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7.2: Molecular Compounds

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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 chemical compounds that take the form of discrete molecules. Examples include such familiar substances 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. 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 them 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. This discovery was discussed in the Atomic Structure chapter in the context of the law of multiple proportions. Consider the elements nitrogen and oxygen. They combine to make several binary compounds, including NO, NO2, and N2O. 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.

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 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.
  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 example 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 like boron generally form molecular compounds instead of ionic compounds.

Lesson Summary

  • Molecular compounds exist as a collection of individual molecules.
  • 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

Reviewing Concepts

  1. What type of elements typically form binary molecular compounds?
  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 that would be used in the name of a molecular compound for each of the following quantities of atoms.
    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 the 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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