular orbitals.
• Option D (Different Symmetry): Incorrect, as orbits with different symmetries cannot effectively combine.

Given these considerations, the most appropriate options that ensure the combination of atomic orbitals are A and C only. Hence, the correct answer is:

Answer: A and C only

Answer 2

* Molecular orbitals should have maximum overlap.
* Symmetry about the molecular axis should be similar for effective linear combination.