11/9/2010
Chapter 10
The Shapes of Molecules
Chapter 10
p
Chapter 10
Molecular Lewis Structures
¾ We can use Lewis dot symbols to build molecules, to
show the bonding between atoms, and to predict the
three-dimensional molecular structure of the resulting
compound
¾ Powerful tool to predict molecular structure
¾ Start thinking in 3
3-D
D
¾ How to determine the central atom for polyatomic
molecules (H2O, CH4)?
¾ Use a systematic approach
Molecular Lewis Structures
A Systematic Approach
Step
IIdentify the central atom based on electronegativities of the constituents
1.
atoms. The central atom is the one with low electronegativity value.
Exception: Hydrogen (always be the terminal atom)
2.
Recognize the number of bonds each atom should make to satisfy it’s
octet
3.
Determine the total number of valence electrons in the molecule.
Step II
4.
Place one pair of electrons between each pair of bonded atoms to
form a single bond.
5.
Use any remaining pairs as lone pairs around each terminal atom
(except H) so that each terminal atom is surrounded by eight
electrons
Step III
6.
If the central atom has fewer than eight electrons at this point, move
one or more of the lone pairs on the terminal atoms into a position
intermediate between the center and the terminal atom to form
double or triple bonds
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Chapter 10
Molecular Lewis Structures
Number of valence electrons
Usual number of covalent bonds
Group 8A
8 e–
Group 1A
1 e–
H
1 bond
¾ Hydrogen needs one e- to
achieve a noble gas
configuration – thus it
forms one covalent bond
Group 3A
3 e–
Group 4A
4 e–
B
3 bonds
C
4 bonds
Group 5A
5 e–
Group 6A
6 e–
N
O
3 bonds 2 bonds
Si
P
4 bonds 3 bonds
(5)
S
2 bonds
(4, 6)
¾ Carbon
¾ Nitrogen
Group 7A
7 e–
He
0 bonds
F
1 bond
Ne
0 bonds
Cl
1 bond
(3, 5)
Ar
0 bonds
Br
1 bond
(3, 5)
Kr
0 bonds
I
1 bond
(3, 5, 7)
Xe
0 bonds
¾ Oxygen
¾ Fluorine
Chapter 10
Molecular Lewis Structures
¾
Formaldehyde, CH2O
¾ Which is the central atom?
¾ Total valence electrons = ?
¾ Check the octet of C, is it satisfied?
Chapter 10
Molecular Lewis Structures
¾
Ammonia, NH3
¾ Which is the central atom?
¾ Total valence electrons = ?
¾ Check the octet of N, is it satisfied?
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Molecular Lewis Structures
¾
NF3
¾ Which is the central atom?
¾ Total valence electrons = ?
¾ Bonding electrons
¾ How many electrons remain and where do they
go?
¾ Check the octets of N
and F, are they satisfied?
Molecular Lewis Structures
¾
Ethane, C2H6
¾ Total valence electrons = ?
¾ Bonding electrons
¾ Check the “octets”, are they satisfied?
Molecular Lewis Structures
¾
Ethylene, C2H4
¾ Total valence electrons = ?
¾ Bonding electrons
¾ How many electrons are left over and where do they
go?
¾ Check the “octets”, are they satisfied?
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Molecular Lewis Structures
¾
Phosgene, COCl2
Molecular Lewis Structures
¾
Hydrazine, N2H4
¾
Hydrogen peroxide, H2O2
Polyatomic Ions
¾
¾
Covalent bonding can also occur in polyatomic
ions
¾ Sulfate (SO4 2-) , nitrate (NO3‐), ammonium
(NH4+) for example
To generate Lewis structures, use the systematic
approach except …
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Polyatomic Ions
¾
Nitrate, NO3¯
¾ Total valence electrons = ?
¾ Bonding electrons
¾ How many electrons are left over and where do
they go?
¾ Check the octets of O and N, are they satisfied?
¾ Place brackets around the structure: overall
charge is 1-
Polyatomic Ions
¾
Another example, PO43‐
Polyatomic Ions
¾
Chlorate, ClO3¯
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Delocalized Electron-pair Bonding:
Resonance
¾
O3
¾
CO32‐
Which One is the Real Structure?
Delocalized Electron-pair Bonding:
Resonance
Actual representation
of the molecule is a
hybrid of many
different
d
e e t Lewis
e s
Structures i.e.
resonance forms
“Resonance Hybrid”
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Which One is the Real Structure?
Or,
O
O
O
O
O
O
O
O
O
Formal Charge
¾
What is it?
Formal charge = Group number - [LPE+ ½ (BE)]
LPE: lone pair electrons
BE: number of bonding electrons
¾
Examples:
i) H2O
ii) OH-
Overall charge of a molecule or polyatomic ion = summation of formal charges of all the atoms in a molecule / polyatomic ion
Formal Charge
¾ Hypochlorous acid: HOCl or HClO?
¾ ClO-
¾ Hydrogen cyanide: HNC or HCN?
¾ CN-
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Formal Charge
¾
Formal charge and resonance structures
i) NO3-
ii) CH3COO-
Octet Rule Exceptions
Electron-deficient Molecules:
¾
BF3
¾
BeH2
Expanded Octets
¾
PCl5
¾
SiF5¯
¾
SF6
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More Exceptions
¾
ClF4
¾
BrF5
¾
NO2
¯
Molecule with odd number of electrons: free radicals
Chemical Bonding and Molecular Geometry
DNA
Hemoglobin
Molecular Shape
¾
Most molecules are not flat as they appear in twodimensional Lewis structures
¾
Molecules have three-dimensional shapes
¾
Shape can be predicted using Valence Shell
Electron Pair Repulsion (VSEPR) theory
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Shape via VSEPR
¾
¾
First obtain the Lewis structure
Then apply the VSEPR rules:
¾ All valence shell electron pairs (electron charge
clouds) are treated equally whether they are
bonding or nonbonding
¾ Stereochemically significant electron pairs
¾ Double and triple bonds are treated as single
bonds for predicting molecular shapes
¾ Electron pairs orient in space to get as far away
from neighbors as possible
Balloon models of electron-pair
geometries
Electron-pairs:
2
3
4
5
6
Molecular Shapes with Two Electron Pairs
¾CO2
180o
O
C
O
linear
¾BeCl2
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Molecular Shapes with Three Electron Pairs
¾BF3
F
120o
trigonal planar
B
F
F
¾ Molecular shape is determined by the
positions of atoms
Molecular Shapes with Four Electron Pairs
¾CH4
H
109.5
109
5o
tetrahedral
C
H
H
H
Molecular Shapes with Five Electron Pairs
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Molecular Shapes with Four Electron Pairs
A Summary of Common Molecular Shapes
Shape via VSEPR
¾
NH4+
¾
PF5
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Shape via VSEPR
¾
H2 O
Structure is ______________
¾
ClF2+
Structure is ______________
Shape via VSEPR
¾
Central atoms with expanded octets and lone pairs
¾
ClF4+
¾
ICl4¯
¾
BrF5
Molecular Shapes with More Than One Central Atom
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Molecule Polarity
¾
We can use the polarity of individual bonds to predict
the polarity of overall molecules
¾ Polar Molecule = a molecule that contains
polarized bonds AND has the resulting partial
charges distributed unsymmetrically
δ+
δ−
H
O
δ+
H
¾ Polarity induces a dipole or a separation of
charge
Molecule Polarity and Dipole Moment
¾ A quantitative measure of the degree of charge separation in a
molecule.
¾ Product of partial charges and the distance by which they are
separated.
¾ Unit : Debye (D); 1 D = 3.34 x 10-30 C.m)
¾ A vector quantity, having both a magnitude and a direction
Molecule Polarity
¾
Nonpolar molecule = a molecule that has nonpolar
bonds (like H2 or I2) or a molecule that has polarized
bonds AND has the resulting partial charges
distributed symmetrically
¾
CO2
δ+
δO
δ-
¾
δ-
C
O
F
+
δ+ δ
BF3
B
Fδ
δ+
δ-
F
¾ Dipoles cancel out (point in opposite directions)
resulting in a nonpolar molecule
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Molecule Polarity
Polar or non-polar?
¾ CO2
¾ H2 O
Molecule Polarity
Polar or non-polar?
¾ BF3
¾ NH3
Trigonal planar
Trigonal pyramidal
Molecule Polarity
More examples:
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Molecule Polarity
¾
CS2 – polar or nonpolar?
¾
CCl4 – polar or nonpolar?
¾
SF6 – polar or nonpolar?
Molecule Polarity
¾
NH3 – polar or nonpolar?
¾
SO3 – polar or nonpolar?
¾
SO2Cl2 – polar or nonpolar?
Molecule Polarity
¾
SCl2 – polar or nonpolar?
¾
SF4 – polar or nonpolar?
¾
XeF4 – polar or nonpolar?
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