Investigation III – Molecules in Action
Lesson 2 – Polar Bears and Penguins
III-2 Polar Bears and Penguins
Electronegativity
Think about it
Hydrogen chloride, HCl, is a colorless but very toxic gas. Its smell is described as an
unpleasant, suffocating, acrid odor. Like most other small molecules that smell, it is
polar. But what makes an HCl molecule polar? Where do the partial charges come from
on the atoms in a polar molecule?
What makes a molecule polar?
To answer this question, you will examine:
1. Electronegativity
2. Nonpolar molecules
3. Electronegativity and Bonding
Exploring the Topic
1. Electronegativity
The H atom and the Cl atom in hydrogen chloride, HCl, form a covalent bond by sharing
a pair of electrons. The whimsical drawing below represents HCl as a penguin and a polar
bear. The bonded pair of electrons is represented as two scoops of ice cream. Although
the penguin and the polar bear are sharing the ice cream, they are not sharing the ice
cream equally.
Just like the polar bear and the penguin, the hydrogen atom and the chlorine atom in a
hydrogen chloride molecule do not share the bonded pair of electrons equally. The
chlorine atom attracts the shared electrons much more strongly than the hydrogen atom.
As a result, the shared electrons spend more time near the chlorine atom than they do
near the hydrogen atom. Because of this displacement of the electrons, the hydrogen
atom has a partial positive charge and the chlorine atom has a partial negative charge.
© Living by Chemistry, Unit 2: Smells, 2006
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Investigation III – Molecules in Action
Lesson 2 – Polar Bears and Penguins
[Modify the illustration to show the valence
electrons slightly closer to the chlorine atom.
Show partial positive and partial negative
charge.]
The tendency of an atom to attract electrons is called electronegativity. An atom that
strongly attracts electrons is considered highly electronegative. In this case, the chlorine
atom, like the polar bear, is stronger in attracting electrons, and thus, is more
electronegative than the hydrogen atom. The more electronegative atoms are the ones that
end up with a partial negative charge. The atom with lower electronegativity will end up
with a partial positive charge. The result is a polar bond.
2. Nonpolar Molecules
When two atoms with identical electronegativities bond together, the attraction for the
shared electrons is identical. As a result, the molecule is nonpolar. For example, both H2
and Cl2 are both nonpolar molecules. There is no partial charge on a nonpolar molecule.
The penguins represent two
hydrogen atoms.
The polar bears represent
two chlorine atoms
Cl
H
H
Cl
Two polar bears of identical strength or two penguins of identical
strength get together and arm-wrestle. Neither one would win.
There is another way to end up with a nonpolar molecule besides bonding two identical
atoms together. Examine the illustration below. Why is carbon dioxide, CO2, a nonpolar
molecule?
© Living by Chemistry, Unit 2: Smells, 2006
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Investigation III – Molecules in Action
Lesson 2 – Polar Bears and Penguins
Notice that there are
4 scoops of ice
cream because there
is a double bond
between the oxygen
and carbon atom.
O
C
O
The oxygen atoms attract the shared electrons more strongly in each C=O bond. Thus,
each C=O bond is polar. However, the oxygen atoms on opposite sides of the molecule
both have a partial negative charge. There is no difference in charge on opposite sides of
the molecule. Thus, the overall molecule is nonpolar.
3. Electronegativity and Bonding
There are two types of covalent bonds: polar covalent bonds and nonpolar covalent
bonds. When atoms with slightly different electronegativities bond together, the result is
a polar covalent bond.
Different
electronegativities result
in polar bonds.
When the electronegativities of the atoms that bond are identical, the result is a nonpolar
covalent bond.
Identical
electronegativities result
in nonpolar bonds.
© Living by Chemistry, Unit 2: Smells, 2006
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Investigation III – Molecules in Action
Lesson 2 – Polar Bears and Penguins
If the electronegativities of the two atoms differ greatly, it is possible for electrons to be
pulled entirely toward one of the atoms in a bond. The result is an ionic bond.
Extremely different
electronegativities result
in ionic bonds.
DON’T GET CONFUSED: Ionic compounds are the extreme of a polar bond in which
electrons are transferred to the more electronegative atom in the pair.
Lesson Summary
What makes a molecule polar? Electronegativity is a measure of the tendency of an atom
to attract the electrons that are involved in a bond. Different atoms have different
electronegativity values. When two different atoms bond together, they do not attract the
bonding electrons equally. The electrons are attracted towards the atom with the greater
electronegatviity. The result is a polar covalent bond. When atoms with identical
electronegativities bond together, the electrons are shared equally and the result is a
nonpolar covalent bond. If the two atoms involved in a bond differ significantly in
electronegativity, the result is an ionic bond.
Reading Questions
1. What is the difference between a polar covalent bond and a nonpolar covalent bond?
2. What is the difference between a polar covalent bond and an ionic bond?
3. Explain why carbon dioxide is a nonpolar molecule even though its bonds are polar.
© Living by Chemistry, Unit 2: Smells, 2006
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Investigation III – Molecules in Action
Lesson 2 – Polar Bears and Penguins
Homework
4. Divide the following molecules into two categories: polar covalent and nonpolar
covalent.
a. N2
b. HF
c. F2
d. NO
e. FCl
5. Using polar bears and penguins, create an illustration showing an ammonia molecule,
NH3. (Hint: You may wish to start with a Lewis dot structure.)
6. Is the molecule HOCl polar or nonpolar. Use a Lewis dot structure to explain your
thinking.
7. Use electronegativity to explain why some molecules are attracted to a charged wand.
Glossary

Electronegativity: The tendency of an atom to attract the electrons that are involved
in bonding.
Side bars

You may have noticed a new symbol in the illustrations above. Chemists have a
specific name for a molecule that has two poles — it is called a dipole. ("Di" means
two.) Every polar molecule is a "dipole". The crossed arrow symbol is used to show
the direction of the dipole in these compounds. The crossed end of the arrow indicates
the more positive (+) end of the polar bond and the arrow points in the direction of
the more negative (-) end.

Polar bears and penguins live on opposite poles. (Get the joke?)
© Living by Chemistry, Unit 2: Smells, 2006
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