Ch. 8 Advanced Theories of Covalent Bonding
• Valence Bond Theory
o It describes a covalent bond as the overlap of half-filled atomic orbitals that yield a pair of
electrons shared between two bonded atoms.
o There are two ways orbitals can overlap to form bonds between atoms:
 Sigma (σ) bonds
• Head-to-head overlap
• Cylindrical symmetry of electron density about the internuclear axis
 Pi (π) bonds
• Side-to-side overlap
• Electron density above and below the internuclear axis
• Hybridization
o The mixing of native orbitals to form special orbitals for bonding
o The number of atomic orbitals combined always equals the number of hybrid orbitals formed
o The electrons that were in the originally orbital are distributed to the hybrid orbitals
• sp Hybridization
o Involves mixing of the valence one s and one p orbital yield two equivalent sp hybrid orbitals
o Two hybrid orbitals that are oriented in a linear geometry (180° bond angles)
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• sp Hybridization
o Involves mixing of one s orbital and two p orbitals to produce three identical hybrid orbitals
o valence orbitals of a central atom surrounded by three sp2 hybrid orbitals and one unhybridized p
orbital
o Three identical hybrid orbitals oriented in a trigonal planar geometry (120° bond angles)
o The unhybridized p orbital oriented perpendicular to the plane of the sp2 orbitals
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• sp Hybridization
o Involves mixing of one s orbital and three p orbitals to produce four identical hybrid orbitals
o Gives a tetrahedral arrangement of atomic orbitals
o Each of these hybrid orbitals points toward a different corner of a tetrahedron
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• dsp Hybridization
o Involves mixing of one s orbital and three p orbitals and one of the d orbitals to produce five
identical hybrid orbitals
o A set of five effective pairs around a given atom always requires a trigonal bipyramidal
arrangement
o Only possible for atoms that have d orbitals in their valence subshells
• d2sp3 Hybridization
o Involves mixing of one s orbital and three p orbitals and two of the d orbitals to produce six
identical hybrid orbitals
o A set of six effective pairs around a given atom always requires an octahedral arrangement
o Only possible for atoms that have d orbitals in their valence subshells
• Multiple Bonds
o In a multiple bond, one of the bonds is a σ bond and the rest are π bonds.
 No more than one σ bond and no more than two π bonds.
o Hybrid orbitals overlaps in σ fashion
o Unhybridized orbitals overlap in π fashion
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Molecular Orbitals (MOs)
o Have the same characteristics as atomic orbitals
 Can hold two electrons with opposite spins
 The square of the molecular orbital wave function indicates electron probability
Bonding and antibonding
o Bonding molecular orbital
 Lower in energy than the atomic orbitals from which it is composed
 Electrons in this orbital will favor bonding
o Antibonding molecular orbital
 Higher in energy than the atomic orbitals from which it is composed
 Electrons in this orbital will favor the separated atoms
Molecular Orbital Diagram
o Shows the relative energy levels of atomic and molecular orbitals
o Each horizontal line represents one orbital that can hold two electrons
o For each pair of atomic orbitals that combine, one lower-energy (bonding) molecular orbital and
one higher-energy (antibonding) orbital result
 i.e. σ2s and σ2s*
o Fill the orbitals by the Aufbau principle
 Lower-energy orbitals fill first, electrons spread out among degenerate orbitals before
pairing, and each orbital can hold a maximum of two electrons with opposite spins
Bond Order
o Used to indicate bond strength
number of bonding electrons − number of antibonding electrons
o bond order =
2
o Higher bond order = stronger bond
o Bond order = 0 means the molecule does not exist
Paramagnetism
o Substance is attracted into the inducing magnetic field
o Associated with unpaired electrons
Diamagnetism
o Substance is repelled from the inducing magnetic field
o Associated with paired electrons