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7.2: Molecular Compounds

Created by: CK-12

Lesson Objectives

  • Describe the difference between an ionic compound and a molecular compound.
  • Name a molecular compound given its formula.
  • Write the correct formula for a molecular compound given its name.

Lesson Vocabulary

  • binary molecular compound

Check Your Understanding

Recalling Prior Knowledge

  • Describe the relationships between the following: atom, element, molecule, and compound.

Inorganic chemical compounds can be broadly classified into two groups: ionic compounds and molecular compounds. In the last lesson, we learned that 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 (H2O) and carbon dioxide (CO2) (see Figure below). 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. You will learn more about ionic chemical bonding in the chapter Ionic and Metallic Bonding.

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.

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 H2O. 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 C8H18.

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. This discovery was discussed in the Atomic Structure chapter and led to the law of multiple proportions. Consider the elements nitrogen and oxygen. They combine to make several compounds including NO, NO2, and N2O. 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.

Nitrogen dioxide (NO2) is a reddish-brown toxic gas that is a prominent air pollutant produced by internal combustion engines.

Prefixes are used in the names of binary molecular compounds to identify the number of atoms of each element. Table below show the prefixes up to ten.

Numerical Prefixes
Number of Atoms Prefix
1 mono-
2 di-
3 tri-
4 tetra-
5 penta-
6 hexa-
7 hepta-
8 octa-
9 nona-
10 deca-

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 below.

Examples of Molecular Compounds
Formula Name
NO nitrogen monoxide
N2O dinitrogen monoxide
S2Cl2 disulfur dichloride
Cl2O7 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 S2Cl2 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.

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 B2H6. Notice that metalloids such as boron generally form molecular rather than ionic compounds.

Lesson Summary

  • Molecular compounds can be described as individual molecules.
  • The formula of a binary molecular compound shows the number of each atom in the molecule with the less electronegative element written first.
  • Prefixes are used in the names of molecular compounds to designate the number of each atom in the molecule.

Lesson Review Questions

Reviewing Concepts

  1. What are the elements that make up a binary molecular compound?
  2. What determines the order of the elements in the formula of a molecular compound?
  3. Based on your answer to number 2, which of the formulas in each pair is correct?
    1. BrCl or ClBr
    2. ON2 or N2O
  4. Give the prefix for each of the following numbers of atoms of an element in the formula of a molecular compound.
    1. 6
    2. 3
    3. 9
    4. 5
    5. 8

Problems

  1. Name these molecular compounds.
    1. CCl4
    2. ClF3
    3. N2O3
    4. As2O5
    5. B4H10
    6. IF7
  2. Write correct formulas for the following molecular compounds.
    1. xenon hexafluoride
    2. tetraphosphorus decasulfide
    3. oxygen difluoride
    4. tellurium trioxide
    5. germanium monoxide
    6. trisilicon tetranitride
  3. Name the compounds below. Some are ionic and some are molecular.
    1. N2O
    2. Na2O
    3. K2Cr2O7
    4. P2O5
    5. CuSO4
    6. SnO
    7. BF3
    8. W2O3
  4. Write formulas for the following compounds. Some are ionic and some are molecular.
    1. sodium hydrogen sulfide
    2. carbon disulfide
    3. tetraselenium tetranitride
    4. iron(II) dihydrogen phosphate
    5. scandium(III) nitrate
    6. ammonium hydroxide
    7. bromine pentafluoride
    8. aluminum selenide

Further Reading / Supplemental Links

Points to Consider

Acids and bases are a special and important class of inorganic compounds. When an acid or a base dissolves in water, it breaks apart into ions.

  • What ion must be present in its water solution for a compound to be considered an acid? A base?
  • How are acids and bases named?

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