In which of the following pairs of molecules / ions, both the species are not likely to exist?
H22+ , He2 Bond order = 0
so both species are not likely to exist.
This question tests your understanding of molecular orbital theory (MOT) and bond order calculations to determine the stability of diatomic molecules/ions. A species is considered stable and likely to exist if it has a positive bond order (B.O. > 0). A bond order of zero or negative indicates no bond formation, making the species unstable and unlikely to exist.
Step 1: Recall the Molecular Orbital Energy Diagram for H₂ and He₂
The molecular orbital configuration for a molecule is built by combining atomic orbitals. For H₂ and He₂ (and their ions), the order is:
σ1s < σ*1s
The bond order is calculated using the formula:
, where Nb is the number of electrons in bonding orbitals and Na is the number of electrons in antibonding orbitals.
Step 2: Analyze Each Pair
Pair 1: H2+ and He22–
Conclusion: Both are not likely to exist? No, because H₂⁺ does exist.
Pair 2: H2– and He22+
Conclusion: Both are not likely to exist? No, both exist.
Pair 3: H2– and He22–
Conclusion: Both are not likely to exist? No, because H₂⁻ does exist.
Pair 4: H22+ and He2
Conclusion: Both are not likely to exist? Yes, neither H₂²⁺ nor He₂ are stable molecules.
Final Answer: The correct pair where both species are not likely to exist is H22+, He2.
Molecular Orbital Theory (MOT): A model that describes the electronic structure of molecules in terms of molecular orbitals that are delocalized over the entire molecule. These orbitals are formed by the linear combination of atomic orbitals (LCAO).
Bond Order Formula:
Key Principle: A positive bond order (B.O. > 0) indicates a stable bond and a molecule that can exist. A bond order of zero or negative means no bond is formed, and the molecule is unstable.