http://www.ck12.org Chapter 7. Chemical Nomenclature
Naming Binary Molecular Compounds
Abinary molecular compoundis a molecular compound that is composed of two elements. In general, the elements
that combine to form binary molecular compounds are both nonmetals. This contrasts with ionic compounds, which
usually involve bonds between metal ions and nonmetal ions. Because ionic charges cannot be used to name these
compounds or to write their formulas, a different naming system must be used for molecular compounds. Another
difference between ionic and molecular compounds is that two nonmetal atoms will frequently combine with one
another in a variety of ratios. For example, nitrogen and oxygen combine to make several binary compounds,
including NO, NO 2 , and N 2 O. Obviously they can’t all be called nitrogen oxide! How would someone know which
one you were talking about? Each of the three compounds has very different properties and reactivity. A system to
distinguish between compounds such as these is necessary.
Prefixes are used in the names of binary molecular compounds to identify the number of atoms of each element.
Table7.5 shows the prefixes for one to ten atoms:
TABLE7.5: Numerical Prefixes
Number of Atoms Prefix
1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-The rules for using the prefix system of nomenclature of binary molecular compounds can be summarized as follows:
- Generally, the less electronegative element is written first in the formula, though there are a few exceptions.
Carbon is almost always first in a molecular formula, and hydrogen is usually written after nitrogen in a
formula such as NH 3. The order in which common nonmetals are written in binary compound formulas is the
same as their order in the following series: C, P, N, H, S, I, Br, Cl, O, F. - The first element in the formula is written first in the name of the compound, along with the appropriate prefix.
No prefix is used if there is only one atom of the first element. - The second element is named after the first, but the ending of the element’s name is changed to -ide. The
appropriate prefix is always used for the second element, even if there is only one atom of that element. Even
though the -idesuffix is also used to name anions, it is important to remember that molecules are held together
by covalent bonds and do not contain cations and anions. - Theaoroat the end of a prefix is usually dropped from the name when the name of the element begins with
a vowel. As an example, four oxygen atoms is tetroxide instead of tetraoxide.
Some examples of molecular compounds are listed inTable7.6.
TABLE7.6: Examples of Molecular Compounds
Formula Name
NO nitrogen monoxide