two bonding theories valence bond theory (localized electron model) –electrons in a molecule still...
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Two Bonding Theories
• Valence Bond Theory (localized electron model)– Electrons in a molecule still occupy
orbitals of individual atoms.
• Molecular Orbital Theory– Says that atomic orbitals no longer
exist. Molecular orbitals are available for occupation by electrons.
Valence Bond Theory
• Electrons in a molecule still occupy orbitals of individual ATOMS.
• Half-filled orbitals of two atoms overlap.
VB Theory: Formation of H2S
VB Theory: Formation of H2S
VB Theory: Carbon
2s 2p
Would suggest only two bonds possible
VB Theory: Carbon
2s 2p
What if we just bump an electron up?
Would suggest two different types of bonds form (one with an s orbital and three with p orbitals)
Valence Bond Theory• Atomic Orbitals Hybridize.
• These are still atomic orbitals.• Two or more nonequivalent orbitals
of same atom combine.• When X number of atomic orbitals
hybridize, X number of hybrid orbitals will result.
2s 2p
Four atomic orbitals (one s and three p) hybridize into four degenerate hybrid orbitals, as a group called sp3
2sp3
VB Theory: Carbon
How will four degenerate orbitals exist around the central atom?
How will four degenerate orbitals exist around the central atom?
VB Theory: NH3
VB Theory: sp3 hybridization
VB Theory: sp2 hybridization
VB Theory: sp2 hybridization
VB Theory: sp2 hybridization
C2H4, sp2 hybridization
Valence Bond TheoryHybridization is directly connected to
the Electron-pair Geometry
sp3 sp2 sp = linear
sp3d = trig bipryramidalsp3d2 = octahedral
= tetrahedral= trigonal planar
Molecular Orbital Theory
• Says that atomic orbitals no longer exist.
• Molecular orbitals are available for occupation by electrons.
• Atomic orbitals from all atoms overlap and become an equivalent number of molecular orbitals.
Molecular Orbital TheoryConstructive vs. Destructive
Interference
Molecular Orbital Theory
Molecular Orbital Energy Diagram
for H2
Molecular Orbital Energy Diagramfor He2
Molecular Orbital Theory
Molecular Orbital Theory
Molecular Orbital Theory
Molecular Orbital Theory
• Sigma Bond – electron density along line joining two nuclei
- from the side, would look like an s orbital
- first bond that forms between two nuclei (single bond)
bonding: s1s antibonding: s*1s
s*2ps2p
Molecular Orbital Theory
• Pi Bond – formed by sideways overlapping of orbitals, electron density above and below plane of nuclei
– second / third bond that forms between two nuclei (double / triple bonds)
- less overlap means weaker than sigma bonds
sp
C Atom
H
C
HHH
2s 2p
Promotion Step
C Atom
2s 2p
Hybridization step ( leads to energy release after bonding, since more bonds can be formed)
sp3
For 4 groups: tetrahedral
electron pair geometry
C C
Hybridization of Carbon
For 3 groups: trigonal planar
electron pair geometry
H
H
H
H
For 2 groups: linear
electron pair geometry
sp2 pz
C
py
leftover
H
H
sp py, pzTwo hybridized
AO'sleftover
sp
Example: CH4
Use to form 3 single () bonds
Use pz to form 1
pi () bond
H C
Example: C2H4
bond
C
Each C forms three bonds and
one bond. Bond angles are 120o.
H
xy plane
Use to form 2 single ()
bonds
Use py and pz to
form 2 pi() bonds
Example: C2H2
Each C forms two bonds
and two bonds (which are
perpendicular to each other).
Bond angles are 180o.
( requires energy)
Three hybridized AO's
Four hybridized AO's
pzxy plane
C
H
pz
H
C CH H
Use to form 4 single (sigma = ) bonds
sp2
sp2sp2
sp3 sp3
sp3
sp3
C forms four bonds.
Bond angles are 109.5o.
from Susan Piepho,
Sweet Briar College
http://lhs2.lps.org/staff/squiring/chemistry/Chap15_16.2/CarbonHybridization.html
http://lhs2.lps.org/staff/squiring/chemistry/Chap15_16.2/CarbonHybridization.html