21. Which of the following statements is true about bonding molecular orbitals?
A) They are formed from the combination of orbitals with opposite phases.
B) They are higher in energy than the atomic orbitals that formed them.
C) They stabilize the molecule.
D) They have no electron density between the nuclei.
Answer: C
Explanation: Bonding molecular orbitals lower the energy of the system and stabilize the molecule.
22. What is the electron configuration for the molecular ion O2⁻?
A) 1s² 2s² 2p⁶
B) 1s² 2s² 2p⁵
C) 1s² 2s² 2p⁴ 3s²
D) 1s² 2s² 2p⁵ 3s²
Answer: D
Explanation: O2⁻ has one additional electron compared to O2, so its configuration is 1s² 2s² 2p⁵.
23. Which element has the highest bond order in diatomic molecules?
A) He
B) H
C) N
D) O
Answer: C
Explanation: N2 has a bond order of 3, the highest among common diatomic molecules.
24. Which molecular orbital is filled last in the O2 molecule?
A) 1s
B) 2s
C) 2p
D) 3s
Answer: C
Explanation: In O2, the 2p orbitals are filled after the 1s and 2s orbitals, and contain unpaired electrons contributing to its paramagnetic property.
25. Which of the following species is isoelectronic with Ne?
A) O2⁻
B) F2
C) Na⁺
D) All of the above
Answer: D
Explanation: O2⁻, F2, and Na⁺ all have the same number of electrons as Ne (10 electrons).
26. Which molecular orbital has no nodal planes?
A) 1s
B) 2p
C) σ*
D) π*
Answer: A
Explanation: The 1s molecular orbital has no nodal planes, meaning it has continuous electron density without any regions of zero probability.
27. In which molecule is the bond order 3?
A) O2
B) N2
C) F2
D) He2
Answer: B
Explanation: N2 has a bond order of 3, indicating a triple bond due to three pairs of shared electrons.
28. Which of the following orbitals is non-bonding?
A) σ2s
B) π2p
C) σ2p
D) n2p
Answer: D
Explanation: Non-bonding orbitals do not contribute to bonding and typically have no electron density between the nuclei.
29. In which molecular orbital diagram do you expect a change in energy ordering for the p orbitals?
A) Diatomic molecules of the first period
B) Diatomic molecules of the second period
C) Diatomic molecules of the third period
D) None of the above
Answer: B
Explanation: In the second period (e.g., B2, C2, N2), the π2p orbitals are lower in energy than the σ2p orbital, whereas in O2 and F2, the order reverses.
30. Which of the following species has unpaired electrons?
A) N2
B) O2
C) CO
D) He2
Answer: B
Explanation: O2 has two unpaired electrons in its molecular orbital configuration, making it paramagnetic.
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