TO4/025 : COVALENT BOND FORMATION Flashcards
Define the valence bond theory
The Valence Bond (VB) Theory explains chemical bonding as the result of atomic orbital overlap, where bonding electrons pair up in overlapping orbitals between two nuclei, and hybridization of orbitals determines molecular geometry and bond strength.
What are the key principles of the valence bond theory?
Key Principles of Valence Bond Theory:
Atomic Orbital Overlap: A chemical bond is formed when the atomic orbitals of two atoms overlap. The greater the overlap, the stronger the bond.
Electron Pairing: The electrons involved in bonding are usually in opposite spins and pair up in the overlapping orbitals.
Hybridization: Atomic orbitals can mix to form hybrid orbitals of equivalent energy, which can explain the geometry of molecules (e.g., sp, sp², sp³ hybridizations).
Localized Bonds: According to VB theory, the bonding electrons are localized between the two nuclei of the bonded atoms, unlike the delocalization concept in Molecular Orbital Theory.
Bond Strength: The strength of a bond is influenced by the extent of overlap between the atomic orbitals. Sigma (σ) bonds are typically stronger due to head-on overlap, while pi (π) bonds result from the side-to-side overlap.
what is a sigma bond?
type of covalent bond formed by head on overlap of bonding orbitals along the internuclear axis.
what are the different types of sigma bonds?
- s bond
- pi bond
what are the characteristics of a sigma s bond?
1) From head-on
overlap
2) Lie along the
bond axis
3) Account for
the first bond
4) Can freely
rotate around
bond
What are the features of the pi bond?
– from lateral overlap by
adjacent p or d orbitals
– pi bonds are perpendicular to
bond axis
– account for the second and
third bonds in a multiple
bond
– Cannot undergo rotation
around bond
Explain hybridization as a quantum mechanical phenomenon.
Hybridization is the mathematical combination of atomic orbitals on the same atom to form degenerate hybrid orbitals, which align with observed molecular geometries.
Describe the geometry and bond angle associated with sp² hybridization.
Molecules with sp² hybridization exhibit a trigonal planar geometry with bond angles of approximately 120°.
What are sigma (σ) and pi (π) bonds?
Sigma bond: Formed by head-on overlap of orbitals, with electron density along the internuclear axis.
Pi bond: Formed by sideways overlap of p-orbitals, with electron density above and below the internuclear axis.
How does the Valence Shell Electron Pair Repulsion (VSEPR) theory explain molecular geometry?
VSEPR theory posits that electron pairs (bonding and lone) around a central atom arrange themselves to minimize repulsion, defining molecular shape and bond angles.
How do lone pairs affect bond angles in molecular geometries?
Lone pairs exert greater repulsion than bonding pairs, leading to compressed bond angles in geometries like trigonal pyramidal and bent shapes.
What determines the bond strength and stability in Molecular Orbital Theory (MOT)?
Bond strength and stability depend on the bond order, calculated as half the difference between bonding and antibonding electrons in molecular orbitals.
Contrast bonding and antibonding molecular orbitals.
Bonding orbitals: Formed by constructive interference of atomic orbitals, lowering energy.
Antibonding orbitals: Formed by destructive interference, raising energy.
Explain the bond order of O₂ and its implications.
The bond order of O₂ is 2, indicating a double bond. Its unpaired electrons in antibonding orbitals confer paramagnetic properties.
Define the concept of delocalization in molecular orbitals.
Delocalization refers to electrons spreading over multiple atoms in molecular orbitals, enhancing stability as seen in resonance structures.
Explain the sp³ hybridization and provide an example
sp³ hybridization involves mixing one s orbital and three p orbitals, forming four equivalent orbitals. Example: CH₄ exhibits tetrahedral geometry.
Discuss the hybridization in XeF₄.
XeF₄ exhibits sp³d² hybridization with a square planar geometry, where lone pairs occupy axial positions to minimize repulsion.
What is the bond order formula in MOT?
Bond Order = (Number of Bonding Electrons − Number of Antibonding Electrons) / 2.
How does MOT describe the magnetic properties of diatomic molecules?
Magnetic properties are determined by the presence of unpaired electrons in molecular orbitals: paramagnetic if unpaired (e.g., O₂), diamagnetic if all electrons are paired (e.g., N₂).
Explain the hybridization of carbon in ethene (C₂H₄).
Carbon in ethene is sp² hybridized, with a planar structure and one unhybridized p-orbital per carbon forming a π-bond.
What role do lone pairs play in the molecular geometry of NH₃?
NH₃ has a trigonal pyramidal geometry due to sp³ hybridization. The lone pair compresses the bond angles from 109.5° to approximately 107°.
