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.
Naming and Writing
Formulas for 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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Naming and Writing
Formulas for Molecular
Compounds
>
A prefix tells how many atoms of each
element are present in one molecule.
Prefix
Number
Prefix
Number
mono-
1
hexa-
6
di-
2
hepta-
7
tri-
3
octa-
8
tetra-
4
nona-
9
penta-
5
deca-
10
carbon monoxide (CO)
carbon dioxide (CO2)
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Naming and Writing
Formulas for Molecular
Compounds
Molecular Naming> Rules:
1) Name the elements in the order
listed in the formula.
2) Use prefixes to show the number
of each atom.
3) The suffix of second element is -ide.
4) No prefix mono- for first element.
CCl4
N 2O
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SO3
H 2O
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Naming and Writing
Formulas for 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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Quick 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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Quick Quiz.
2. Which of the following molecular compounds
is named INCORRECTLY?
A. AsCl3, arsenic trichloride
B. C2O5, dicarbon pentoxide
C. CF4, carbon tetrafluoride
D. H3As, hydrogen arsenide
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Quick Quiz.
3. The correct formula for
tetraphosphorus trisulfide is
A. P3S4
B. S3P4
C. P4S3
D. S4P3
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