chemistry
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9.3
Naming and Writing Formulas for
Molecular Compounds
One milligram of gold is worth only about one cent,
but one kilogram of gold is worth approximately
$12,500. The correct prefix ( milli- or kilo-) makes
quite a difference! Prefixes are important in
chemistry, too. The prefixes in the name of a
binary molecular compound tell you its
composition.
Objectives: Interpret the prefixes in the names of
molecular compounds in terms of their chemical
formulas
Apply the rules for naming and writing formulas for
binary molecular compounds
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9.3
Naming and Writing
Formulas for Molecular
Compounds
>
Naming Binary Molecular
Compounds
Naming Binary Molecular Compounds
What does a prefix in the name of a
binary molecular compound tell you
about the compound’s composition?
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9.3
Naming and Writing
Formulas for Molecular
Compounds
>
Naming Binary Molecular
Compounds
Carbon and oxygen combine to form carbon
monoxide (CO) and carbon dioxide (CO2), but
these two invisible gases are very different.
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9.3
Naming and Writing
Formulas for Molecular
Compounds
>
Naming Binary Molecular
Compounds
Sitting in a room with small amounts of CO2 in
the air would not present any problems. If the
same amount of CO were in the room, you could
die of asphyxiation. A naming system that
distinguishes between these two compounds is
needed.
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9.3
Naming and Writing
Formulas for Molecular
Compounds
>
Naming Binary Molecular
Compounds
A prefix in the name of a binary molecular
compound tells how many atoms of an
element are present in each molecule
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9.3
Naming and Writing
Formulas for Molecular
Compounds
>
Naming Binary Molecular
Compounds
To name binary molecular compounds:
1. Name the elements in the order listed
in the formula.
2. Use prefixes to indicate the number of
each kind of atom.
3. Omit the prefix mono- when the
formula contains only one atom of the
first element in the name.
4.The suffix of the name of the second
element is -ide.
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Naming and Writing
Formulas for Molecular
Compounds
>
Molecular compounds
Example:
Name the following binary molecular
compounds:
1. P2O3
2. CCl4
3. N2O
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9.3
Naming and Writing
Formulas for Molecular
Compounds
>
Writing Formulas for Binary
Molecular Compounds
Writing Formulas for Binary Molecular
Compounds
How do you write the formula for a binary
molecular compound?
1. Write the symbol of each anion
2. Use the prefix in front of each name to
assign a subscript to that particular cation
3. DO NOT CRISSCROSS!
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9.3
Naming and Writing
Formulas for Molecular
Compounds
>
Writing Formulas for Binary
Molecular Compounds
Silicon carbide is a hard material like diamond.
The name silicon carbide has no prefixes, so the
subscripts of silicon and carbon must be one.
Thus, the formula for silicon carbide is SiC.
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Naming and Writing
Formulas for Molecular
Compounds
>
Molecular compounds
Example:
Write the formula for each of the following:
1. Sulfur trioxide
2. Silicon tetrachloride
3. Carbon monoxide
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9.3 Section Quiz.
Assess students’ understanding of the
concepts in Section 9.3.
Continue to:
-or-
Launch:
Section Quiz
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9.3 Section Quiz.
1. Which of the following compounds is named
INCORRECTLY?
a. CS2, carbon disulfide
b. BCl3, boron trichloride
c. IF7, iodine heptafluoride
d. PCl5, phosphorus hexachloride
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9.3 Section Quiz.
2. Which of the following molecular compounds
is named INCORRECTLY?
a. SbCl3, antimony trichloride
b. C2O5, dicarbon pentoxide
c. CF4, carbon tetrafluoride
d. H3As, hydrogen arsenide
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9.3 Section Quiz.
3. The correct formula for tetraphosphorus
trisulfide is
a. P3S4
b. S3P4
c. P4S3
d. S4P3
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