4.5 Characteristics of Molecules

Learning Objectives

  1. Determine the molecular mass of a molecule.
  2. Predict the general shape of a simple covalent molecule.

Unlike the ions in ionic compounds, which are arranged in lattices called crystals, molecules of covalent compounds exist as discrete units with a characteristic mass and a certain three-dimensional shape.

Molecular Mass

The mass of a molecule—the molecular mass (sometimes called the molecular weight)—is simply the sum of the masses of its atoms. As with formula masses, it is important that you keep track of the number of atoms of each element in the molecular formula to obtain the correct molecular mass. (For more information about formula masses, see Chapter 3 “Ionic Bonding and Simple Ionic Compounds”, Section 3.5 “Formula Mass”.)

Example 7

What is the molecular mass of each covalent compound?

  1. H2O
  2. C6H6
  3. NO2
  4. N2O5

Solution

Skill-Building Exercise

What is the molecular mass of each covalent compound?

  1. C2H2
  2. CO
  3. CO2
  4. BF3

Molecular Shape: VSEPR Theory

Unlike ionic compounds, with their extended crystal lattices, covalent molecules are discrete units with specific three-dimensional shapes. The shape of a molecule is determined by the fact that covalent bonds, which are composed of negatively charged electrons, tend to repel one another. This concept is called the valence shell electron pair repulsion (VSEPR) theory. For example, the two covalent bonds in BeCl2 stay as far from each other as possible, ending up 180° apart from each other. The result is a linear molecule:

image

The three covalent bonds in BF3 repel each other to form 120° angles in a plane, in a shape called trigonal planar:

image

The molecules BeCl2 and BF3 actually violate the octet rule; however, such exceptions are rare and will not be discussed in this text.

Note

Try sticking three toothpicks into a marshmallow or a gumdrop and see if you can find different positions where your “bonds” are farther apart than the planar 120° orientation.

The four covalent bonds in CCl4 arrange themselves three dimensionally, pointing toward the corner of a tetrahedron and making bond angles of 109.5°:

image

Molecules with lone electron pairs around the central atom have a shape based on the position of the atoms, not the electron pairs. For example, NH3 has one lone electron pair and three bonded electron pairs. These four electron pairs repel each other and adopt a tetrahedral arrangement:

image

However, the shape of the molecule is described in terms of the positions of the atoms, not the lone electron pairs. Thus, NH3 is said to have a pyramidal shape, not a tetrahedral one. Similarly, H2O has two lone pairs of electrons around the central oxygen atom, in addition to the two bonded pairs:

image

Although the four electron pairs adopt a tetrahedral arrangement due to repulsion, the shape of the molecule is described by the positions of the atoms only. The shape of H2O is bent.

In determining the shapes of molecules, it is useful to first determine the Lewis diagram for a molecule. The shapes of molecules with multiple bonds are determined by treating the multiple bonds as one bond. Thus, CH2O has a shape similar to that of BF3:

image

Example 8

Describe the shape of each molecule.

  1. PCl3
  2. CO2

Solution

Skill-Building Exercise

Describe the shape of each molecule.

  1. CBr4

  2. BCl3

Concept Review Exercises

  1. How do you determine the molecular mass of a covalent compound?

  2. How do you determine the shape of a molecule?

Key Takeaways

  • A molecule has a certain mass, called the molecular mass.
  • Simple molecules have geometries that can be determined from VSEPR theory.

Exercises

  1. What is the molecular mass of each compound?

    1. H2S
    2. N2O4
    3. ICl3
    4. HCl

  2. What is the molecular mass of each compound?

    1. O2F2
    2. CCl4
    3. C6H6
    4. SO3

  3. Aspirin (C9H8O4) is a covalent compound. What is its molecular mass?

  4. Cholesterol (C27H46O) is a biologically important compound. What is its molecular mass?

  5. What is the shape of each molecule?

    1. H2S
    2. COCl2
    3. SO2

  6. What is the shape of each molecule?

    1. NBr3
    2. SF2
    3. SiH4

  7. Predict the shape of nitrous oxide (N2O), which is used as an anesthetic. A nitrogen atom is in the center of this three-atom molecule.

  8. Predict the shape of acetylene (C2H2), which has the two carbon atoms in the middle of the molecule with a triple bond. What generalization can you make about the shapes of molecules that have more than one central atom?