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Unformatted text preview: Chem 210 / Chapter 9B
Valence Bond Theory Chapter 9.4 – 9.6
Bonding Theories: Valence Bond Theory Valence Bond Theory (VBT) Orbital overlap and spin pairing in 3 diatomic molecules.
The bond in H2 is
the overlap of 1s
(H#1) and (H#2). Theory to explain covalent
bonding. Basic idea:
“Covalent bonds are formed by
the overlap of atomic orbitals,
one from each of the two
bonding atoms.” 9.4 Covalent Bonding and Orbital Overlap
9.5 Hybrid orbitals
9.6 Multiple Bonds Hydrogen fluoride, HF
The bond in HF is the overlap
of the 1s (H) and the 2p (F).
2p Two wave functions are in
phase so the amplitude
increases between the nuclei. The Central Themes of VB Theory
A set of overlapping orbitals has a maximum of
two electrons that must have opposite spins.
The greater the orbital overlap, the stronger
(more stable) the bond.
The valence atomic orbitals in a molecule are
different from those in isolated atoms.
• A hybridization of atomic orbitals may occur so
that the molecule has the observed shape
(which is predictable by the VSEPR theory).
VSEPR Orbital Hybridization
Mixing of two or more atomic
orbitals to form a new set of hybrid
orbitals with new shape / orientation.
1. Two or more atomic orbitals (nonequivalent,
such as s ↔ p) mix to yield hybrid orbitals of
very different shape from the original atomic
orbitals. Fluorine, F2
The bond in F2 is the overlap
of 2p(F#1) & 2p(F#2).
of 2p(F#1) Hybrid Orbitals ↔ Molecular Shape
Key Points The number of hybrid orbitals obtained equals the number
of atomic orbitals mixed.
The type of hybrid orbitals obtained depends on the
atomic orbitals mixed.
The set of hybrid orbitals determines molecular shape.
Types of Hybrid Orbitals sp sp2 sp3 sp3d sp3d2 2. Number of hybrid orbitals is equal to number of
original atomic orbitals used in the hybridization
process. Movie on hybridization
http://www.chem.ufl.edu/~myers/chm2045/shapes.htm The sp
sp two hybrid orbitals
sp & sp
two atomic orbitals:
2s & 2p The sp
sp Hybridization of orbitals
characteristics of each atomic orbital. hybrid orbitals Energy hybrid orbitals in gaseous BeCl2 (continued). sp Hybrid Orbitals in BeCl2
Valence orbitals undergo sp
hybridization in Be so to make two
sp orbitals at 180o to overlap with
the p orbital in each Cl atom. orbital box
contours in gaseous BeCl2. two hybrid orbitals
sp & sp Each covalent bond: Be-Cl is made by
the overlap of: 3p (Cl) & sp (Be).
3p (Cl) F09 / Wang / UM-SJTU JI paired in 2s “promotion”
for overlap /
energy. unpaired in sp This explains why BeCl2
is linear, with 180o
bond angle. 1 Chem 210 / Chapter 9B
Valence Bond Theory The sp2 Each
Each bond is
made by the
sp2 (B) ↔ 2p(F). 3 electrons in
2s & 2p
unpaired in 3
energy of sp2
lower than 2p,
higher than 2s. The sp3
sp sp2 Hybrid
Orbitals in BF3
BF hybrid orbitals in BF3. • Valence orbitals undergo sp2
hybridization in B so to make
2 orbitals at 120o to
overlap with the p orbital in each
• Molecular shape is planar triangle
on the plane defined by the three
sp The sp3
sp hybrid orbitals in CH4.
in Four atomic
One s orbital
Three p orbitals
Mix (hybridize) to
orbitals of sp3
Each has the same
They spread in
orbital hybridization The sp3 hybrid orbitals in NH3.
in hybrid orbitals in H2O. 2s22p2 one
unpaired to occupy 4
equal sp2 orbitals; energy
between 2s and 2p. 4 bonds made each by the
overlap of a sp3 (C) and 1s (H).
Accounts for the tetrahedral
geometry of the molecule. sp3 Hybrid Orbitals in water & ammonia 1. Where is the lone pair located?
2. Point out differences between
these diagrams and those from
CH4. The sp3d Oxygen undergoes sp3 hybridization
to yield 4 sp3 orbitals.
2 of the 4 is each occupied by a lone
2 covalent bonds are made by the
overlap of sp3 (O) and 1s (H).
1s (H). hybrid orbitals in PCl5 (and SF4 & ClF3.)
in The sp3d2 hybrid orbitals in SF6.
promotion Lone pairs exert stronger repulsion than
bonding pairs. Bond angles in H2O and NH3 are
less than the normal tetrahedral angles: 109.5o. F09 / Wang / UM-SJTU JI promotion SF4
SF4 ClF3 2 Chem 210 / Chapter 9B
Valence Bond Theory VBT: orbital hybridization Hybrid Orbitals When s and one p orbital mix
2 orbitals at 180° (linear)
When s and two p orbitals mix
3 orbitals at 120° (trigonal planar)
When s and three p orbitals mix
4 orbitals at 109.5° (tetrahedral)
When s, three p orbitals and one d orbital mix
5 orbitals at 120° and 90° (trigonal bipyramidal)
When s, three p and two d orbitals mix
6 orbitals at 90° (octahedral) The conceptual steps from molecular formula to the
hybrid orbitals used in bonding.
formula Step 2
structure Step 3
and e- group
orbitals Hybrid Orbitals Ethane, CH3CH3 Ethane, CH3-CH3
• Four single bonds around each carbon
•Single bonds are called σ (sigma) bonds.
• Tetrahedral geometry around each carbon C linear 2 orbitals
trigonal planar 3 orbitals sp2 hybrid
tetrahedral 4 orbitals sp3 hybrid
trigonal bipyramidal 5 orbitals
octahedral 6 orbitals sp3d2 hybrid
orbitals sp • Each Carbon has sp3 hybridized orbitals!
sp The σ bonds in ethane(C2H6).
both C are sp3 hybridized
s-sp3 overlaps to σ bonds SAMPLE PROBLEM 1(a) Use partial orbital diagrams to describe mixing of the
atomic orbitals of the central atom leads to hybrid
orbitals in each of the following:
(a) Methanol, CH3OH The groups around C are
arranged as a tetrahedron. H
HH sp3-sp3 overlap to form a σ bond 1s orbital
of H sp3
hybrids Each C-H bond is
made by the
overlap of sp3 (C)
and 1s (H). C The C-C bond in the middle is made by the
overlap of sp3 (1st C) and sp3 (2nd C) .
All overlaps are of the σ (sigma) type, in which
Overlapping occurs between the two nuclei.
PROBLEM 1(b) Use partial orbital diagrams to describe mixing of the
atomic orbitals of the central atom leads to hybrid orbitals: (b) Sulfur tetrafluoride, SF4
SF4 has a seesaw shape with 4 bonding & 1 nonbonding epairs. F
F O also has a tetrahedral
arrangement with 2
nonbonding e- pairs.
3d relatively even
distribution of electron
density over all σ bonds. 2p
single C atom 2s sp3 hybridized
C atom 2p sp3 hybridized
O atom 3p
sp3d 2s single O atom 3s F09 / Wang / UM-SJTU JI 3d S atom hybridized S
atom 3 Chem 210 / Chapter 9B
Valence Bond Theory Multiple Bonds = 1σ + 1 or 2π
All Single bonds ← σ bonds
→ σ-Bond: electron density lies on the axis between Two types of
pi (π) bond
bond Multiple Bonds = 1σ + 1 or 2π
π-Bonds: electron density lies above and below
the plane of the nuclei. SideSide-ways overlap the nuclei. Pi overlap:
axis Sigma overlap: right on the axis endend-on overlap Compare and contrast the two types
of orbital overlaps in VBT:
The sigma (σ) type The Pi (π) type 1st bond formed occurs after sigma bond
The 2nd in a double bond
The 2nd and 3rd in a triple bond Ethene (ethylene), CH2=CH2
• trigonal planer geometry
• 3 single bonds Overlap region:
cross the line
between 2 nuclei and π bonds in ethylene (C2H4).
in 2 σ (sigma) bonds around each C
Need 3 hybrid orbitals
1 C-to-C π (pi) bond
less stable p overlap
-π Not on the line Stronger
More stable • The σ overlap in one position - σ Explain why “π electrons” are more
reactive than “σ electrons.” π electron density Restricted rotation of π-bonded molecules
in C2H2Cl2 (as well as C2H4).
π electrons” are more reactive than “σ
electrons” because they are more
“exposed” to other chemical species, which
may approach from the top or bottom of the
plane of ethene molecule. “
“ F09 / Wang / UM-SJTU JI Ethyne (acetylene), CHΞCH
• Linear geometry π electron cloud” in this diagram is found
both above and below the plane of the atoms. 2 sigma bonds around each carbon
• 2 C-C pi bonds CIS TRANS These are two different compounds – They can
interconvert only by first breaking the pi bond!
pi bond! 4 Chem 210 / Chapter 9B
Valence Bond Theory The σ and π bonds in acetylene (C2H2).
in Electron density and bond order. VBT Diagram of Acetylene, H-CΞC-H overlap in one position - σ p overlap - π Each carbon
Each carbon in C2H2 makes two sp
hybrid orbitals by mixing one s and
one p orbital. The sp orbitals are
then used to make two sigma bonds.
Two 2p orbitals are left alone in each
carbon, which overlaps to make two
pi bonds SAMPLE PROBLEM 2
ANSWER VBT explanation of ethyne
(acetylene) 1. There are:
2 sigma bonds made by sp (C) overlap with
1 sigma bond made by sp(1st C) overlap with
2.Each carbon in ethyne has two p orbitals left
over (not hybridized). These p orbitals overlap
to make two pi (π) bonds between the two
3.The pi electrons are more reactive than the
sigma electrons, because they are more
“exposed”. A View of the π bonds in benzene,
H H H All 6 p orbitals overlap to
make 3 pi bonds shared
by all 6 Carbon atoms.
The pi electrons are
delocalized, resulting in
a very stable structure! H σ bonds Best represented by benzene (C6H6)
• a six carbon ring
• three alternating single & double bonds
• electrons in π bonds spread over the
• exists as average, resonance structure
resonance O C
H3 C CH3
π bond Another example of Resonance structures
& delocalized electrons: O3. H H sp3
sp C sp C sp3
sp 3 H
H sp3 Aromatic hydrocarbons 6e in the 3 pi
among all 6
Carbon atoms. H H SAMPLE PROBLEM 2 1. Identify the sigma bonds in ethyne by specifying the
atomic orbitals that are overlapping to form each one.
2. Identify the pi bonds in ethyne. How many are there?
3. Are the pi electrons in ethyne more or less reactive than
the sigma electrons? Why? SAMPLE PROBLEM 3 Describe the types of bonds and orbitals in acetone,
(CH3)2CO. Delocalized π Bonding in C6H6 All C atoms have
sp2 hybridization H Electron density between the two nuclei (related
to bond strength) increases with bond order. Why
then is it easier for the C≡C bond in alkynes and
the C=C bond in alkenes to react chemically than
the C-C bond in in alkanes?
The reactivity of C=C and C≡C bonds is based on
the fact that pi bonds are more exposed and
weaker than sigma bonds, not on the overall bond
strengths, as shown in the above graphs. H H H F09 / Wang / UM-SJTU JI 5 Chem 210 / Chapter 9B
Valence Bond Theory A Third Example of
Delocalized π Bonding
in CO3-2 SAMPLE PROBLEM 4 SAMPLE
PROBLEM 5 Explain the bonding in C3H4 by the VBT theory. 1. Draw Lewis structure for the polyatomic ion.
2. How many sigma bonds and pi bonds are present
in this ion?
3. What orbitals overlap to make the sigma bonds? F09 / Wang / UM-SJTU JI 6 ...
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