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11.2: Covalent Formulas and Nomenclature

Difficulty Level: At Grade Created by: CK-12

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.


  • chemical nomenclature


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, \text{Al}_2\text{S}_3. 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, \text{FeCl}_2.

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 below, 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, \text{CO} is called carbon monoxide, not monocarbon monoxide.
Greek Prefixes
Prefix Number Indicated
Mono- 1
Di- 2
Tri- 3
Tetra- 4
Penta- 5
Hexa- 6
Hepta- 7
Octa- 8
Nona- 9
Deca- 10


\begin{array} {ll}\text{N}_2\text{O} & \text{dinitrogen monoxide}\\\text{NO} & \text{nitrogen monoxide}\\\text{NO}_2 & \text{nitrogen dioxide}\\\text{N}_2\text{O}_3 & \text{dinitrogen trioxide}\\\text{N}_2\text{O}_4 & \text{dinitrogen tetroxide}\\\text{N}_2\text{O}_5 & \text{dinitrogen pentoxide}\\\text{SF}_6 & \text{sulfur hexafluoride}\\\text{CO}_2 & \text{carbon dioxide}\\\text{P}_4\text{O}_{10} & \text{tetraphosphorus decaoxide}\\\text{P}_2\text{S}_5 & \text{diphosphorus pentasulfide}\end{array}

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.

Review Questions

  1. Name the compound \text{CO}.
  2. Name the compound \text{PCl}_3.
  3. Name the compound \text{PCl}_5.
  4. Name the compound \text{N}_2\text{O}_3.
  5. Name the compound \text{BCl}_3.
  6. Name the compound \text{SF}_4.
  7. Name the compound \text{Cl}_2\text{O}.
  8. Write the formula for the compound sulfur trioxide.
  9. Write the formula for the compound dinitrogen tetrafluoride.
  10. Write the formula for the compound oxygen difluoride.
  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.

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