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ChemActivity #3

Lewis Structures

(What Makes a Molecule?)

The properties of a molecule depend on how the electrons are distributed in the molecule. For example, it takes more energy to separate an oxygen atom from a carbon atom in a molecule of carbon monoxide, CO, than it does to separate an oxygen atom from a carbon atom in a molecule of carbon dioxide, CO2. Another example: CO2 is a linear molecule (the three nuclei lie in a straight line), whereas H2O is not linear (the three nuclei do not lie in a straight line). These experimentally determined facts can be predicted by making diagrams of molecules, called Lewis structures. The purpose of Lewis structures is to provide a simple way for chemists to represent molecules that allow reasonable predictions to be made about the structure and properties of the actual molecules.

Model (proposed by G. N. Lewis):

We have seen that a hydrogen atom has a core charge of + 1 and that a neutral hydrogen atom has one valence electron (i.e., the electrons in the outermost shell, which participate in chemical bonding). Also, we have seen that a fluorine atom has a core charge of +7 and seven valence electrons. Thus, we have represented these two atoms as shown in Figure 1. Alternatively, we could represent each atom with the appropriate atomic symbol and a dot for each electron, also shown in Figure 1. The latter designations take up less space, make the atom easily identifiable, and are more concise; the core charge information is lost, however, and it is the responsibility of the reader to keep the core charges in mind.

Critical Thinking Questions

1. Given the shell model of the atom, why do you think Lewis proposed a maximum of two electrons for hydrogen and a maximum of eight for carbon, nitrogen, oxygen, and fluorine atoms?

2. Answer the following for the nitrogen atom:

a) How many valence electrons does N have?

b) What is the Lewis representation for N?

c) How many additional electrons does one N atom require when it forms a molecule?

d) What is the likely formula for a molecule composed of hydrogen atoms and one nitrogen atom? Draw the Lewis structure for this molecule.

3. Answer the following for the sulfur atom:

a) How many valence electrons does S have?

b) What is the Lewis representation for S?

c) How many additional electrons does one S atom require when it forms a molecule?

d) What is the likely formula for a molecule composed of hydrogen atoms and one sulfur atom? Draw the Lewis structure for this molecule.

4. Make a checklist that can be used to determine if a Lewis structure for a molecule is correct.

Exercises

1. How many valence electrons are in the F2 molecule? Write the Lewis structure for F2.

2. How many valence electrons are in the Cl2 molecule? Write the Lewis structure for Cl2.

3. How many valence electrons are there in the SiH4 molecule? Write the Lewis structure for SiH4.

4. How many valence electrons are there in the SiCl4 molecule? Write the Lewis structure for SiCl4.

5. How many valence electrons are there in the PCl3 molecule? Write the Lewis structure for PCl3.

Bond Order and Bond Strength

Critical Thinking Questions

1. Without referring to the Lewis structure, calculate the total number of valence electrons in the H2CO molecule (i.e., count the total number of valence electrons from the atoms only).

2. Count the total number of valence electrons in the Lewis structure for H2CO in Table 1 (i.e., count the total number of valence electron from the Lewis structure only).

3. For H2CO (see Table 1), is the sum of the bonding electrons and lone pair electrons (also known as non-bonding electrons) around the carbon atom consistent with the Lewis model?

4. For H2CO (see Table 1), is the sum of the bonding electrons and lone pair electrons (non-bonding electrons) around the oxygen atom consistent with the Lewis model?

5. Without referring to the Lewis structure, calculate the total number of valence electrons in the N2 molecule.

6. What is the total number of electrons in the Lewis structure for N2 in Table 1?

7. For N2 (see Table 1), is the sum of the bonding electrons and nonbonding electrons around each nitrogen atom consistent with the Lewis model?

8. How does one determine the total number of electrons that should be used to generate a Lewis structure?

9. What is the total number of valence electrons for C2H2? Draw the Lewis structure for C2H2.

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