Chapter 4
Compounds and Their Bonds
1
4.7
Shapes and Polarity of Molecules
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
VSEPR Theory
2
Valence Shell Electron-Pair Repulsion Theory (VSEPR)
• describes the orientation of electron groups around
the central atom
• states that electron groups are arranged as far
apart as possible around the central atom
• states that specific shape of a molecule is
determined by the number of atoms attached to the
central atom
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Central Atom with Two Electron
Groups
3
In a molecule of BeCl2
•
there are only two electron groups around the central
atom, Be, which is an exception to octet rule
•
repulsion is minimized by placing the two groups on
opposite sides of the Be atom, giving this a linear
arrangement with bond angles of 180°
•
the shape with two electron groups around the central
atom is linear
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Central Atom with Three Electron
Groups
4
In a molecule of BF3
•
there are only three electron groups around the central
atom, B, which is an exception to octet rule
•
repulsion is minimized by placing the three groups as far
apart as possible at bond angles of 120°
•
the shape with three electron groups around the central
atom is trigonal planar
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Central Atom with Three Electron
Groups
5
In a molecule of SO2
•
there are three electron groups around the central atom S:
a single-bonded O atom, a double-bonded O atom, and a lone pair
of electrons
•
repulsion is minimized by placing the three groups as far apart as
possible
•
since one group is a lone pair, the shape
is determined by the two O atoms bonded
to S, giving the molecule its bent shape
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Central Atom with Four Electron
Groups
6
In a molecule of CH4
•
there are four electron groups attached to H atoms around
the central atom, C
•
repulsion is minimized by placing the four groups at
corners of a tetrahedron with bond angles of 109°
•
the shape with four bonds on the central atom is called
tetrahedral
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Central Atom with Four Electron
Groups
7
In a molecule of NH3
•
there are three electron groups attached to H atoms and a
lone pair around the central atom, N
•
repulsion is minimized by placing the four groups at
corners of a tetrahedron with bond angles of 109°
•
the shape with three bonds on the central atom is called
trigonal pyrimidal
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Central Atom with Four Electron
Groups
8
In a molecule of H2O
•
there are two electron groups attached to H atoms and
two lone pairs around the central atom, O
•
repulsion is minimized by placing the four groups at
corners of a tetrahedron with bond angles of 109°
•
the shape with two bonds on the central atom is called
bent
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Molecular Shapes for Molecules
9
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Guide to Predicting
Molecular Shape
10
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Predict Molecular Shape of H2S
11
Predict the shape of H2S.
Step 1 Draw the electron-dot formula.
H SH
Step 2 Arrange electron groups around the central atom to
minimize repulsion.
To minimize repulsion, electron groups have a
tetrahedral arrangement.
Step 3 Use the atoms bonded to the central atom determine
the molecular shape.
With two bonds and two lone pairs, the shape is bent.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Learning Check
12
State the number of electron groups and lone pairs and
use VSEPR theory to determine the shape of the
following molecules or ions as tetrahedral, trigonal
pyramidal, or bent.
A. PF3
B. H2S
C. CCl4
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Solution
13
State the number of electron groups and lone pairs and use
VSEPR theory to determine the shape of the following
molecules or ions as tetrahedral, trigonal pyramidal, or bent.
Step 1 Draw the electron-dot formula.
A. PF3
F
P
F
F
B. H2S
S
C. CCl4
Cl
H
Cl
C
Cl
H
Cl
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Solution
14
State the number of electron groups, and lone pairs and use VSEPR
theory to determine the shape of the following molecules or ions as
tetrahedral, trigonal pyramidal, or bent.
Step 2 Arrange electron groups around the central atom to
minimize repulsion.
A. PF3 To minimize repulsion electron groups
have a tetrahedral arrangement.
B. H2S To minimize repulsion electron groups
have a tetrahedral arrangement.
C. CCl4 To minimize repulsion electron groups
have a tetrahedral arrangement.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Solution
15
State the number of electron groups and lone pairs and use
VSEPR theory to determine the shape of the following
molecules or ions as tetrahedral, trigonal pyramidal, or bent.
Step 3 Use the atoms bonded to the central atom to
determine the molecular shape.
A. PF3 With three bonds and one lone pair, the
shape is trigonal planar.
B. H2S With two bonds and two lone pairs, the
shape is bent.
C. CCl4 With four bonds the shape is tetrahedral.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Nonpolar Molecules
16
Nonpolar molecules
• such as H2, Cl2, and O2 are nonpolar because they contain
nonpolar bonds
• with polar bonds such as CO2 can be nonpolar if the polar
bonds cancel each other in a symmetrical arrangement
• dipoles cancel out, which makes the molecule nonpolar
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Polar Molecules
17
Polar molecules such as HCl are polar
• because one end of the molecule is more negatively
charged than the other
• when polar bonds in the molecule do not cancel each
other
• because the electrons are shared unequally in the polar
covalent bond
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Polar Molecules
18
In polar molecules
• with three or more atoms, the shape of the molecule determines
whether the dipoles cancel or not
• there are often lone pairs around the central atom
• such as H2O, the dipoles do not cancel, making the molecule
positive at one end and negative at the other end
• such as H2O, there is a dipole on the central atom
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Learning Check
19
Identify each of the following molecules as polar or nonpolar.
Explain.
A. PBr3
B.
HBr
C. CF4
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.
Solution
20
Identify each of the following molecules as polar or nonpolar.
Explain.
Number Number
Bonds Lone Pairs
Geometry
A. PBr3
3
1
trigonal pyrimidal
The dipoles do not cancel; the molecule is polar.
B. HBr
1
0
linear
The bond has a dipole; the molecule is polar.
C. CF4
4
0
tetrahedral
The bonds have dipoles; the dipoles cancel and the
molecule is nonpolar.
Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition
Copyright © 2012 by Pearson Education, Inc.