4.2 Covalent Compounds: Formulas and Names

Learning Objectives

  1. Determine the chemical formula of a simple covalent compound from its name.
  2. Determine the name of a simple covalent compound from its chemical formula.

What elements make covalent bonds? Covalent bonds form when two or more nonmetals combine. For example, both hydrogen and oxygen are nonmetals, and when they combine to make water, they do so by forming covalent bonds. Nonmetal atoms in polyatomic ions are joined by covalent bonds, but the ion as a whole participates in ionic bonding. For example, ammonium chloride has ionic bonds between a polyatomic ion, NH4+, and Cl− ions, but within the ammonium ion, the nitrogen and hydrogen atoms are connected by covalent bonds as shown below.  Note that the dashed line indicates that the bond extends backward from the page and the wedge indicates that the bond extends forward from the page.  Square brackets are used with a charge on the outside of the brackets to indicate a polyatomic ion with the charge belonging to the polyatomic ion as a whole.

Example 2

Is each compound formed from ionic bonds, covalent bonds, or both?

  1. Na2O
  2. Na3PO4
  3. N2O4

Solution

Skill-Building Exercise

Is each compound formed from ionic bonds, covalent bonds, or both?

  1. Ba(OH)2

  2. F2

  3. PCl3

The chemical formulas for covalent compounds are referred to as molecular formulas because these compounds exist as separate, discrete molecules. Typically, a molecular formula begins with the nonmetal that is closest to the lower left corner. Then the other nonmetal symbols are listed. Numerical subscripts are used if there is more than one of a particular atom. For example, we have already seen CH4, the molecular formula for methane.

Binary (two-element) covalent compounds are named by different rules than those for simple ionic compounds. The first element in the formula is simply listed using the name of the element. The second element is named by taking the stem of the element name and adding the suffix –ide. A system of numerical prefixes is used to specify the number of atoms in a molecule. Table 4.1 “Numerical Prefixes for Naming Binary Covalent Compounds” lists these numerical prefixes. Normally, no prefix is added to the first element’s name if there is only one atom of the first element in a molecule. If the second element is oxygen, the trailing vowel is usually omitted from the end of a polysyllabic prefix but not a monosyllabic one (that is, we would say “monoxide” rather than “monooxide” and “trioxide” rather than “troxide”).

Table 4.1 Numerical Prefixes for Naming Binary Covalent Compounds
Number of Atoms in Compound Prefix on the Name of the Element
1 mono-*
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-
*This prefix is not used for the first element’s name.

Let us practice by naming the compound whose molecular formula is CCl4. The name begins with the name of the first element—carbon. The second element, chlorine, becomes chloride, and we attach the correct numerical prefix (“tetra-”) to indicate that the molecule contains four chlorine atoms. Putting these pieces together gives the name carbon tetrachloride for this compound.

Example 3

Write the molecular formula for each compound.

  1. chlorine trifluoride
  2. phosphorus pentachloride
  3. sulfur dioxide
  4. dinitrogen pentoxide

Solution

Skill-Building Exercise

Write the molecular formula for each compound.

  1. nitrogen dioxide

  2. dioxygen difluoride

  3. sulfur hexafluoride

  4. selenium monoxide

Note

Because it is so unreactive, sulfur hexafluoride is used as a spark suppressant in electrical devices such as transformers.

Example 4

Write the name for each compound.

  1. BrF5
  2. S2F2
  3. CO

Solution

Skill-Building Exercise

Write the name for each compound.

  1. CF4

  2. SeCl2

  3. SO3

For some simple covalent compounds, we use common names rather than systematic names. We have already encountered these compounds, but we list them here explicitly:

  • H2O: water
  • NH3: ammonia
  • CH4: methane

Methane is the simplest organic compound. Organic compounds are compounds with carbon atoms and are named by a separate nomenclature system that we will introduce in Section 4.6 “Introduction to Organic Chemistry”.

Concept Review Exercises

  1. How do you recognize a covalent compound?

  2. What are the rules for writing the molecular formula of a simple covalent compound?

  3. What are the rules for naming a simple covalent compound?

Answers

Key Takeaways

  • The chemical formula of a simple covalent compound can be determined from its name.
  • The name of a simple covalent compound can be determined from its chemical formula.

Exercises

  1. Identify whether each compound has covalent bonds.

    1. NaI
    2. Na2CO3
    3. N2O
    4. SiO2

  2. Identify whether each compound has covalent bonds.

    1. C2H6
    2. C6H5Cl
    3. KC2H3O2
    4. Ca(OH)2

  3. Identify whether each compound has ionic bonds, covalent bonds, or both.

    1. Na3PO4
    2. K2O
    3. COCl2
    4. CoCl2

  4. Identify whether each compound has ionic bonds, covalent bonds, or both.

    1. FeCl3
    2. Fe(NO3)3
    3. (NH2)2CO
    4. SO3

  5. Which is the correct molecular formula: H4Si or SiH4? Explain.

  6. Which is the correct molecular formula: SF6 or F6S? Explain.

  7. Write the name for each covalent compound.

    1. SiF4
    2. NO2
    3. CS2
    4. P2O5

  8. Write the name for each covalent compound.

    1. CO
    2. S2O3
    3. BF3
    4. GeS2

  9. Write the formula for each covalent compound.

    1. iodine trichloride
    2. disulfur dibromide
    3. arsenic trioxide
    4. xenon hexafluoride

  10. Write the formula for each covalent compound.

    1. boron trichloride
    2. carbon dioxide
    3. tetraphosphorus decoxide
    4. germanium dichloride

  11. Write two covalent compounds that have common rather than systematic names.

  12. What is the name of the simplest organic compound? What would its name be if it followed the nomenclature for binary covalent compounds?

Answers