Download Bonding and Molecular Structure: Orbital Hybridization and

Covalent Bonding: Orbitals
Chemical bonding
• Valence bond theory (VB)
– Bonding electron pairs
– Lone pairs of electrons localized on a particular atom
– Provides qualitative, visual picture of molecular structure
and bonding in ground state
• Molecular orbital theory (MO)
– Molecular orbitals spread out ,“delocalized”, over the
molecule
– Atomic orbitals combine to form a set of orbitals which are
property of molecule
– Provides quantitative picture of bonding, describe molecules
in excited states, and only way to describe bonding for a few
molecules (NO and O2)
Valence Bond Theory
• Bonds are formed by overlap of atomic orbitals
• Electron cloud distorted
• Electrons have higher probability of being in region between
two bonded atoms
• σ (sigma) bond: electron density greatest along bond axis
• H2
• F2
Exercise
• Draw the Lewis structure for methane
– What is the shape of the methane molecule
– What are the bond angles
EXERCISE!
Draw the Lewis structure for methane, CH4.
– What is the shape of a methane molecule?
tetrahedral
– What are the bond angles?
109.5o
H
H C H
H
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CONCEPT CHECK!
What is the valence electron configuration of a
carbon atom?
s2p2
Why can’t the bonding orbitals for methane be
formed by an overlap of atomic orbitals?
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Bonding in Methane
• Assume that the carbon atom has four equivalent
atomic orbitals, arranged tetrahedrally.
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Hybridization of Atomic Orbitals
• CH4
– 4 equivalent bonding pairs, bond angle = 109.5°
– C: 1s22s22p2, 2 unpaired electrons in p orbitals are at 90 °
– Need a different way to look at bonding
• Orbital Hybridization
– Hybrid orbitals : created by combining s, p and/or d orbitals
– 2 important principles:
• Number of hybrid orbitals is always the same as the
# of atomic orbitals used to create the hybrid orbitals
( 1s + 3p gives 4 sp3 hybridized orbitals)
• Hybrid orbitals are more directed from the central atom
toward terminal atom than atomic orbitals
Hybrid Orbitals
– A hybrid orbital is required for each electron pair on central
atom
https://www.youtube.com/watch?v=SJdllffWUqg
An Energy-Level Diagram Showing the Formation of
Four sp3 Orbitals
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The Formation of sp3 Hybrid
Orbitals
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Tetrahedral Set of Four sp3 Orbitals
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sp Hybrid orbital
• One s and one p orbital gives 2 sp hybrid orbitals
sp2 Hybrid orbital
– One s and 2 p orbitals gives 3 sp2 hybrid orbitals
sp3 Hybrid orbital
• One s and 3 p orbitals gives 4 sp3 orbitals
NH3 and H2O
• 4 sp3 hybridized orbitals
• Lone pairs occupy hybrid orbitals
Hybrid orbitals involving d orbitals
• Maximum of 4 hybrid orbitals can be created from s
and p orbitals
• How explain PCl5 or SF6
• Hybridize s, p, and d orbitals
– d orbitals are valence shell orbitals for main group elements
period 3 and higher
sp3d hybrid orbitals
• PCl5
–
–
–
–
Electron pair geometry = 5
5 sp3d hybridized orbitals
Can’t form π bonds
Electron pair geometry is trigonal bipyramidal
sp3d2 hybrid orbitals
• SF6
– Electron pair geometry = 6
– 6 sp3d2 hybridized orbitals
– Electron pair geometry is octahedral
Multiple Bonds
• Where is unhybridized p orbital
• Sigma (σ)bonds form from overlap of orbitals along bond axis
• Pi (π) bond form between adjacent p orbitals above and below
bond axis
C2H4
• One C-C σ bond
• One C-C π bond
• Four C-H σ bonds
C2H2
• One C-C σ bond
• Two C-C π bond
• Two C-H σ bonds
H-CC-H
7.35
Identify each of the following sets of hybrid orbitals:
The following ball-and-stick molecular model is a representation of acetaminophen,
the active ingredient in such over-the-counter headache remedies as Tylenol (red =
O, gray = C, blue = N, ivory = H):
(a) What is the molecular formula of acetaminophen?
(b) What is the geometry around each carbon and nitrogen? (The lines between
atoms indicate connections only, not whether the bonds are single, double, or
triple.)
(c) What is the hybridization of each carbon and nitrogen?
Molecular Orbital Theory
• Valence Bond Theory explains a lot about bonding.
• MO theory assumes atomic orbitals combine to form
molecular orbitals which are spread out or “delocalized” over
several atoms
• MO theory can account for properties not explainable by
valence bond theory
– Oxygen’s paramagnetism
– Resonance
– Metal conductivity
• MO theory
– Determine sets of molecular orbitals
– Molecular orbitals encompass all atoms of molecule
– Valence electrons for all atoms assigned to molecular orbitals using
same principles as assigning electrons to atomic orbitals.
H2
• 2 s orbitals form 2 molecular orbitals
– Addition of atomic orbitals results in increased probability
of electron reside in bond region between atoms (bonding
molecular orbital) σ bond
– Subtraction of atomic orbitals results in reduced probability
of finding electron between nuclei, but higher elsewhere
(antibonding molecular orbital) σ* bond
The Molecular Orbital Model (H2)
Energy
MO2 (σ*)
1s
MO1(σ)