AP Chemistry
Mr. Markic
Chapter 10 - Molecular Geometry and Hybridization of Atomic Orbitals
Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding)
pairs.
Molecules in Which the Central Atom Has One or More Lone Pairs
• If the central atom has both lone pairs and bonding pairs
• There are 3 types of repulsive forces
1. Those between bonding pairs
2. Those between lone pairs
3. Those between a bonding pair and a lone pair
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AP Chemistry
Mr. Markic
Predicting Molecular Geometry
1. Draw Lewis structure for molecule.
2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom.
3. Use VSEPR to predict the geometry of the molecule.
Sample Exercise
Use the VSEPR model to predict the geometry of the following molecules and ions:
(a) AsH3
(c) AlCl4-
(b) OF2
(d) I3-
(e) C2H4
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AP Chemistry
Mr. Markic
Use the VSEPR model to predict the geometry of
(a) SiBr4
(b) CS2
(c) NO3-
Review of Concepts
Which of the following geometries has a greater stability for tin (IV) hydride (SnH4)?
Dipole Moments
• Bond polarity is a measure of how equally the e- in a bond are shared between the two atoms of the bond
• As the difference in electronegativity between the two atoms increases, so does the bond polarity
• Dipole moment is a quantitative measurement of the amount of charge separation in the molecule
• The dipole moment depends on both the polarities of the individual bonds and the geometry of the molecule
• Bond dipoles and dipole moments are vector quantities (magnitude and direction)
Dipole Moments and Polar Molecules
electron poor
region
H
electron rich
region
F


Behavior of Polar Molecules
(a) In the absence of an external electric field
(b) When the electric field is turned on.
• Nonpolar molecules are not affected by an electric field
Bond moments and resultant dipole moments in NH3 and NF3. The electrostatic potential maps show the electron
density distributions in these molecules
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AP Chemistry
Sample Exercise
Predict whether each of the following molecules has a dipole moment:
(a) BrCl
(b) BF3 (trigonal planar)
Mr. Markic
(c) CH2Cl2 (tetrahedral)
Does the AlCl3 molecule have a dipole moment?
Review of Concepts
Carbon dioxide has a linear geometry and is nonpolar. Yet we know that the molecule executes bending and stretching
motions that create a dipole moment. How would you reconcile these conflicting descriptions about CO2?
Valence Bond Theory
• When atoms share e• Sharing concentrates e- density between the nuclei
• The valence orbital of one atom merges with that of another atom
• The valence orbitals are then said to share a region of space, or to overlap
• The overlap of the orbitals allows 2e- of opposite spin to share a common space between the nuclei
• Two quantum mechanical theories are used to describe covalent bond formation and the electronic structure of
molecules
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AP Chemistry
Mr. Markic
1. Valence bond theory
• Electrons in a molecule occupy atomic orbitals
of the individual atoms
• A picture of individual atoms taking part in bond
formation
2. Molecular orbital
• Formation of molecular orbitals from the
atomic orbitals
• As atoms approach each other, each electron is
attracted by the nucleus of the other atom, at
the same time, the electrons repel each other,
as do the nuclei
• Attraction is stronger than repulsion – the
potential energy of the system decreases
(stable)
Change in potential energy of two H atoms with their distance of separation. At the point of minimum potential energy,
the H2 molecule is in its most stable state and the bond length is 74 pm. The spheres represent the 1s orbitals.
Top to bottom: As two H atoms approach each other, their 1s
orbitals begin to feel the attraction of the other proton.
Gradually, the electron density builds up in the region
between the two nuclei (red color). Eventually, a stable H2
molecule is formed when the internuclear distance is 74 pm
Hybridization of Atomic Orbitals
• Explains both the formation of e- pair bonds and the observed geometries of the molecules
• Atomic orbitals (s, p, and sometimes d) on an atom mix to form new orbitals called hybrid orbitals
• These hybrid orbitals have different shapes than the original atomic orbital
• The process of mixing (thereby changing the atomic orbital shape) as atoms form bonds is called hybridization
• Hybrid orbitals can accommodate nonbonding pairs
• The total number of atomic orbitals remain constant
• A mode of hybridization can be associated with each of the five common e- domain geometries
Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals.
1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from
original atomic orbitals.
2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process.
3. Covalent bonds are formed by:
a. Overlap of hybrid orbitals with atomic orbitals
b. Overlap of hybrid orbitals with other hybrid orbitals
How do I predict the hybridization of the central atom?
1. Draw the Lewis structure of the molecule.
2. Count the number of lone pairs AND the number of atoms bonded to the central atom
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AP Chemistry
# of Lone Pairs
+
# of Bonded Atoms
2
3
4
5
6
Mr. Markic
Hybridization
sp
sp2
sp3
dsp3
d2sp3
Examples
BeCl2
BF3
CH4, NH3, H2O
PCl5
SF6
Sample Exercise
Indicate the hybridization of orbitals employed by the
central atom (underlined) in each of the following:
• BeH2
•
AlI3
•
PF3
Sample Exercise
• Describe the hybridization state of phosphorous in phosphorous pentabromide (PBr5)
Sigma (σ) and Pi Bonds (π)
Single Bond
Double Bond
Triple Bond
1 sigma bond
1 sigma bond and 1 pi bond
1 sigma bond and 2 pi bonds
Sample Exercise
Describe how the bonds of Carbon in formaldehyde are formed in terms of overlaps, of appropriate hybridization, and
unhybridized orbitals. Assume that the O atom is sp2 -hybridized
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AP Chemistry
Mr. Markic
Delocalized π Bonding
• Σ and π e- are associated totally with the two atoms that form the bond
• When molecules have two or more resonance structures involving π bonds, they are called delocalized bonds
• Delocalization of the e- in its π bonds gives a molecule a special stability
• Delocalization of π bonds is also responsible for the color of many organic molecules
The Benzene Molecule consists of a Ring of Six Carbon Atoms with one Hydrogen Atom Bound to Each Carbon (b) Two of
the Resonance Structures for Benzene Molecule
Big Idea 2: Chemical and Physical Properties of materials can be explained by the structure and the arrangement of
atoms, ions, or molecules and the forces between them.
Duration: Middle December/Early January
Textbook Chapter: 10
Enduring Understanding
Essential Knowledge
2.C: The strong electrostatic forces of attraction holding
atoms together in a unit are called chemical bonds.
2.C.1: In covalent bonding, electrons are shared between the nuclei
of two atoms to form a molecule or polyatomic ion.
Electronegativity differences between the two atoms account for
the distribution of the shared electrons and the polarity of the
bond.
2.C.4: The localized electron bonding model describes and predicts
molecular geometry using Lewis diagrams and the VSEPR model.
Learning Objectives
2.13 The student is able to describe the relationships between the structural features of polar molecules and the forces of
attraction between the particles.
2.18 The student is able to rank and justify the ranking of bond polarity on the basis of the locations of the bonded atoms in
the periodic table.
2.20 The student is able to explain how a bonding model involving delocalized electrons is consistent with macroscopic
properties of metals (e.g., conductivity, malleability, ductility, and low volatility) and the shell model of the atom.
2.21 The student is able to use Lewis diagrams and VSEPR to predict the geometry of molecules, identify hybridization, and
make predictions about polarity.
5.9 The student is able to make claims and/or predictions regarding relative magnitudes of the forces acting within collections
of interacting molecules based on the distribution of electrons within the molecules and the types of intermolecular forces
through which the molecules interact.
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