Wrap-Up

In this lesson we explored the various kinds of bonds that determine the physical properties of materials and we learned how to tell from its chemical formula which bonds were present in any given material.

We saw that in network materials the bonds that hold the atoms together are the same ones that determine the material's physical properties - covalent bonds between non-metal atoms, ionic bonds between metal and non-metal atoms, or metallic bonds between metal atoms. We also learned that these are all relatively strong bonds, giving these materials very high melting and boiling points.

We found that for molecular materials, however, the covalent bonds holding the atoms together within molecules are different from the intermolecular forces that determine the material's physical properties. These intermolecular bonds - dispersion bonds, dipole-dipole forces, and hydrogen bonds, are far weaker than interatomic bonds so that molecular materials have far lower melting and boiling points than network materials.

We learned to use a molecule's Lewis structure to determine its shape, and, from its shape and composition, to predict its polarity, which, in turn helped us determine the kinds of intermolecular forces that were present. These, in turn, allowed us to make reasonable predictions about the physical properties of molecular materials: that water (H₂O), for example, should have a higher boiling point than methyl chloride (CH₃Cl); or that methyl alcohol (CH₃OH) should dissolve in water but methane (CH₄) should not.

In lab this week, you will enhance your understanding of molecular shapes by building 3D models for a number of molecules. You will use these models to confirm your analysis of their geometry and polarity, and use that information to determine the intermolecular forces they would experience.

Review the objectives to make sure that you have covered everything that you should have for this lesson, then test yourself by taking the self-quiz.

Self Quiz

1. The molecules of NH₃ are: (What would have to change in the molecules to change the answer?)

a. nonpolar

b. polar

2. What is the shape of CH₄ molecules?

a. bent

b. triangular pyramidal

c. trigonal planar

d. linear

e. tetrahedral

3. Which of the following chemicals has the lowest boiling point: (What factors cause this to be so?)

a. CO₂

b. H₂O₂

c. PbO₂

d. SO₂

e. SiO₂

4. The strongest forces between particles (i.e., molecules, ions, atoms, etc.) is found in materials which are at room temperature.

a. solids

b. liquids

c. gases

5. Which of the following would you expect to have polar linear molecules?

a. CO₂

b. H₂O

c. ICN

d. NaCl

e. SiO₂

f. SO₂

6. Alcohol evaporates more readily than water.

Which has stronger intermolecular forces?

Which has the higher melting point?

Which has the higher boiling point?

7. In water the interatomic bonds are (ionic/covalent/metallic) and the intermolecular forces are (dispersion/dipole-dipole/hydrogen bond).

8. What shape, polarity and IMF type do H₂S molecules have?

9. What type of bonding attracts NaCl to H₂O in a mixture of the two?

10. Put the following in order of increasing IMF strength. (What factors caused you to select this order?)

NH₃ N₂ NF₃

Self Quiz Answers

1. The molecules of NH₃ are: (What would have to change in the molecules to change the answer?)

b. polar

2. What is the shape of CH₄ molecules?

e. tetrahedral

3. Which of the following chemicals has the lowest boiling point: (What factors cause this to be so?)

a. CO₂

CO₂ is a linear and symmetric molecule, so that it is non-polar. The resulting Van der Waal's bonds are the weakest of the various kinds of intermolecular bonds, giving it the lowest boiling point of the compounds listed.

4. The strongest forces between particles (i.e., molecules, ions, atoms, etc.) is found in materials which are at room temperature.

a. solids

5. Which of the following would you expect to have polar linear molecules?

c. ICN

6. Alcohol evaporates more readily than water.

Which has stronger intermolecular forces? water

Which has the higher melting point? water

Which has the higher boiling point? water

7. In water the interatomic bonds are (ionic/covalent/metallic) and the IMFs are (disperson /dipole-dipole/hydrogen bonds).

8. What shape, polarity and IMF type do H₂S molecules have?

They are bent (angular), polar, and have dipole-dipole forces

9. What type of bonding attracts NaCl to H₂O in a mixture of the two? Ion-dipole forces

10. Put the following in order of increasing IMF strength. (What factors caused you to select this order?)

NH₃ N₂ NF₃

N₂ < NF₃ < NH₃

N₂ is non-polar and so has dispersion forces, the weakest IMF.

NF₃ is a trigonal pyramid and polar, so has dipole-dipole forces, the next strongest.

NH₃ has N-H bonds and so has hydrogen bonding, the strongest of the three.