Polarity & Lewis Structures
Name: ___________________
Period: __________________
Date: ____________________
Objectives
To use the Lewis structure of a molecule and its symmetry to classify whether or not the molecule is polar or nonpolar.
To compare two molecules, and determine which is the most polar.
Background
Covalent bonds involve the sharing of electrons between nonmetals and nonmetals.
Nonpolar bonds mean electrons are shared equally between atoms (think two polar bears or two penguins).
Polar bonds mean electrons are shared unequally between atoms, and the more electronegative
element hogs the electrons more (think polar bear vs. penguin).
You can also have a symmetrical, nonpolar molecule that has polar bonds within it. Because of
symmetry, the poles cancel each other out. CCl4 is an example of this. The C-Cl bond is polar, but the
molecule is symmetrical, and therefore, the polarity cancels and the overall molecule is nonpolar.
In a nutshell, symmetrical molecules are nonpolar, and nonsymmetrical molecules are polar.
Directions
For each of the following pairs of molecules, circle which is most polar and explain your choice. You should draw each
molecule to determine its shape (linear, bent, etc.). An example has been done for you.
Ex.
carbon disulfide (CS2)
OR
Lewis Structure
sulfur difluoride (SF2)
Lewis Structure
..
.. . S . .. :
: ..F
F
..
Carbon disulfide is nonpolar because it is symmetrical (shape is linear). Sulfur difluoride is polar because it is
not symmetrical (shape is bent). Therefore, sulfur difluoride is more polar.
1.
water (H2O)
OR
Lewis Structure
carbon tetrachloride (CCl4)
Lewis Structure
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2.
nitrogen trichloride (NCl3)
Lewis Structure
OR
oxygen dichloride (OCl2)
Lewis Structure
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3.
boron trihydride (BH3)*
OR
ammonia (NH3)
*Note: BH3 breaks octet rule.
Lewis Structure
Lewis Structure
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4.
chlorine (Cl2)
OR
phosphorus trichloride (PCl3)
Lewis Structure
Lewis Structure
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5.
silicon dioxide (SiO2)
OR
carbon dioxide (CO2)
Lewis Structure
Lewis Structure
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6.
methane (CH4)
OR
CH2Cl2
Lewis Structure
Lewis Structure
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7.
silicon tetrabromide (SiBr4)
Lewis Structure
OR
HCN
Lewis Structure
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