Which one of the following conditions is not correct for the combination of atomic orbitals to form molecular orbitals ?
The combining atomic orbitals must have same or nearly the same energy.
The combining atomic orbitals must show lateral overlap with each other.
The combining atomic orbitals must have the same symmetry about the molecular axis.
The combining atomic orbitals must overlap to the maximum extent.
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UPSC Geoscientist – 2024
1. **The combining atomic orbitals must have comparable energies.** This allows for effective interaction and mixing of orbitals. (Statement A is correct)
2. **The combining atomic orbitals must have the same symmetry about the molecular axis.** Orbitals with different symmetries (e.g., s orbital and p$_x$ orbital if the molecular axis is z) cannot effectively overlap. (Statement C is correct)
3. **The combining atomic orbitals must overlap to the maximum possible extent.** Significant overlap leads to strong bonding (and antibonding) molecular orbitals. (Statement D is correct)
Statement B claims that combining atomic orbitals *must* show lateral overlap. This is incorrect. Atomic orbitals can overlap axially (end-to-end), which leads to the formation of sigma ($\sigma$) molecular orbitals, or laterally (side-by-side), which leads to the formation of pi ($\pi$) molecular orbitals. Both types of overlap are valid and common ways for atomic orbitals to combine. It is not a requirement that *only* lateral overlap must occur. For instance, the formation of H₂ molecule involves axial overlap of two 1s orbitals.