AP Chemistry
Molecular Models Lab
Covalent Compounds
Text Reference: Brown, LeMay, Bursten, and Murphy, The Central Science Chapters 8 and 9
Introduction
The purpose of this lab is to evaluate the geometries, polarities, resonance, and hybridization of
molecules. Many physical and chemical properties of compounds depend upon the shape or
geometry of the molecule and its polarity. A hemoglobin molecule, for example, becomes
ineffective as an oxygen transport vehicle if its shape is altered. Molecular shape and polarity
helps determine a compound’s boiling point, freezing point, and viscosity.
Overview: While Lewis dot structures are fairly easy to do for simple compounds such as Cl2
and H2O, there are other compounds for which it is not immediately obvious how a dot structure
should be drawn. You will gain experience in how to write dot structures for complex molecules
and ions.
For example, if you attempt to draw a dot structure for nitric acid (HNO3), you may find it
difficult. The final answer is given below:
There is another way that you could draw nitric acid. If you switched the double bond on the
oxygen to the other singly bonded oxygen you would have drawn a resonance structure, another
equally legitimate way of drawing the Lewis Structure. In fact, both resonance structures are
necessary for the complete Lewis Structure diagram. After you practice drawing a few
molecules without resonance structures, you will draw some with resonances.
General rules of bonding: (These don't quite have the "rule" status.)
HYDROGEN: always forms one and only one bond.
CARBON: almost always forms four bonds. Never more than four!
OXYGEN: usually forms two bonds and has two lone pairs. Sometimes forms one
bond and has three lone pairs.
NITROGEN: can form three bonds with one lone pair, or four bonds with no lone pair.
HALOGENS: usually form one bond, but when they are the central atom in a polyatomic
ion, can form up to 4 (or even more) bonds
(Note: a double bond counts as 2 bonds)
1
Revised: 15 Aug 2010
AP Chemistry
Molecular Polarity:
Covalent molecules can be classified as polar or nonpolar. A polar molecule contains polar
covalent bonds that are asymmetrically arranged. This lack of symmetry causes an uneven
electron distribution around the central atom that leaves part of the molecule with a more
negative region, and part of the molecule with a more positive region. Remember that polar
bonds are covalent bonds between 2 atoms that have different electronegativities. (In the 0.5 to
1.9 range.)
Model Kits:
The model kits are used to demonstrate the shapes of small molecules. They are useful because
they represent the molecules in three dimensions, as opposed to two-dimensional drawings. The
different colored balls represent certain atoms, according to the chart:
Color
Black
White
Red
Blue
Purple
Green
Green
Green
Yellow
Element
Carbon
Hydrogen
Oxygen
Nitrogen
Phosphorous
Bromine
Chlorine
Iodine
Sulfur
# of holes
4
1
2
3
3
1
1
1
6
The short grey links are used to represent single covalent bonds. Two flexible grey links are
used to represent a double bond, and three flexible grey links are needed for a triple bond.
Prelab Assignment:
1. Describe resonance.
2. Describe how formal charge can be used to determine which resonance structure is the most
probable.
3. Draw the resonance structures for the nitrate ion and show the formal charges on each atom
of each ion.
Lab Procedure:
1. Draw Lewis Dot Structures for the following compounds (the octet rule may not apply in all
cases). Then build models for each compound and sketch their shapes. Name each basic
shape. Note that complex molecules such as the ones you are making here may have a
combination of shapes. You should point to specific shapes and label appropriately.
Determine the hybridization of each central atom(s) in each molecule. Label the bond
angles and bond polarity. Indicate whether a molecule is polar or not.
a.
b.
c.
d.
HBrO
CH3I
CH3NHCH3
H2O2
2
Revised: 15 Aug 2010
AP Chemistry
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
BF3
HCOOH
HCN
HCOCH3
SF6
HClO4
COCl2
CO(NH2)2
C2H4
C2H2
XeF2
2. For any molecule or ion with resonance structure, draw each resonance structure. Use formal
charge to determine which structure is the most probable.
a.
b.
c.
d.
SO42ClO4SCNCO32In this polyatomic ion, all three C—O bonds have identical bond lengths of 136 pm.
Discuss how this supports the idea of resonance? (Consider the following bond
lengths: C—O 143 pm; C==O 123 pm; C==O 109 pm)
3. There are three possible structures for substances with the formula C2H2Cl2, two of which are
polar overall and one of which is not. Draw the three structures and indicate approximate
bond angles. Explain how the molecules differ. Explain why the C to C bond does not
rotate.
4. Consider the structure for glycine, an amino acid, H2NCH2COOH. Complete the structure to
show glycine’s Lewis structure. What is the hybridization for each carbon?
5. The molecule C4H4 has an unusual structure. Draw possible Lewis dot structure(s) for the
molecule. Indicate the hybridization of each carbon. Is the shape of the molecule planar or
non-planar?
6. Examples with alternative structures that break the octet rule to reduce formal charge.
Draw Lewis Dot Structures for the following compounds (the octet rule may not apply in all
cases). Then build models for each compound and sketch their shapes. Name each basic
shape. Note that complex molecules such as the ones you are making here may have a
combination of shapes. You should point to specific shapes and label appropriately.
Determine the hybridization of each central atom(s) in each molecule. Label the bond
angles and bond polarity. Indicate whether a molecule is polar or not. Determine the
formal charge on each atom in the structure.
a.
b.
c.
d.
HClO4 perchloric acid
H2SO4
PO43SO2Cl2
3
Revised: 15 Aug 2010