Prescriptive Activities: Facet Cluster 1.6: PF#54
Teacher Page
Prescriptive #54:
54 The student does not use the periodic table to accurately predict polar bonds.
*Note: Misconception found is that in all covalent bonds electron pairs were shared equally
Materials:
Student handout
Internet access
Background:
When nonmetal atoms bond together to form covalent compounds, they will either share
the electrons equally between the atoms, or the electrons will be more attracted to one atom
than another. This unequal attraction results in a polar bond and is very important in
determining the properties of substance.
Electronegativity is defined as the ability of an atom in a molecule to attract shared
electrons toward itself. This ability depends on the nuclear charge (number of
protons) and the distance of the outermost electrons from the nucleus.
Electronegativity values have been experimentally determined and can be found in
the table provided on the next page.
In a covalent bond, the more electronegative atom will attract the
shared electrons more strongly towards itself and away from the
less electronegative atom. (Remember, covalent bonds share
electrons, while ionic bonds do not) This unequal sharing will create
more “electron density” or partial negative charge near the more
electronegative atom, as shown for HF at right. Fluorine is more
electronegative than hydrogen, and so the shared electrons spend
more time around the fluorine atom. The arrow is pointing towards
the slightly negative side of the molecule.
We say that this more electronegative side of the bond has a partial negative charge,
symbolized by the lowercase Greek letter delta (δ-). The side of the bond with the less
electronegative atom has a partially positive charge, symbolized by “δ+”.
H
H
If two atoms have the same electronegativity then they both attract
shared electrons equally and the bond is a nonpolar covalent bond. The
hydrogen (H2) molecule shown at left is an example of a nonpolar
molecule.
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Prescriptive Activities: Facet Cluster 1.6: PF#54
Teacher Page
To determine if a bond is polar or nonpolar, you look at the electronegativity values of the
two atoms (shown on the periodic table below).



If the numbers are equal, or their difference is under .5, they are nonpolar.
If the numbers are not equal or under .5, then the atom with the higher
electronegative value has a partial negative charge
If the atoms have a large difference in electronegativity, more than 1.6, the bond is an
ionic bond and they do not share electrons.
Note: The difference in electronegativity is symbolized by EN
Electronegativity
difference
0-0.4
0.5-1.6
1.7 or more
Bond type
Nonpolar covalent
Polar covalent
Ionic
One example of a polar covalent bond is the Si-C bond. The electronegativity difference
between silicon and carbon is 2.5-1.8 = 0.7 (ΔEN = 0.7). From the table above you can see
that this would create a polar covalent bond. Because carbon (2.5) is more electronegative
than silicon (1.8), the carbon side of the bond would have a partially negative charge and the
silicon side would have a partially positive charge. This is diagrammed below.
δ+
Si
C
δ-
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Prescriptive Activities: Facet Cluster 1.6: PF#54
Teacher Page
Electronegativity Table
Directions of Activity:
Watch the video about electronegativity located here and
then answer the questions on the next page:
Title: Electronegativity (1.25)
http://www.youtube.com/watch?v=Kj3o0XvhVqQ&feat
ure=player_detailpage
Note: You may notice that the value ranges given in the
video for ionic, polar and nonpolar bonds are slightly
different than those given in this activity. It is difficult to
make an absolute distinction between these types of bonds
and therefore you will find that different scientists, books
and resources will give different values. This is acceptable
as long as you understand that there is a continuum that
exists in going from nonpolar to polar to ionic bonds and
this is determined by the ability of an atom to attract
electrons to itself within a bond.
Table I
Bond
H-N
Nonpolar
Polar
Ionic
H-C
H-F
N-O
K-S
S-F
F-F
Mg-Cl
H-H
Br-Cl
Check with your teacher to find out which values you are
expected to use in class.
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Prescriptive Activities: Facet Cluster 1.6: PF#54
Teacher Page
Questions:
1. Use the electronegativity table to determine if each of the bonds shown in Table I is polar
covalent, nonpolar covalent or ionic.
2. Use the electronegativity table to determine which side of the bond will be slightly negative
and which side slightly positive in a covalent bond between the pairs of nonmetals shown
below. Circle the atom that will have the partial negative charge.
a. P-Cl
b. H-Cl
c. O-S
d. O-F
3. Why are no electronegativity values shown for the noble gases?
Teacher Notes:
The students can do this activity without watching the video about electronegativity. The video
provide a dynamic visual to help students understand the concept, and it includes a good
explanation.
A note about the continuum of bond types:
There are many opinions on where to draw the line between polar covalent bonds and ionic
bonds. Here’s one option for resolving this issue.
You can consider 0.5-1.6 to be polar covalent, and > 1.9 ionic.
Between 1.7 and 1.9, inclusive, it is polar covalent if the bond is between two nonmetals, and
ionic if the bond involves a metal.
References:
Electronegativity Table from:
http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Resonance
Video: Electronegativity, http://www.youtube.com/watch?v=Kj3o0XvhVqQ
Computer animation about nonpolar, polar and ionic bonds
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Prescriptive Activities: Facet Cluster 1.6: PF#54
Teacher Page
HF molecule image from:
http://darkwing.uoregon.edu/~ch111/L13.htm
Answer Key
Question 1:
Table I
Bond
H-N
Nonpolar
H-C
X
Polar
X
H-F
X
N-O
X
K-S
Ionic
a. P-Cl
X
S-F
Question 2:
Use the electronegativity table to determine which
side of the bond will be slightly negative and which
side slightly positive in a covalent bond between the
pairs of nonmetals shown below. Circle the atom
that will have the partial negative charge.
b. H-Cl
c. O-S
X
d. O-F
F-F
X
Mg-Cl
X
H-H
X
Br-Cl
X
Question 3:
The noble gases do not tend to form bonds because
they have a full valence energy level and are very
stable. Therefore, they do share electrons and thus do
not have electronegativity values.
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