Molecular Compounds
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Printed: January 14, 2015
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C HAPTER
Chapter 1. Molecular Compounds
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Molecular Compounds
Lesson Objectives
•
•
•
•
Define a molecule and give examples of molecules.
Be able to name a molecular compound when given its formula.
Be able to write the formula for a molecular compound when given its name.
Know the common names for some simple molecular compounds, such as methane (CH4 ), ammonia (NH3 ),
phosphine (PH3 ), water (H2 O), and hydrogen sulfide (H2 S).
Lesson Vocabulary
•
•
•
•
covalent bond: Two or more atoms bonded together by sharing electrons.
molecule: A group of atoms are joined together by covalent bonds.
molecular formula: Designates how many of each atom are in a single molecule of that substance.
binary molecular compound: A molecular compound that is composed of two elements.
Check Your Understanding
• Give an example of a cation and an anion.
• Give an example of an ionic compound.
• Name the following compounds: MgO, CuO.
Introduction
So far we have looked at ionic compounds, in which atoms of various elements gain or lose electrons to produce
ions. The resulting ions are held together by strong attractions between oppositely charged particles. However, this
only works for bonds between atoms in which one partner (the metal) has a tendency to lose electrons, and the other
(the nonmetal) has a tendency to gain them. Then how might two nonmetals, such as nitrogen and oxygen, form
chemical bonds? Neither is likely to lose electrons and become a cation, but both require more electrons to reach
a noble gas configuration. Instead of a complete transfer of electrons, these atoms can bond by sharing electrons,
producing what is called a covalent bond. When a group of atoms are joined together by covalent bonds, the
resulting structure is called a molecule. Molecules are generally much smaller than the extended three-dimensional
networks of ions that are seen in ionic compounds. We will look much more at covalent bonding and molecules in
future chapters, but for now, we will focus on the ways in which molecules are named.
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Naming Binary Molecular Compounds
A binary molecular compound is 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, NO2 , and N2 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.
Table 1.1 shows the prefixes for one to ten atoms:
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 almost always first in a molecular formula, and hydrogen is usually written after nitrogen in a
formula such as NH3 . 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.
2. 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.
3. 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 -ide suffix 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.
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 tetrooxide.
Some examples of molecular compounds are listed in Table 1.2.
TABLE 1.2: Examples of Molecular Compounds
Formula
NO
2
Name
nitrogen monoxide
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Chapter 1. Molecular Compounds
TABLE 1.2: (continued)
Formula
N2 O
S2 Cl2
Cl2 O7
Name
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. Also, the o of mono- and the a of hepta- are dropped from the name when paired with
oxide.
The example S2 Cl2 emphasizes another difference between ionic and molecular substances. Because ionic substances 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, the molecular formula designates how many
of each atom are in a single molecule of that substance. S2 Cl2 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.
1.
2.
3.
4.
5.
BF3
NO
N2 O 5
PCl5
P4 O 6
Answer:
1.
2.
3.
4.
5.
boron trifluoride
nitrogen monoxide
dinitrogen pentoxide
phosphorous pentachloride
tetraphosphorus 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 B2 H6 . 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 following Table 1.3.
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TABLE 1.3: Common Names of Covalent Compounds
Formula
NO
N2 O
N2 H4
H2 O
NH3
PH3
H2 S
H2 O2
Common name
nitric oxide
nitrous oxide (laughing gas)
hydrazine
water
ammonia
phosphine
hydrogen sulfide
hydrogen peroxide
IUPAC Name
nitrogen monoxide
dinitrogen monoxide
dinitrogen tetrahydride
dinitrogen monoxide
nitrogen trihydride
phosphorus trihydride
dihydrogen monosulfide
dihydrogen dioxide
Nomenclature for Organic Molecules
Carbon has a unique ability to form an extremely large variety of molecules with just a few other common elements.
In fact, most of the molecules that make up living beings are composed of just carbon, hydrogen, oxygen, and
nitrogen (with a little sulfur and phosphorus as well). Knowing only the molecular formula for an organic molecule
is not enough to identify it; we also need to indicate how the atoms are arranged within the molecule. For example,
dimethyl ether and ethanol are two molecules with very different properties that both have the molecular formula
C2 H6 O. One is an extremely flammable gas, and the other is the intoxicating liquid found in alcoholic beverages.
As a result, most carbon-based molecules have a separate, more complex system of naming that we will cover in
another chapter. However, the formulas for a few common organic compounds can be found in the Table 1.4.
TABLE 1.4: Names of Common Organic Compounds
Formula
CH4
CH3 OH
C2 H6
C2 H5 OH
C3 H7 OH
C6 H12 O6
C12 H22 O11
Name
methane
methanol
ethane
ethanol
isopropanol (rubbing alcohol)
glucose
sucrose
Lesson Summary
• Molecular compounds are formed when atoms are held together by covalent bonds, which involve sharing
electrons rather than transferring them.
• The formula of a binary molecular compound shows how many of each atom are present in the molecule. The
less electronegative element is generally written first.
• Prefixes are used in the names of molecular compounds to designate how many of each atom are in the
molecule.
Lesson Review Questions
1. How is a covalent bond characterized?
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2.
3.
4.
5.
Chapter 1. Molecular Compounds
What differs between ionic compounds and molecules?
What are binary molecular compounds?
What differs between a molecular formula and an empirical formula?
Give the formula for each of the following binary covalent compounds:
a.
b.
c.
d.
e.
f.
carbon dioxide
phosphorus triiodide
sulfur dichloride
boron trifluoride
dioxygen difluoride
xenon trioxide
6. Name the following binary covalent compounds:
a.
b.
c.
d.
e.
f.
N2 F 4
HBr
SF4
BCl3
P2 O 5
ClF3
7. Is "nitrogen oxide" an appropriate name for the compound NO? Why or why not?
8. Is "calcium oxide" an appropriate name the compound CaO? Why or why not?
Further Reading / Supplemental Links
• Winter, M. (1993-2011). WebElements: the periodic table on the WWW, from http://www.webelements.com/
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