Lesson 9: 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 (called "interatomic" or "intramolecular" bonds) are different from the bonds between molecules (the "intermolecular" bonds) that determine the material's physical properties. These intermolecular bonds - Van der Waal's bonds, dipole-dipole bonds, 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 bonds 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.
Your lab work this week is to complete Exercise 15; you do not have a formal lab report to write. You will start by drawing the Lewis diagram for each molecule, then build a model of the molecule and have the lab instructor check both the Lewis diagrams and the models. Once you have your models completed, you should be able to determine the molecular shape, polarity, and intermolecular bonding.
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.
There is no problem set for this lesson, but do remember to complete the lab exercise before you go on.
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. angular
b. triangular pyramidal
c. flat triangular
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 bonding 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 bonds?
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 bonds are (van der Waals/dipole-dipole/hydrogen).
8. What shape, polarity and intermolecular bond type do H2S molecules have?
9. What type of bonding attracts NaCl to H2O?
10. Put the following in order of increasing bond 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 bonding 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 bonds? 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 intermolecular bonds are (van der Waals/dipole-dipole/hydrogen).
8. What shape, polarity and intermolecular bond type do H2S molecules have?
They are bent (angular), polar, and have dipole-dipole bonds
9. What type of bonding attracts NaCl to H2O? Ion-dipole bonds
10. Put the following in order of increasing bond strength. (What factors caused you to
select this order?)
NH3 N2 NF3
N2 < NF3 < NH3
N2 is non-polar and so has Van der Waal's, the weakest intermolecular bonding.
NF3 is a trigonal pyramid and polar,
so has dipole-dipole bonds, the next strongest.
NH3 has N-H bonds and so has
hydrogen bonds, the strongest of the three.