7.2. Molecular Compounds http://www.ck12.org
TABLE7.6:(continued)
Formula Name
N 2 O dinitrogen monoxide
S 2 Cl 2 disulfur dichloride
Cl 2 O 7 dichlorine heptoxideNotice that the mono- prefix is not used with the nitrogen in the first compound, but is used with the oxygen in both
of the first two examples. Also, theoof mono- and theaof hepta- are dropped from the name when paired with
oxide.
The example S 2 Cl 2 emphasizes another difference between ionic and molecular substances. Because ionic sub-
stances exist as extended networks, we simply give the lowest whole-number ratio of cations to anions, which is the
empirical formula. A molecular formula is not simply a ratio. Rather, themolecular formuladesignates how many
of each atom are in a single molecule of that substance. S 2 Cl 2 cannot be reduced to SCl, because each molecule of
disulfur dichloride contains two sulfur atoms and two chlorine atoms.
Example 7.2
Name the following binary compounds.
- BF 3
- NO
- N 2 O 5
- PCl 5
- P 4 O 6
Answer:
- boron trifluoride
- nitrogen monoxide
- dinitrogen pentoxide
- phosphorous pentachloride
- tetraphosphorous hexoxide
Writing Formulas for Binary Molecular Compounds
When you know the name of a molecular compound, the prefixes directly tell you which subscript to place with that
element in the formula. If there is no prefix, only one atom of that element is present and no subscript is used. For
example, if given the name diboron hexahydride, you would realize that the molecule must contain two atoms of
boron and six atoms of hydrogen. Its formula is B 2 H 6. Notice that metalloids like boron generally form molecular
compounds instead of ionic compounds.
Other Ways of Naming Molecules
Common Names
Some compounds (generally very common ones) are better known by names that are different than the "official"
names, which are designated by the International Union of Pure and Applied Chemistry (IUPAC). A few examples
can be found in the followingTable7.7.