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The bonding characteristics of inorganic molecular compounds are different from ionic compounds, and they are named using a different system as well. The charges of cations and anions dictate their ratios in ionic compounds, so specifying the names of the ions provides sufficient information to determine chemical formulas. However, because covalent bonding allows for significant variation in the combination ratios of the atoms in a molecule, the names for molecular compounds must explicitly identify these ratios.
When two nonmetallic elements form a molecular compound, several combination ratios are often possible. For example, carbon and oxygen can form the compounds CO and CO 2 . Since these are different substances with different properties, they cannot both have the same name (they cannot both be called carbon oxide). To deal with this situation, we use a naming method that is somewhat similar to that used for ionic compounds, but with added prefixes to specify the numbers of atoms of each element. The name of the more metallic element (the one farther to the left and/or bottom of the periodic table) is first, followed by the name of the more nonmetallic element (the one farther to the right and/or top) with its ending changed to the suffix – ide . The numbers of atoms of each element are designated by the Greek prefixes shown in [link] .
| Nomenclature Prefixes | ||||
|---|---|---|---|---|
| Number | Prefix | Number | Prefix | |
| 1 (sometimes omitted) | mono- | 6 | hexa- | |
| 2 | di- | 7 | hepta- | |
| 3 | tri- | 8 | octa- | |
| 4 | tetra- | 9 | nona- | |
| 5 | penta- | 10 | deca- |
When only one atom of the first element is present, the prefix mono - is usually deleted from that part. Thus, CO is named carbon monoxide, and CO 2 is called carbon dioxide. When two vowels are adjacent, the a in the Greek prefix is usually dropped. Some other examples are shown in [link] .
| Names of Some Molecular Compounds Composed of Two Elements | ||||
|---|---|---|---|---|
| Compound | Name | Compound | Name | |
| SO 2 | sulfur dioxide | BCl 3 | boron trichloride | |
| SO 3 | sulfur trioxide | SF 6 | sulfur hexafluoride | |
| NO 2 | nitrogen dioxide | PF 5 | phosphorus pentafluoride | |
| N 2 O 4 | dinitrogen tetroxide | P 4 O 10 | tetraphosphorus decaoxide | |
| N 2 O 5 | dinitrogen pentoxide | IF 7 | iodine heptafluoride |
There are a few common names that you will encounter as you continue your study of chemistry. For example, although NO is often called nitric oxide, its proper name is nitrogen monoxide. Similarly, N 2 O is known as nitrous oxide even though our rules would specify the name dinitrogen monoxide. (And H 2 O is usually called water, not dihydrogen monoxide.) You should commit to memory the common names of compounds as you encounter them.
(a) SF 6
(b) N 2 O 3
(c) Cl 2 O 7
(d) P 4 O 6
(a) sulfur hexafluoride
(b) dinitrogen trioxide
(c) dichlorine heptoxide
(d) tetraphosphorus hexoxide
(a) phosphorus pentachloride
(b) dinitrogen monoxide
(c) iodine heptafluoride
(d) carbon tetrachloride
(a) PCl 5 ; (b) N 2 O; (c) IF 7 ; (d) CCl 4
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