Covalent Formulas and Nomenclature Say Thanks to the Authors (No sign in required)

Covalent Formulas and
Nomenclature
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Printed: October 12, 2014
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C HAPTER
Chapter 1. Covalent Formulas and Nomenclature
1
Covalent Formulas and
Nomenclature
Lesson Objectives
The student will:
• list the Greek prefixes from 1 to 10.
• provide the correct formulas for binary covalent compounds.
• name binary covalent compounds using the IUPAC nomenclature system.
Vocabulary
• chemical nomenclature
Introduction
The systematic procedure for naming chemical compounds, or the chemical nomenclature, is different for different
types of compounds. In the chapter “Ionic Bonds and Formulas,” we have discussed the procedures for naming
binary ionic compounds, ionic compounds involving polyatomic ions, and ionic compounds involving metals with
variable oxidation states. In this section, we will describe chemical nomenclature for covalently bonded compounds.
Because of the large numbers of covalent compounds that may form between the same two elements, the nomenclature system for covalent compounds is somewhat different to the nomenclature system for ionic compounds.
In naming ionic compounds, there is no need to indicate the number of atoms of each element in a formula because,
for most cases, there is only one possible compound that can form from the ions present. When aluminum combines
with sulfur, the only possible compound is aluminum sulfide, Al2 S3 . The only exception to this is a few metals
with variable oxidation numbers, and these are handled by indicating the oxidation number of the metal with Roman
numerals, such as in iron(II) chloride, FeCl2 .
With covalent compounds, however, we have a very different situation. There are six different covalent compounds
that can form between nitrogen and oxygen, and for two of them, nitrogen has the same oxidation number. Therefore,
the Roman numeral system will not work. Instead, chemists devised a nomenclature system for covalent compounds
that would indicate how many atoms of each element is present in a molecule of the compound.
Greek Prefixes
In naming binary covalent compounds, four rules apply:
1. The first element in the formula is named first using the normal name of the element.
2. The second element is named as if it were an anion. There are no ions in these compounds, but we use the
“-ide” ending on the second element as if it were an anion.
3. Greek prefixes, shown in Table 1.1, are used for each element to indicate the number of atoms of that element
present in the compound.
4. The prefix "mono-" is never used for naming the first element. For example, CO is called carbon monoxide,
not monocarbon monoxide.
1
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TABLE 1.1: Greek Prefixes
Prefix
MonoDiTriTetraPentaHexaHeptaOctaNonaDeca-
Number Indicated
1
2
3
4
5
6
7
8
9
10
Examples:
N2 O
NO
NO2
N2 O3
N2 O4
N2 O5
SF6
CO2
P4 O10
P2 S5
dinitrogen monoxide
nitrogen monoxide
nitrogen dioxide
dinitrogen trioxide
dinitrogen tetroxide
dinitrogen pentoxide
sulfur hexafluoride
carbon dioxide
tetraphosphorus decaoxide
diphosphorus pentasulfide
Lesson Summary
• Covalently bonded molecules use Greek prefixes in their nomenclature.
Further Reading / Supplemental Links
This website reviews the rules for naming binary covalent compounds and includes a quiz to test your understanding
of these rules.
• http://www.chem.purdue.edu/gchelp/nomenclature/covalent_2009.htm
Review Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
2
Name the compound CO.
Name the compound PCl3 .
Name the compound PCl5 .
Name the compound N2 O3 .
Name the compound BCl3 .
Name the compound SF4 .
Name the compound Cl2 O.
Write the formula for the compound sulfur trioxide.
Write the formula for the compound dinitrogen tetrafluoride.
Write the formula for the compound oxygen difluoride.
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Chapter 1. Covalent Formulas and Nomenclature
11. Write the formula for the compound dinitrogen pentoxide.
12. Write the formula for the compound sulfur hexafluoride.
13. Write the formula for the compound tetraphosphorus decaoxide.
Answer:
1. carbon monoxide
2. phosphorus trichloride
3. phosphorus pentachloride
4. dinitrogen trioxide
5. boron trichloride
6. sulfur tetrachloride
7. dichlorine monoxide
8. SO3
9. N2 F4
10. OF2
11. N2 O5
12. SF6
13. P4 O10
3