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Lab 9.1 Shapes and Polarity of Covalent Molecules
Name ______________
The most common chemical bond between two atoms is a covalent bond. The
covalent bond consists of a pair of shared electrons, one
from each atom. If this pair of electrons is shared between two atoms of
equal electronegativities, the bond would be called a nonpolar
covalent bond. However, in most cases, the pair of electron is shared by two
atoms of different electronegativities. Thus, the pair of
electrons is sifted toward the more electronegative element. A partial
negative charge results on one side of the bond and a partial positive
charge on the other. This type of covalent bond is called polar covalent.
Molecules composed of covalent bonded atoms may also be polar or nonpolar.
For the molecule to be polar, it must, of course have
polar bonds. But the key factor for determining the polarity of a molecule is
its shape. If the polar bonds (dipoles) are symmetrical around
the central atom, they offset each other and the resulting molecule is
nonpolar. However, if the dipoles are not symmetrical around the
central atom, the electrons will be pulled to one end of the molecule and the
resulting molecule is polar.
Ball and stick models are often used to demonstrate molecular shape. In this
exercise you will build several covalent molecules and
predict each molecule’s polarity on the basis of its molecular shape.
16 springs
1 orange – nitrogen
Objectives
Materials
- Build models of some simple molecules
4 black – carbon
6 red – oxygen
1 yellow w/ black circles
- Predict each molecule’s polarity on the basis of its shape 9 white –
hydrogen 2 green – chlorine
around holes – aluminum
Procedure
1. Build models of the molecules listed in the data table (when in doubt,
check teacher’s models).
Don’t disassemble the models until directed to do so. When you disassemble
them, twist the springs clockwise.
2. Use the correct color sphere based on the materials list above.
3. Remember that some atoms, such as carbon and oxygen, can form multiple
bonds.
Use the longer springs for double and triple bonds.
4. Include in your table the formula, the Lewis dot structure, a diagram of
the ball and stick model, the shape of the molecule
(linear, angular, tetrahedral, and so on), and the molecular polarity (polar
or nonpolar) for each molecule.
The answers for the first substance are given, but still make the model, for
comparison to the second example.
When two different atoms are bonded together, the bond will be polar. The
molecule will be polar if it’s not symmetric.
No Special Models!!! Only build those indicated by the data table.
Formula
Electron Dot
Ball and Stick
Shape of
Molecular
Structure
Structural Model
Molecule
Polarity
HCl
H2
O2
(Use long springs
for double bonds)
Question 1) Why is HCl polar and H2 nonpolar? (Hint: The answer is in number
4 above!)
Question 2) HCl is a solid at room temperature, whereas H 2 is a gas at room
temperature.
Can you draw a conclusion about polarity and physical state (solid, liquid,
gas)?
Formula
H2O
Dot Structure
Structural Model
Shape
CO2
(Use long springs
for double bonds)
HClO
Question 3) What causes the H2O to be bent rather than linear?
(Hint: It has to do with the O in the H2O. You can’t see them, but they are
there!)
Disassemble your models. Remember to twist the springs clockwise as you pull.
Polarity
Formula
NH3
Dot Structure
Structural Model
Shape
Polarity
AlH3
H2CO
(One double bond)
Question 4) Why does the NH3 bend downward, while the other 2 models lie flat
as a pancake?
(Hint: It has do do with the same reason the H2O is bent rather than linear.
You can’t see them, but they are there!)
Disassemble your models. Remember to twist the springs clockwise as you pull.
Formula
Dot Structure
Structural Model
Shape
CH4
Polarity
CH3Cl
C2H2
(One triple bond)
Question 5) Between the CH4 and the CH3Cl, one is polar and one is nonpolar,
yet they have the same shape.
Explain why difference.
Conclusions
1. On the basis of this experiment and your classwork, predict the
(a) type of bonding (ionic or covalent), (ionic: metals and non-metals,
covalent: two non metals)
(b) if it’s covalent, the molecular shape and polarity
for the following:
Substance
Type of Bonding
Shape
Polarity
a. BaCl2
b. Cl4
c. SCl2
d. PH3
e. AlCl3
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1. Science
2. Chemistry
Lab9.1 Shapes and Polarity.doc
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