Molecular Shape and Polarity
The Importance of Geometry in
Determining Physical Properties
Structure Determines Properties
• It is important to understand structure in
order to predict properties
• Structure can be predicted
• Predicting molecular structure begins with
Lewis Dot Diagrams
Drawing Lewis Dot Structures
First step: decide what atoms are bonded together
– This is not always an easy choice
•
•
For most binary covalent compounds and polyatomic ions
the central atom will be the atom that occurs only once in
the formula.
When in doubt use the most symmetrical atom
arrangement.
Using CH4 and CO3 2- as examples
H
O O
H C H
and
C
H
O
Lewis Dot Structures- Step 2
2. Count the total number of valence electrons
in the ion or molecule
– CH4 has 8 valence electrons, 4 from Carbon and
1 from each of the four H atoms
– CO3 2- has 24 valence electrons, 4 from Carbon, 6
from each of the three Oxygen, plus 2 added to
give the ion a charge of 2-
Lewis Dot Structures- Step 3
• Place a pair of electrons (dots) between the
central atom and each of the other atoms.
H
H C H
H
and
O O
C
O
Lewis Dot Structures- Step 4
• Distribute the remaining outer electrons so that
each atom has a filled outer energy level, a filled
octet (exceptions H only 2, B only 6)
A complete Structure
This structure does
not work it needs 2e-
Lewis Dot Structures- Step 5
• When there are not enough electrons to
satisfy the octet rule, make double or triple
bonds
• When there are extra electrons, place these
on the central atom.
Check up from the Neck Up!
• Draw Lewis Structures for each of the
following.
1. H2S
2. NH4+
3. NO34. CO
5. CCl4
6. HCOOH (one H bonds to C, One to O)
Answers
Valence Electron Pair Repulsion
• Valence electron pair charge clouds repel
each other
• The electron pairs stay as far away from each
other as possible to minimize repulsion
• This theory explains many of the shapes of
molecules.
Demonstration Of Water Molecule
•What element is represented
by the balloons?
•What does the fist represent?
•What does the string
represent?
•What do you expect to occur?
•Why?
Molecular Shapes
• Are determined by electron repulsion
• The geometry or shape which allows the electrons to
be the farthest away from each other is the shape of
the compound
• The shape which minimizes the repulsive force
between electrons will be formed
• Nonbonded pairs are more repulsive than bonded pairs
so take up more “space” around the atom
– This forces the bonded pairs a little closer than they would
like
• Perfect tetrahedral angles – 109.5°, the two bonds in water are at
~ 105° the distortion of this angle is because of the nonbonded
pair repulsive force.
Let’s Use These Molecules to Explore
Molecular Structure
Using the previous table, predict what shape you
would expect for each of these molecular structures?
Check Your Predictions!
Tetrahedral Bent structure;
2 bonding domains, 2 unbound domains
3 Bonding Domains;
Trigonal Planar structure
4 Bonding domains;
Tetrahedral Structure
Multiple Bond Molecular Shapes
• Double and triple bonds form 1 bonding domain
• Carbon atom can form 4 chemical bonds with several
arrangements
– These can be single bonds in a tetrahedral
arrangement (4 bonding Domains)
– Can form 1 double bond and 2 single bonds having trigonal
planar geometry
(2 bonding domains,
1 nonbonded domain)
– 2 bonding domains creates a linear structure
-double bonds are arranged in a linear molecule
-1 single bond and 1 triple bond
Pre-Lab: Model Molecular Shape 9.2
Purpose: To investigate the three-dimensional shape of
molecules by building models
Question: What molecular shapes will be formed because
of electron repulsion in these molecules?
Background information:
• The geometry or shape which allows the electrons to
be the farthest away from each other is the shape of
the compound
• The shape which minimizes the repulsive force
between electrons will be formed
• Nonbonded pairs are more repulsive than bonded pairs
so take up more “space” around the atom
Students will complete the pre-lab table and construct
their hypothesis
Polarity
• Most chemical bonds are not 100% ionic nor
100% covalent because of differing
electronegativities.
• Unequal sharing creates polar molecules or
polar covalent bonds
– H-Cl is a polar bond; H Cl , an arrow is used to
represent a polar bond, pointing toward the more
electronegative element.
– Not all polar bonds create polar molecules,
geometry plays a role
• Symmetrical arrangement of polar bonds can cancel polar
effect.
• Unsymmetrical arrangements can produce polar molecules
Polarity
• Unsymmetrical arrangements occur in water,
creating a polar molecule
• Many Physical properties, such as melting and
boiling points and solubility are affected by
degree of polarity of molecules
Apply what we’ve learned about
polarity.
• Determine the polarity of HCN and SOCl2
– First draw the Lewis structures and identify the
geometry
– Then use electronegativity to determine polarity
(Take out your handout for reference)
– Draw the appropriate arrows and geometry
Answers
• HCN en H=2.1, en C=2.5, en N=3.0
Linear
• OSCl en S=2.5, en Cl= 3.0, en O=3.5
Cl
trigonal pyramid
Both Geometry and Polarity
Determine Physical Properties