PRINCIPLES OF CHEMISTRY I

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PRINCIPLES OF CHEMISTRY I CHEM 1211CHAPTER 10

DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 10 MOLECULAR STRUCTURE AND BONDING
THEORIES
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ELECTRON PAIRS
Valence Shell Electron Pair Repulsion (VSEPR)
Theory - Used to predict molecular structure
(geometry) - That is the three-dimensional
arrangement of atoms within molecules - The
specific arrangements depend on the number of
valence electron pairs present Stearic Number
number of lone pairs on central atom number
of atoms bonded to central atom
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ELECTRON PAIRS
Two Electron Pairs (2 Electron Domains) -
Predicted to be as far apart as possible from one
another - Gives 180o angles to one another
(opposite sides of the central atom) - This
electron pair arrangement is said to be linear
180o


central atom
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ELECTRON PAIRS
Three Electron Pairs (3 Electron Domains) -
Predicted to be as far apart as possible -
Found at the corners of an equilateral triangle
(separated by 120o angles) - This electron pair
arrangement is said to be trigonal planar
120o


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ELECTRON PAIRS
Four Electron Pairs (4 Electron Domains) -
Predicted to be as far apart as possible -
Found at the corners of a tetrahedron (separated
by 109o angles) - This electron pair arrangement
is said to be tetrahedral

109o



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ELECTRON PAIRS
Five Electron Pairs (5 Electron Domains) -
Separated by 90o and 120o - This electron pair
arrangement is said to be trigonal bipyramidal
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ELECTRON PAIRS
Six Electron Pairs (6 Electron Domains) -
Separated by 90o - This electron pair
arrangement is said to be octahedral
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VSEPR MODEL
VSEPR ELECTRON GROUPS - Electrons present in a
specific localized region about a central
atom Single bond - VSEPR electron group
containing two electrons - Represents one
electron group Double bond - VSEPR electron
group containing four electrons - Represents one
electron group
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VSEPR MODEL
VSEPR ELECTRON GROUPS Triple bond - VSEPR
electron group containing six electrons -
Represents one electron group Nonbonding
Electron Pair Included when determining the
number of electron groups - Each pair represents
one electron group
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VSEPR MODEL
Molecules with Two VSEPR Electron Groups - These
molecules are linear Examples CO2 (carbon
dioxide) HCN (hydrogen cyanide) BeCl2 (beryllium
chloride)
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VSEPR MODEL
Molecules with Three VSEPR Electron Groups These
molecules are - trigonal planar (all electron
groups are bonding) H2CO (formaldehyde) -
angular/bent/V-shaped (one electron group is
nonbonding) SO2 (sulfur dioxide)
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VSEPR MODEL
Molecules with Four VSEPR Electron Groups These
molecules are - tetrahedral (all electron
groups are bonding) CH4 (methane) - trigonal
pyramidal (one electron group is nonbonding)
NH3 (ammonia) - angular/bent/V-shaped (two
electron groups are nonbonding) H2O (water)
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VSEPR MODEL
Molecules With Five VSEPR Electron Groups These
molecules are - trigonal bipyramidal (all
electron groups are bonding) PCl5 - seesaw
(one electron group is nonbonding) SF4 -
T-shaped (two electron groups are nonbonding)
ClF3 - linear (three electron groups are
nonbonding) XeF2
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VSEPR MODEL
Molecules With Six VSEPR Electron Groups These
molecules are - octahedral (all electron groups
are bonding) SF6 - square pyramidal (one
electron group is nonbonding) BrF5 - square
planar (two electron groups are nonbonding)
XeF4
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VSEPR MODEL
Molecules with More Than One Central Atom -
Determined by considering each central atom
separately and combining the results C2H2
(acetylene) and H2O2 (hydrogen peroxide)
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BOND ANGLES
- Bond angles decrease as the number of
nonbonding electron pairs increases -
Nonbonding electron pairs tend to exert greater
repulsive forces on adjacent electron domains
and compress bond angles - Multiple bonds
also decrease bond angles (greater repulsive
forces)
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MOLECULAR POLARITY
Nonpolar Molecule - There is a symmetrical
distribution of electron charge Polar Molecule
- There is an unsymmetrical distribution of
electron charge - Molecular polarity depends on
bond polarity and molecular geometry -
Symmetrical molecules cancel polar bond effects
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MOLECULAR POLARITY
Generally - Molecules with lone pair of electrons
on the central atom are polar - Molecules
without lone pairs and with identical atoms on
the central atom are nonpolar
Diatomic Molecule - polar bond results in polar
molecule - nonpolar bond results in nonpolar
molecule
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MOLECULAR POLARITY
CO2
O
C
O
Linear, symmetrical and nonpolar
O
H2O
H
H
Nonlinear and polar
HCN
H
C
N
Linear but polar
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HYBRID ORBITALS
- The assumption that atomic orbitals on an atom
mix to form new orbitals of different
shapes - The process is called
hybridization - The number of hybrid orbitals
equals the number of atomic orbitals mixed
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HYBRID ORBITALS
sp Hybrid Orbitals (sp hybridization) - Two
hybrid orbitals arranged at 180o involving one s
orbital and one p orbital - Each hybrid
orbital has two lobes (one small and one
large) - Results in a linear arrangement of
electron domains BF2, BeCl2, CO2
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HYBRID ORBITALS
sp2 Hybrid Orbitals (sp2 hybridization) - Three
identical hybrid orbitals involving one s orbital
and two p orbitals (at 120o) - Three large
lobes point towards the corners of an
equilateral triangle - Results in trigonal
planar geometry BF3
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HYBRID ORBITALS
sp3 Hybrid Orbitals (sp3 hybridization) - Four
identical hybrid orbitals involving one s orbital
and three p orbitals (at 109o) - Four large
lobes point towards the vertex of a
tetrahedron - Results in a tetrahedral
arrangement of electron domains CH4
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HYBRID ORBITALS
sp3d Hybrid Orbitals (sp3d hybridization) - Five
hybrid orbitals arranged at 90o and 120o
involving one s orbital, three p orbitals, and
one d orbital - Large lobes point towards the
vertices of a trigonal bipyramid PF5, SF4
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HYBRID ORBITALS
sp3d2 Hybrid Orbitals (sp3d2 hybridization) -
Six hybrid orbitals arranged at 90o involving one
s orbital, three p orbitals, and two d
orbital - Large lobes point towards the vertices
of an octahedron SF6, ClF5
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SIGMA (s) BONDS
- The overlap of two orbitals (electron density)
along the internuclear axis (line connecting
nuclei) - The overlap of two s orbitals (H2) -
The overlap of an s and a p orbital (HCl) - The
overlap of two p orbitals (Cl2) - The overlap of
a p orbital and an sp hybrid orbital (BeF2)
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PI (p) BONDS
- Sideways overlap between two p orbitals
(perpendicular to the internuclear axis) - The
regions overlapping lie above and below the
internuclear axis - Weaker than s bonds (less
total overlap) - Most common in atoms having sp
or sp2 hybridization (small atoms in period 2
C, N, O)
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MULTIPLE BONDS
- Single bonds are s bonds (H2) - Double bonds
are comprised of one s and one p bonds
(C2H4) - Triple bonds are comprised of one s
and two p bonds (C2H2 , N2)
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DELOCALIZATION
- Observed in resonance structures with p
bonds - Results in greater stability -
Responsible for colors of many organic
compounds Benzene (C6H6) - Delocalized p bonds
among the six carbon atoms - Bond lengths are
identical and are between the C C single bonds
and the C C double bonds
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MOLECULAR ORBITALS (MO)
- Most characteristics are the same as atomic
orbitals - Can hold a maximum of two electrons
with opposite spins - Atomic orbitals are
associated with a single atom - Molecular
orbitals are associated with the entire
molecule - The number of molecular orbitals
formed is equal to the number of atomic
orbitals combined
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MOLECULAR ORBITALS (MO)
s1s
1s
1s
Energy
H atom
H atom
s1s
H2 molecule
- Molecular orbital diagram for H2 (electron
configuration is s1s2) - Two atomic orbitals
overlap to form two molecular orbitals - Energy
level of one MO is lower than the atomic orbitals
(filled with the two 1s electrons and is called
bonding molecular orbital (s1s) - Energy level of
the other MO is higher than the atomic orbitals
(empty and is called antibonding molecular
orbital (s1s) - Electrons occupy lower energy
and explains why hydrogen is diatomic
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MOLECULAR ORBITALS (MO)
s1s
1s
1s
Energy
He atom
He atom
s1s
He2 molecule
- Molecular orbital diagram for He2 (electron
configuration is s1s2 s1s2) - Bonding molecular
orbital (s1s) is filled - Antibonding molecular
orbital (s1s) is also filled - Energy decrease
in s1s is offset by energy increase in s1s - He2
is therefore unstable
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BOND ORDER
- Determines the stability of covalent bonds
- Single bonds bond order is 1 - Double bonds
bond order is 2 - Triple bonds bond order is
3 - Bond order is 1 for H2 and 0 for He2 (no
bond exists)
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MOLECULAR PROPERTIES
Paramagnetism - Molecules with unpaired electrons
are attracted into a magnetic field - Force
of attraction increases with increasing number of
unpaired electrons Diamagnetism - Molecules
without unpaired electrons are weakly repelled
from a magnetic field
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MOLECULAR PROPERTIES
Experimental Determination - Weigh samples in
the presence and absence of a magnetic
field - Paramagnetic substances will weigh more
in the magnetic field - Diamagnetic
substances will weigh less in the magnetic
field