Transcript Chapter 4 Compounds and Their Bonds
Chapter 4 Compounds and Their Bonds 4.7
Shapes and Polarity of Molecules
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
VSEPR
In the
valence-shell electron-pair repulsion theory (VSEPR),
the electron groups around a central atom • are arranged as far apart from each other as possible.
• have the least amount of repulsion of the negatively charged electrons.
• have a geometry around the central atom that determines molecular shape.
2
Guide to Predicting Molecular Shape (VSEPR Theory)
1. Write the electron-dot structure.
2. Arrange the electron groups around the central atom to minimize repulsion.
3. Use the atoms bonded to the central to determine the shape of the molecule.
3
Four Electron Groups
• • • In a molecule of CH 4 , there are four electron groups around C. repulsion is minimized by placing four electron groups at angles of 109 ° , which is a
tetrahedral
arrangement.
the shape with four bonded atoms is
tetrahedral
.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
4
Three Bonding Atoms and One Lone Pair
• • • In a molecule of NH 3 , three electron groups bond to H atoms and the fourth one is a lone (nonbonding) pair.
repulsion is minimized with 4 electron groups in a
tetrahedral
arrangement.
with three bonded atoms, the shape is
pyramidal
.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
5
Two Bonding Atoms and Two Lone Pairs
• • • In a molecule of H 2 O, two electrons groups are bonded to H atoms and two are lone pairs (4 electron groups).
four electron groups minimize repulsion in a
tetrahedral
arrangement.
the shape with two bonded atoms is
bent(~109
).
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
6
Shapes with 4 Electron Groups Electron Group Bonded Atoms Lone Pairs Bond Angles Molecular Shape Example
4 4 0 109 tetrahedral CH 4 4 4 3 1 ~109 Pyramidal NH 3 2 2 ~109 Bent Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings H 2 O
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Learning Check
State the number of electron groups, lone pairs, and use VSEPR theory to determine the shape of the following molecules or ions.
1) tetrahedral 2) pyramidal 3) bent A. PF 3 B. H 2 S C. CCl 4
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Solution
A. PF 3 4 electron groups, 1 lone pair, (2) pyramidal B.
H 2 S 4 electron groups, 2 lone pairs, (3) bent C. CCl 4 4 electron groups, 0 lone pairs, (1) tetrahedral
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Polar Molecules
• • • A
polar molecule
contains polar bonds.
has a separation of positive and negative charge called a dipole indicated with
+
has dipoles that do not cancel.
+
H –Cl
• •
H — N — H and
-
.
dipole
H
dipoles do not cancel 10
Nonpolar Molecules
• A
nonpolar molecule
contains nonpolar bonds.
Cl –Cl H –H • or has a symmetrical arrangement of polar bonds.
O=C=O Cl Cl –C–Cl Cl
dipoles cancel 11
Determining Molecular Polarity STEP 1: STEP 2: STEP 3:
Write the electron-dot formula.
Determine the polarity of the bonds.
Determine if dipoles cancel.
Example: H 2 O
. .
H ─O
:
│ H
dipoles do not cancel
H 2 O is polar
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Learning Check
Identify each of the following molecules as 1) polar or 2) nonpolar. Explain.
A. PBr 3 B. HBr C. Br 2 D. SiBr 4
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Solution
Identify each of the following molecules as 1) polar or 2) nonpolar. Explain.
A. PBr 3 B. HBr C. Br 2 D. SiBr 4 1) pyramidal; dipoles don’t cancel;
polar
1) linear; one polar bond (dipole);
polar
2) linear; nonpolar bond;
nonpolar
2) tetrahedral; dipoles cancel;
nonpolar 14