Naming Binary Molecular
Compounds
Ck12 Science
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Printed: August 18, 2016
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Naming Binary Molecular Compounds
1
Naming Binary Molecular
Compounds
• Define molecular compound.
• Explain how molecular compounds are different from ionic compounds.
• Be able to name the compound when given the formula.
Why do so many relatives in royalty share the same name?
Some families name a son (usually the firstborn) after his father. So it is somewhat common to find John Smith,
Jr. named after John Smith the father. A few families may take it further and name the grandson John Smith
III. Countries with long histories of royalty take the naming even further. One line of kings named Henry goes up
to Henry the Eighth (not the nicest guy in the world - he had six wives and two of them came to untimely ends). The
use of numbering for names adds clarity to a system -we always know which Henry we are talking about.
Inorganic chemical compounds can be broadly classified into two groups: ionic compounds and molecular compounds. The structure of all ionic compounds is an extended three-dimensional array of alternating positive and
negative ions. Since ionic compounds do not take the form of individual molecules, they are represented by empirical
formulas. Now we will begin to examine the formulas and nomenclature of molecular compounds.
Molecular Compounds
Molecular compounds are inorganic compounds that take the form of discrete molecules. Examples include such
familiar substance as water (H2 O) and carbon dioxide (CO2 ). These compounds are very different from ionic
compounds like sodium chloride (NaCl). Ionic compounds are formed when metal atoms lose one or more of their
electrons to nonmetal atoms. The resulting cations and anions are electrostatically attracted to each other.
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So what holds the atoms of a molecule together? Rather than forming ions, the atoms of a molecule share their
valence electrons in such a way that a bond forms between pairs of atoms. In a carbon dioxide molecule, there are
two of these bonds, each occurring between the carbon atom and one of the two oxygen atoms.
FIGURE 1.1
Carbon dioxide molecules consist of a central carbon atom bonded to 2
oxygen atoms.
Larger molecules can have many, many bonds that serve to keep the molecule together. In a large sample of a given
molecular compound, all of the individual molecules are identical.
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 H2 O. A molecule of octane, which is
a component of gasoline, contains 8 atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane
is C8 H18 .
FIGURE 1.2
Nitrogen dioxide (NO2 ) is a reddish-brown toxic gas that is a prominent air
pollutant produced by internal combustion engines.
A binary molecular compound is a molecular compound that is composed of two elements. The elements that
combine to form binary molecular compounds are both nonmetal atoms. This contrasts with ionic compounds,
which were formed from a metal ion and a nonmetal ion. Therefore, binary molecular compounds are different
because ionic charges cannot be used to name them or to write their formulas. Another difference is that two
nonmetal atoms will frequently combine with one another in a variety of ratios. Consider the elements nitrogen
and oxygen. They combine to make several compounds including NO, NO2 , and N2 O. They all can’t 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.
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Chapter 1. Naming Binary Molecular Compounds
Prefixes are used in the names of binary molecular compounds to identify the number of atoms of each element. Table 1.1 shows the prefixes up to ten.
TABLE 1.1: Numerical Prefixes
Number of Atoms
1
2
3
4
5
6
7
8
9
10
Prefix
monoditritetrapentahexaheptaoctanonadeca-
The rules for using the prefix system of nomenclature of binary molecular compounds can be summarized as follows.
1. Generally, the less-electronegative element is written first in the formula, though there are a few exceptions. Carbon is always first in a formula and hydrogen is after nitrogen in a formula such as NH3 . The
order of common nonmetals in binary compound formulas is C, P, N, H, S, I, Br, Cl, O, F.
2. When naming, the appropriate prefix is used only if there are more than one atom of that element in the
formula.
3. The second element is named after the first, but with the ending of the element’s name changed to -ide. The
appropriate prefix is always used for the second element.
4. The a or o at 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 in Table 1.2.
TABLE 1.2:
Formula
NO
N2 O
S2 Cl2
Cl2 O7
Name
nitrogen monoxide
dinitrogen monoxide
disulfur dichloride
dichlorine heptoxide
Notice 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. The S2 Cl2 emphasizes that the formulas for molecular compounds are not reduced to their
lowest ratios. The o of mono- and the a of hepta- are dropped from the name when paired with oxide.
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Summary
• Molecular compounds are inorganic compounds that take the form of discrete molecules.
• The atoms of these compounds are held together by covalent bonds.
• Prefixes are used to indicate the number of atoms of an element that are in the compound.
Review
1.
2.
3.
4.
5.
What are molecular compounds?
How do they differ from ionic compounds?
What holds the atoms of a molecule together?
Are the elements of molecular compounds metals or non-metals?
List the prefixes for the number of atoms of an element when there are four atoms of that element in the
compound.
6. List the prefixes for the number of atoms of an element when there are seven atoms of that element in the
compound.
7. Name the following compounds:
1. ClF3
2. As2 O5
3. B4 H10
• molecule: Two or more atoms that have been chemically combined.
• bond: By sharing valence electrons, bonds hold the atoms of a molecule together.
• valence electron: Can form molecules by bonding with atoms.
References
1. Anonymous. Edward I .
2. Ben Mills (Wikimedia: Benjah-bmm27). http://commons.wikimedia.org/wiki/File:Carbon-dioxide-3D-vdW
.png .
3. User:Greenhorn1/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Nitrogendioxide.jpg .
4