Daniel L. Reger
Scott R. Goode
David W. Ball
http://academic.cengage.com/chemistry/reger
Chapter 10
Molecular Structure and
Bonding Theories
VSEPR
Valence-Shell Electron-Pair
Repulsion Model (VSEPR) predicts
shape from Lewis Structures.
• VSEPR Rule 1: A molecule has a shape
that minimizes electrostatic repulsions
between valence-shell electron pairs.
• Minimum repulsion results when the electron
pairs are as far apart as possible.
Steric Number
• Steric number = (number of lone
pairs on central atom) + (number of
atoms bonded to central atom)
• The steric number is determined from
the Lewis structure.
• Steric number determines the
bonded-atom lone-pair
arrangement, the shape that
maximizes the distances between
the valence-shell electron pairs.
Geometric Arrangements
Geometric Arrangements
Steric Number = 2
• In the Lewis structure of BeCl2,
Cl
Be
Cl
beryllium has two bonded atoms and no
lone pairs, steric number = 2.
• A linear geometry places the two pairs of
electrons on the central beryllium atom as
far apart as possible.
Molecules with Multiple Bonds
• The Lewis structure of HCN (H-CN:)
shows that the carbon atom is bonded to
two atoms and has no lone pairs, steric
number = 2.
• The bonded-atom lone-pair arrangement
is linear.
• The number of bonded atoms, not the
number of bonds, determines the steric
number.
Steric Number = 3
• The Lewis structure of BF3
F
B
F
F
shows the boron atom has a steric
number = 3; the bonded-atom lone-pair
arrangement is trigonal planar.
Steric Number = 4
• The Lewis structure
of CH4
H
H
C
H
H
shows the carbon
atom has a steric
number = 4; the
bonded-atom lonepair arrangement is
tetrahedral.
Steric Number = 5
• The phosphorus
atom in PF5 has
a steric number
= 5; the bondedatom lone-pair
arrangement is
trigonal
bipyramidal.
Steric Number = 6
• The sulfur atom in
SF6 has a steric
number = 6; the
bonded-atom
lone-pair
arrangement is
octahedral.
Central Atoms with Lone Pairs
• The Lewis structure of H2O is
H
O
H
• Steric number = 4, 2 bonded atoms and
2 lone pairs.
• The bonded-atom lone-pair arrangement
is tetrahedral.
Molecular Shape of H2O
• Molecular shape is the
arrangement of the
atoms in a species.
• The bonded-atom lonepair arrangement of
H2O is tetrahedral (top);
the molecular shape is
bent or V-shaped
(bottom).
Molecular Shape of NH3
• What is the electron pair geometry and
molecular shape of NH3?
Electron Pair Repulsions
• The measured bond angle in H2O (104.5o)
is smaller than the predicted angle (109.5o)
• Explanation: repulsions vary lone pair-lone
pair > lone pair-bonding pair > bonding
pair-bonding pair
Location of Lone Pair in SF4
Two structures
are possible:
• The favored structure for a trigonal
bipyramid minimizes 90o lone pair
interactions – the one on the right.
Lone Pairs in Trigonal Bipyramids
• Lone pairs always
occupy the
equatorial positions
in a trigonal
bipyramid so that
lone pair-lone pair
repulsions are
oriented at 120o.
Location of Lone Pairs in XeF4
• The structure on right has no 90o lone
pair-lone pair interactions and is favored.
Test Your Skill
• What is the steric number, the bondedatom lone-pair arrangement, and the
molecular shape of ClF3?
Multiple Central Atoms
• The geometry of each central atom is
determined separately.
• The CH3 carbon in CH3CN has
tetrahedral geometry and the other
carbon has linear geometry.
Shapes of Molecules
• What are the bonded-atom lone-pair
arrangements and the shapes about each
central atom in NH2SH?
• Draw the Lewis structure.
H
H
N S
H
• The bonded-atom lone-pair arrangements of
both are tetrahedral, the nitrogen shape is
trigonal pyramidal and sulfur is “V” shaped.
Overall Shape of C2H4
• Ethylene, C2H4 , could be planar (left)
or nonplanar (right). The VSEPR
model does not predict which is
preferred.
Polarity of Molecules
• The bond dipoles in CO2 cancel
because the linear shape orients the
equal magnitude bond dipoles in
exactly opposite directions.
Polarity of Molecules
• The bond dipoles do not cancel in COSe;
they are oriented in the same direction
and are of unequal length. They do not
cancel in OF2 because the V-shape of the
molecule does not orient them in opposite
directions.
Polarity of Molecules
• The bond dipoles in BCl3 and CCl4
cancel because of the regular shape
and equal magnitude.
Polarity of Molecules
• The bond dipoles in BCl2F and CHCl3 do
not cancel because they are not of the
same magnitude.
Test Your Skill
• Are the following molecules polar or
nonpolar: H2S, SiF4, CH2Cl2?
Valence Bond Theory
• Valence bond
theory describes
bonds as being
formed by overlap
of partially filled
valence orbitals.
Test Your Skill
• Identify the orbitals that form the bond in
HCl.
Bonding in NH3
• The observed bond angles of 107.5o in
NH3 are not consistent with the angles of
90o expected if the bonds formed from N
2p orbitals.
Hybrid Orbitals
• Hybrid orbitals are orbitals obtained by
mixing two or more atomic orbitals on
the same central atom.
• Appropriate hybrid orbitals formed by
mixing one s and xp atomic orbitals
make bonds at either 180o (x = 1), 120o
(x = 2), or 109.5o (x = 3).
Analogy for Hybrid Orbitals
sp Hybrid Orbitals
Shape of Hybrid Orbitals
• For clarity, hybrid orbitals are pictured as
elongated with the small lobe omitted.
Bonding in BeCl2
• The bonds in BeCl2 arise from the
overlap of two sp hybrid orbitals on the
beryllium atom with the 3p orbitals on the
two chlorine atoms.
sp2 Hybrid Orbitals
Bonding in BF3
• The bonds in BF3 arise from the overlap
of three sp2 hybrid orbitals on the boron
atom with 2p orbitals on the three
fluorine atoms.
sp3 Hybrid Orbitals
Bonding in CH4
• The bonds in CH4 arise from the overlap
of four sp3 hybrid orbitals on the carbon
atom with 1s orbitals on the four
hydrogen atoms.
Lone Pairs and Hybrid Orbitals
• Hybrid orbitals can hold lone pairs
as well as make bonds.
Hybridization with d Orbitals
• Hybrid orbitals of central atoms with steric
numbers of 5 or 6 involve d orbitals.
Hybrid Orbitals
Steric
Number
2
3
4
5
6
Electron pair Hybridization
geometry
sp
linear
2
trigonal planar
sp
tetrahedral
sp3
trigonal
sp3d
bipyramid
3 2
octahedral
sp d
Test Your Skill
• Identify the hybrid orbitals on the central
atoms in SiH4 and HCN.
Types of Bonds: Sigma Bonds
• Sigma bonds (s): the
shared pair of
electrons is
symmetric about the
line joining the two
nuclei of the bonded
atoms.
Bonding in C2H4
• The C-C sigma bond in C2H4 arises from
overlap of sp2 hybrid orbitals and the four C-H
sigma bonds from overlap sp2 hybrid orbitals on
C with 1s orbitals on H.
• The second C-C bond forms from sideways
overlap of p orbitals.
Types of Bonds: Pi Bonds
• Pi bonds (p) places electron density
above and below the line joining the
bonded atoms – they form by sideways
overlap of p orbitals.
Bonding in C2H4
• The double bond in C2H4 is one
sigma bond and one pi bond – each
bond is of similar strength.
Proof of Pi Bonds: Shape of C2H4
• C2H4 is planar (A) because pi overlap is at a
maximum. Rotation of one end by 90o (B)
reduces pi overlap to zero.
Triple Bonds
• The triple bond in C2H2 is one sigma
bond and two pi bonds between the sp
hybridized carbon atoms.
Sigma Bonds in Benzene
• Each carbon atom in benzene, C6H6,
forms three sigma bonds with sp2 hybrid
orbitals.
Pi Bonds in Benzene
• The remaining p orbital on each carbon
atom (top) overlap to form three pi bonds.
Test Your Skill
• Describe the bonds made by the
carbon atom in HCN.
Molecular Orbital Theory
• Molecular orbital theory is a model that
combines atomic orbitals to form new
molecular orbitals that are shared over the
entire molecule.
• A bonding molecular orbital concentrates
electron density between atoms in a
molecule.
• An antibonding molecular orbital reduces
electron density between atoms in a
molecule.
Hydrogen Molecule
• Addition of the 1s orbitals of two H atoms
forms a sigma bonding molecular orbital
and subtraction forms a sigma antibonding
molecular orbital, indicated with a * symbol.
Molecular Orbital Diagram: H2
• Bonding molecular orbitals are more stable
and antibonding molecular orbitals are less
stable than the atomic orbital that are
combined.
Bond Order
• Bond order = 1/2 [number of electrons
in bonding orbital - number of electrons
in antibonding orbitals]
• Bond order in H2 = 1/2 [2 - 0] = 1
Molecular Orbital Diagram: He2
• Bond order in He2 = 1/2 [2 - 2] = 0;
the molecule does not form.
Sigma Molecular Orbitals from p
Pi Molecular Orbitals from p
MO Diagram Second-Period Diatomics
Molecular Orbital Diagram: N2
• The electron
configuration is
(s2s)2(s*2s)2(p2p)4(s2p)2.
• The bond order in N2 is
three and there are no
unpaired electrons.
• Lewis theory (:NN:)
predicts the same
result.
Molecular Orbital Diagram: Be2
• The electron
configuration is
(s2s)2(s*2s)2 .
• Bond order in Be2
is zero and the
molecule does not
exist.
Molecular Orbital Diagram for O2
• Draw the molecular orbital diagram
of O2. What is the electron
configuration, the bond order and
how many unpaired electrons are
present?
Test Your Skill
• Draw the molecular orbital diagram of
B2. What is the electron configuration,
the bond order and number of unpaired
electrons?