VBT Flashcards
Q: How is a covalent bond formed in Valence Bond Theory (VBT)?
A: A covalent bond is formed by the overlap of partially filled atomic orbitals from two atoms, where electrons are shared with opposite spins.
Q: What is the relationship between the number of bonds and unpaired electrons in VBT?
A: The number of bonds formed is equal to the number of unpaired electrons in the valence shell of the atoms involved.
Q: What does VBT say about the symmetry of orbitals involved in bonding?
A: Orbitals must have the same symmetry with respect to the bond axis (the line between the nuclei) for effective overlap and bond formation.
Q: What are the different types of bonds in VBT and how are they formed?
σ (Sigma) Bond: Formed by head-on overlap of orbitals (stronger).
π (Pi) Bond: Formed by sideways overlap of orbitals (weaker).
Q: According to VBT, what happens during bond formation in terms of energy?
A: Energy is released during bond formation because the system becomes more stable. This process is exothermic.
Q: What does VBT say about the spins of electrons involved in bond formation?
A: The electrons in the overlapping orbitals must have opposite spins to form a stable covalent bond.
Q: What limitation does VBT have regarding coordinate covalent bonds?
A: VBT cannot explain the formation of coordinate covalent bonds, where both electrons in the bond come from the same atom (e.g., in NH₄⁺).
Q: Why can’t Valence Bond Theory (VBT) explain molecules like NO and NO₂?
Because VBT assumes electrons are paired in bonds. It fails to explain molecules with odd numbers of electrons, like NO and NO₂, which have unpaired electrons and paramagnetic behavior.
Q: Why doesn’t VBT explain paramagnetism?
A: VBT does not explain why molecules like O₂ are paramagnetic (having unpaired electrons), even though it predicts they should be diamagnetic.
Q: What is the limitation of VBT regarding delocalized electrons?
A: VBT doesn’t explain the delocalization of electrons in molecules like benzene, where electrons are spread over multiple atoms (resonance).
Q: What is the limitation of VBT in explaining molecular energy levels?
A: Unlike Molecular Orbital Theory (MOT), VBT does not provide energy diagrams to show molecular bonding and anti-bonding orbitals.
Q: Why doesn’t VBT explain excited states?
A: VBT doesn’t fully explain how electrons are promoted to higher energy orbitals, which is needed to form bonds in certain molecules (e.g., in BeCl₂).
Q: Who first proposed Valence Bond Theory and what did they explain?
A: Heitler and London (1927) proposed VBT, explaining the bond formation in H₂ using quantum mechanics and orbital overlap.
Q: How did Linus Pauling expand on VBT?
A: Pauling (1931) introduced hybridization to explain bond angles and molecular shapes, and further developed VBT for multiple bonds and bond strength.
