Chapter 9 — Hybrid Orbitals
Two Theories of Bonding
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•  VALENCE BOND THEORY
Sigma Bond Formation by
Orbital Overlap
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Two s orbitals
overlap
— Linus Pauling
•  valence electrons are localized
between atoms (or are lone pairs).
•  half-filled atomic orbitals overlap to
form bonds.
Change in electron
density as two
hydrogen atoms
approach each other.
•  Molecular Orbital Theory
•  valence electrons are delocalized
•  Valence electrons are in molecular
orbitals spread over entire molecule
Linus Pauling,
1901-1994
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Sigma Bond Formation
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Valence Bond Theory
Two s
orbitals
overlap
One s and
one p orbital
overlap
Atomic Orbitals; Hybridization
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Formation of sp Hybrid Orbitals
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•  In molecules, the orbitals occupied by
electron pairs are seldom “pure” s or p
orbitals.
•  Instead, they are “hybrid” orbitals, formed by
combining s, p, and d orbitals.
2s
•  s orbital + p orbital → two sp hybrids
2s
2p
Be in BeCl2 (↑↓) ( )( )( )
Two p
orbitals
overlap
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2p
hydridize
rearrange
electrons
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Chapter 9 — Hybrid Orbitals
Hybridization – mixing of two or more atomic
orbitals to form a new set of hybrid orbitals.
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1.  Mix at least 2 nonequivalent atomic orbitals (e.g. s and p).
Hybrid orbitals have very different shape from original
atomic orbitals.
How do I predict the hybridization of the
central atom?
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3.  Covalent bonds are formed by:
a.  Overlap of hybrid orbitals with atomic orbitals
b.  Overlap of hybrid orbitals with other hybrid orbitals
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sp
sp2
sp3
sp3d
BeCl2
BF3
CH4, NH3, H2O
PCl5
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sp3d2
SF6
•!•!
•!•!
•!•!
•!
Ø •!
•!O = C = O •! sp hybridization for carbon
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•  sigma bond: the electron density is concentrated
along the bond axis and is symmetrical about it.
•  pi bonds: the “extra” electron pairs in a multiple bond
are located in unhybridized orbitals which are not
concentrated along the bond axis. Instead, they are
concentrated in lobes “north” and “south” of the axis.
•  CO2 two sigma bonds; two pi bonds
•  SO2 two sigma bonds; one pi bond
•  N2 one sigma bond; two pi bonds
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•!•!
•!•!
Sigma and Pi Bonds
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•!•!
2
Ø •!
•!O – S = O •!
•! sp hybridization for sulfur
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•  Unshared pairs can be hybridized (H2O, NH3).
•  Only one of the electron pairs in a multiple
bond is hybridized.
Count the number of lone pairs AND the number of atoms
bonded to the central atom
# of Lone Pairs +
# of Bonded Atoms
Hybridization
Examples
2.  Number of hybrid orbitals is equal to number of pure
atomic orbitals used in the hybridization process.
Unshared Pairs
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Chapter 9 — Hybrid Orbitals
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Pi bond (π) – electron density above and below plane of nuclei
of the bonding atoms
Sigma bond (σ) – electron density between the 2 atoms
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sp3d
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sp3d
•  atom with 5 electron domains
– trigonal bipyramid electron
geometry
•  use empty d orbitals from valence
shell
•  unhybridized d orbitals can be
used to make π bonds
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Chapter 9 — Hybrid Orbitals
sp3d2
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sp3d2
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•  atom with 6 electron domains
– octahedral electron geometry
•  use empty d orbitals from valence
shell
There is restricted rotation around C=C bond.
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Experiments show O2 is paramagnetic
O
O
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Consequences of Multiple
Bonding: cis-, trans- isomers
No unpaired eShould be diamagnetic
Liquid Nitrogen v. Liquid Oxygen
You Tube Video
PLAY Kotz MOVIE
Molecular orbital theory – bonds are formed from interaction
of atomic orbitals to form molecular orbitals.
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Consequences of Multiple
Bonding
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