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10-1
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The Shapes of Molecules
10-2
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The steps to follow in converting a molecular formula into LEDS.
1st: Place the atoms relative to each other. [Atom withthe lowest electronegativity(EN)]
2nd: Determine the total number of valence electronsavailable. (Recall that the number of valence e-equals the A-group number)
3rd: Draw a single bond from each surrounding atomto the central atom and subtract two valenceelectrons for each bond.
4th: Distribute the remaining electrons in pairs so thateach atom ends up with 8 electrons (or 2 for H).First place the lone pairs to the surrounding (moreelectronegative) atoms to give each an octet.
10-3
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The steps to follow in converting a molecular formula into LEDS.
5th : CASES INVOLVING MULTIPLE
BONDS
If after step 4, a central atom still does NOT have an octet, make MULTIPLE bond by changing a lone pair from one of the surrounding atoms into a bonding pair in the central atom.
10-4
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Figure 10.1
The steps in converting a molecular formula into a Lewis structure.
Molecular
formula
Atom
placement
Sum of
valence e-
Remaining
valence e-
Lewis
structure
Place atom
with lowest
EN in center
Add A-group
numbers
Draw single bonds.
Subtract 2e- for each bond.
Give each
atom 8e-
(2e- for H)
Step 1
Step 2
Step 3
Step 4
10-5
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Molecular
formula
Atom
placement
Sum of
valence e-
Remaining
valence e-
Lewis
structure
NF3
NFF
F
N 5e-
F 7e-
X 3 = 21e-Total 26e-
:
: :
:
: :
:: ..
N 5e- X 1 = 5e-
F 7e-
NF
F
F
:
10-6
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SAMPLE PROBLEM 1 Writing Lewis Structures for Molecules with
One Central Atom
SOLUTION:
PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds
responsible for the depletion of stratospheric ozone.
PLAN: Follow the steps outlined in Figure 10.1 .
Step 1: Carbon has the lowest EN and is the central atom.
The other atoms are placed around it.C
Steps 2-4:
C has 4 valence e-, Cl and F each have 7. The
sum is 4 + 4(7) = 32 valence e-.
Cl
Cl F
F
C
Cl
Cl F
F
Make bonds and fill in remaining valence
electrons placing 8e- around each atom.
:
::
::
:
:
::
: ::
10-9
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There are some compounds that undergo theprocess of chemical bonding that form more or lessthan 8 electrons are are considered EXCEPTIONSTO THE OCTET RULE.
1) Electron Deficient molecules
- gaseous compounds containing Be or B as the central atom.
Ex. BF3; BeCl2
2) Odd-Electron molecules
-most have central atoms from an odd-numbered group.
Ex. N (Group 5A -15); Cl (Group 7A -17)
NO2 ( free radical contain a lone electron.)
10-10
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There are some compounds that undergo theprocess of chemical bonding that form more or lessthan 8 electrons are are considered EXCEPTIONSTO THE OCTET RULE.
3) Expanded Valence Shells
- molecules having MORE than 8 valence electrons
around the central atom.
- occur around a central NON METAL atom from
period 3 or higher, those in which d orbitals are available.
Ex. SF6
10-12
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• VSEPR focuses not only on
electron pairs it also focus on
electron groups as a whole.
• An electron group is an
electron pair, a lone pair, a
single unpaired electron, a
double bond or a triple bond
on the central atom.
The valence-shell electron-pair repulsion (VSEPR)
theory states that electron pairs repel each other
whether they are in bond pairs or in lone pairs. Electron
pairs will spread themselves as far from each other as
possible to minimize repulsion.
A model for predicting the geometry of molecules
10-13
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VSEPR - Valence Shell Electron Pair Repulsion Theory
has a its general formula.
A Xm En
A - central atom
X -surrounding atom
or
Bonding pairsE -nonbonding
valence electron-
group (lone pairs)
integers
Understanding the molecular structure of a compound can help determine
the polarity, reactivity, phase of matter, color, magnetism, as well as the
biological activity.
10-14
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The actual shape of a molecule can be determined by
the location of the nuclei and the distribution of
electrons.
CATEGORIES:
1) Electron-group geometry
is determined by the
NUMBER of
ELECTRON GROUPS
2) Molecular Geometry
(specific geometry)
depends on the NUMBER
OF LONE PAIRS.
Number of electron
groups
Name of electron
group geometry
2 linear
3 trigonal-planar
4 tetrahedral
5trigonal-
bipyramidal
6 octahedral
ELECTRON GROUP GEOMETRY
10-15
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Number
of
Electron
Groups
Electron-
Group
Geometry
# of
Lone
Pairs
VSEPR
Notation
(Type of
Shape)
Molecular
Geometry
Ideal
Bond
Angles
Examples
2 linear 0 AX2 180° BeH2
3Trigonal
planar
0 AX3 120° CO32-
1 AX2E 120° O3
10-16
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Number
of
Electron
Groups
Electron-
Group
Geometry
# of
Lone
Pairs
VSEPR
Notation
(Type of
Shape)
Molecular
Geometry
Ideal
Bond
Angles
Examples
4
Tetrahe
-dral
0 AX4 109.5° S042-
1 AX3E 109.5° H3O+
2 AX2E2 109.5° H2O
10-17
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.No. of
Electro
n
Groups
Electron-
Group
Geometry
# of
Lone
Pairs
VSEPR
Notation
(Type of
Shape)
Molecular
Geometry
Ideal
Bond
Angles
Examples
5Trigonal-
Bipyramidal
0
AX5 90°,
120° PF5
1 AX4E
90°,
120° TeCl4
2 AX3E2 90°
3 AX2E3180°
I3-
10-18
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.No. of
Electro
n
Groups
Electron-
Group
Geometry
# of
Lone
Pairs
VSEPR
Notation
(Type of
Shape)
Molecular
Geometry
Ideal
Bond
Angles
Examples
6 Octahedral
0 AX6 90° PF6-
1AX5E
90° SbCl52-
2 AX4E290° ICl4
-
10-19
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.2
Electron-group repulsions and the five basic molecular shapes.
linear trigonal planar tetrahedral
trigonal bipyramidal octahedral
10-20
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Figure 10.3 The single molecular shape of the linear electron-group
arrangement.
Examples:
CS2, HCN, BeF2
10-21
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Figure 10.4 The two molecular shapes of the trigonal planar electron-
group arrangement.
Class
Shape
Examples:
SO3, BF3, NO3-, CO3
2-
Examples:
SO2, O3, PbCl2, SnBr2
10-22
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Factors Affecting Actual Bond Angles
Bond angles are consistent with theoretical angles when the atoms
attached to the central atom are the same and when all electrons are
bonding electrons of the same order.
C O
H
Hideal
1200
1200
larger EN
greater
electron
density
C O
H
H
1220
1160
real
Lone pairs repel bonding pairs
more strongly than bonding pairs
repel each other.
Sn
Cl Cl
950
Effect of Double Bonds
Effect of Nonbonding(Lone) Pairs
10-23
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Figure 10.5 The three molecular shapes of the tetrahedral electron-
group arrangement.
Examples:
CH4, SiCl4,
SO42-, ClO4
-
NH3
PF3
ClO3
H3O+
H2O
OF2
SCl2
10-24
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Figure 10.6 Lewis structures and molecular shapes.
10-25
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Figure 10.7 The four molecular shapes of the trigonal bipyramidal
electron-group arrangement.
SF4
XeO2F2
IF4+
IO2F2-
ClF3
BrF3
XeF2
I3-
IF2-
PF5
AsF5
SOF4
10-26
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Figure 10.8 The three molecular shapes of the octahedral electron-
group arrangement.
SF6
IOF5
BrF5
TeF5-
XeOF4
XeF4
ICl4-
10-27
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Figure 10.9 A summary of common molecular shapes with two to six
electron groups.
10-28
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Figure 10.10 The steps in determining a molecular shape.
Molecular
formula
Lewis
structure
Electron-group
arrangement
Bond
angles
Molecular
shape
(AXmEn)
Count all e- groups around central
atom (A)
Note lone pairs and double
bonds
Count bonding and
nonbonding e-
groups separately.
Step 1
Step 2
Step 3
Step 4
See Figure
10.1
10-29
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SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three,
or Four Electron Groups
PROBLEM: Draw the molecular shape and predict the bond angles (relative
to the ideal bond angles) of (a) PF3 and (b) COCl2.
SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding pair
PF F
F
The shape is based upon the tetrahedral arrangement.
The F-P-F bond angles should be <109.50 due
to the repulsion of the nonbonding electron
pair.
The final shape is trigonal pyramidal.
PF F
F
<109.50
The type of shape is
AX3E
10-30
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SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three,
or Four Electron Groups
continued
(b) For COCl2, C has the lowest EN and will be the center atom.
There are 24 valence e-, 3 atoms attached to the center atom.
CCl O
Cl
C does not have an octet; a pair of nonbonding
electrons will move in from the O to make a
double bond.
The shape for an atom with three atom
attachments and no nonbonding pairs on the
central atom is trigonal planar.CCl
O
Cl The Cl-C-Cl bond angle will
be less than 1200 due to
the electron density of the
C=O.
C
Cl
O
Cl
124.50
1110
Type AX3
10-31
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SAMPLE PROBLEM 10.7 Predicting Molecular Shapes with Five or Six
Electron Groups
PROBLEM: Determine the molecular shape and predict the bond angles
(relative to the ideal bond angles) of (a) SbF5 and (b) BrF5.
SOLUTION: (a) SbF5 - 40 valence e-; all electrons around central
atom will be in bonding pairs; shape is AX5 - trigonal
bipyramidal.
F
SbF
F F
FF Sb
F
F
F
F
(b) BrF5 - 42 valence e-; 5 bonding pairs and 1 nonbonding pair on central
atom. Shape is AX5E, square pyramidal.
BrF
F F
F
F