Structure & Reactivity in Chemistry

Structure-Property Relationships

SP3. Kinetic-molecular theory

Temperature is an indicator of how much energy is available in the surroundings. The higher the temperature, the more energy there is available, and the faster molecules can go. Different kinds of matter require different amounts of energy to get moving. That's why some compounds are gases at room temperature while others are solids.

There are many reasons for these differences. One factor is the mass of a molecule. If you hit a tennis ball with a tennis racket, you can be sure that it will sail through the air, but the same thing might not be true if you hit a bowling ball with a tennis racket. By putting an equal amount of energy into a tennis ball and a bowling ball, you would make the tennis ball move much faster than the bowling ball. Lighter objects need less energy to reach high speeds.

For a molecular example, let's look at halogens. All the elemental halogens are simple diatomic molecules. Their chemical properties have many similarities, but they have very different boiling points and melting points. Iodine, with the greatest mass, is a solid at room temperature, while fluorine has the lowest mass and is a gas at pretty much all the temperatures you are likely to encounter on Earth.

Name & formula MW, g/mol mp, C bp, C
fluorine, F2 38.0 -219.6 -188.4
chlorine, Cl2 70.9 -100.98 -34.6
bromine, Br2 159.8 -7.2 58.8
iodine, I2 253.8 113.5 184.3

Clearly the mass of a molecules influences its state.  Does the mass of a molecule alone allow us to predict its behavior? No. A much more important factor involves another difference between gases, liquids and solids: attraction between molecules. The molecules in solids are not just sitting still; they are sitting still together. Getting a crystal of iodine to evaporate requires coaxing molecules away from each other, and overcoming this attraction between them.

There are many different types of intermolecular attractions. Let's simplify things and focus on just three general types: London dispersion interactions, dipole interactions, and hydrogen bonding.

Problem SP3.1.

Based on the data in the table above, what would you predict for the melting point and boiling point of nitrogen, N2, and oxygen, O2?

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

Creative Commons License
Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License

Send corrections to cschaller@csbsju.edu

Back to Structure & Properties: Intermolecular Attractions

Back to Web Materials on Structure & Reactivity in Chemistry