7.2. Molecular Compounds http://www.ck12.org
FIGURE 7.5
Carbon dioxide molecules consist of a central carbon atom bonded to two
oxygen atoms.Naming Binary Molecular Compounds
Recall that a molecular formula shows the number of atoms of each element that a molecule contains. A molecule of
water contains two hydrogen atoms and one oxygen atom, so its formula is H 2 O. A molecule of octane, which is a
component of gasoline, contains eight atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane
is C 8 H 18.
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 them
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. This discovery was discussed in theAtomic Structurechapter in the context of the law of
multiple proportions. Consider the elements nitrogen and oxygen. They combine to make several binary compounds,
including NO, NO 2 (Figure7.6), 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.
Listed below (Table7.7) are the prefixes up to ten.
TABLE7.7: 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-