Lesson 2: Kinetic Molecular Theory

Lesson 2: Kinetic Molecular Theory

Transcona Community Learning CentreCHEM 30S


When you have completed this lesson, you should be able to:

  • Use the Kinetic Molecular Theory to explain the properties of gases.
  • Use the kinetic molecular theory to explain the properties of solids and liquids

Chemists use the Kinetic Molecular Theory to explain why matter, especially gases, behaves as it does. The major points of the theory are:

  • The particles that make up matter are very small.
  • There are spaces between the particles. In gases, this distance is much larger than the particles themselves.
  • The particles are in constant motion. In gases, the particles are in constant straight-line motion. They only change direction when they collide with another particle or the sides of the container.
  • There are forces of attraction between the particles. In gases, these forces are negligible.
  • As the temperature increases, the speed of the particles increases. As the temperature decreases, the particles' speed decreases. That is, the kinetic energy of the particles increases with increasing temperature and decreases with decreasing temperature.

Elastic Collisions

According to the Kinetic Molecular Theory, the particles of a gas are constantly moving in random straight-line motion. If the gas particles are in a container, the particles must eventually collide with the sides of the container or other gas particles. When the particles collide with the sides of the container, they exert a force upon the container’s walls. We call this force gas pressure. Pressure is defined as force per unit area.

When the particles collide there are 3 possible options that can occur in these collisions:

  1. The collision could result in a loss of energy. That is, the energy the particle contains before the collision is greater than after the collision. If this were true, the particles would eventually slow down and the pressure would decrease.
  2. If the particles have more energy after the collision than before, the particles would gain energy due to collisions. If this were true, the particles would speed up and the pressure would increase.
  3. If the energy of the particles before and after the collisions were equal the pressure inside the container would remain constant, at a constant temperature.

Let us examine these three options by studying a propane barbeque tank. If a propane tank is not used, the tank maintains its pressure (force of gas on the sides of the tank) providing the amount of gas remains constant (that is, there are no leaks) and the temperature remains unchanged. If the particles gained energy with every collision, the force of each collision would increase, increasing the pressure. If the particles lost energy with every collision, the force of each collision would eventually decrease, resulting in a lower pressure. This means that options one and two can not be valid options for collisions and actual events are more reflected by option three.

According to the Kinetic Molecular Theory collisions between particles and between particles and their container are elastic. This means there is no loss of energy. This is illustrated in the diagrams below.

In Figure 1, the bouncing ball loses energy with each bounce and as a result the force of the bounce decreases as does the height of its bounce. In Figure 2, the bounces are perfectly elastic. There is no loss of energy, so the ball returns to its original height.


Using the kinetic molecular theory and the physical characteristics discussed in the previous lesson, we can develop a particulate model for gases.

Gases are easily compressed, as is demonstrated by air compressors. This means the particles of a gas must be very far apart. Gases also have low densities, suggesting the particles are quite loosely packed. In fact, in a container of gas, less than one-tenth of one percent of its total volume is occupied by gas particles. This means more than 99.9% of a container of gas is empty space!

Gases will fill any volume they are given and diffusion is very easy. A good example is that an air freshener or perfume eventually moves throughout a room, even if sprayed in a corner. If gases fill any container, the forces of attraction between the particles must be very low. If the forces of attraction were large, the particles would be held in a defined space like solids and liquids. This means the particles of a gas move freely in rapid straight-line motion and their motion is rarely changed by the interference of other gas particles.

The freedom of motion also suggests that the forces of attraction, or repulsion, between individual gas particles is minimal or non-existent.


Since solids are not easily compressed, the particles must be so close together that they cannot be easily forced any closer. The high density of most solids also suggests there are more particles per unit volume than both liquids and gases.
The definite shape and volume of solids also suggests that the particles are held together quite tightly and must not move very much, in fact they probably just vibrate in place. Therefore, the forces of attraction between particles or intermolecular forces (imfs) in a solid must be very high. It is important to note that forces of attraction and chemical bonds are not the same.

In order for diffusion to occur, particles must easily move through the matter. Since the particles in a solid are very close together other particles cannot easily move through and mingle with solid particles.

Solids come in two main forms: crystalline and amorphous. Crystalline solids, or crystals, have molecules, ions or atoms in an orderly, geometric, three-dimensional arrangement. Each element and compound have unique crystal structures. A simple crystal structure is shown at the right.

A covalent network solid has atoms covalently bonded in a crystal. Examples of covalent networks are graphite and diamond, where carbon atoms are arranged in a regular repeating structure. The covalent bonds make it very difficult to separate the individual atoms.


Ionic solids have a regular arrangement of positive and negative ions in a crystal. The ions are held together by electrostatic forces. Each individual ion is held in the crystal lattice by several other ions.

Molecular solids such as sugar or ice, have molecules as the lattice points. The molecules are held together by intermolecular forces. These forces can be strong or weak, but are usually weaker than covalent solids and ionic solids.

The crystal arrangement holds the individual particles of the solid into that form, so the only movement that is possible is vibrational. As the temperature increases, random motion increases causing the particles to move further apart, decreasing strength of the forces holding the solid together.


Liquids are not easily compressed. The use of hydraulics supports this prediction. Front-end loaders, dump trucks, etc. lift heavy loads with a column of liquid. If liquids were compressible, this would not be possible the force of the load would compress the liquid. Since liquids are not compressed easily and they have relatively high density, the particles must still be very close together and densely packed.

Liquids do not have a definite shape. This means the forces of attraction between liquid particles must not be as strong as those in a solid. The forces are strong enough to hold the particles in an open container so the volume can be measured, but not strong enough to hold the particles in a constant shape.

If the particles in a liquid are not held in a constant shape, the particles must be moving around more than the particles of a solid. The particles of a liquid are able to just slide past each other.

The particles of a liquid are not much further apart than in a solid. There is just enough room for them to slide past each other. They must be close together or they would be easier to compress.

Since the particles of a liquid have spaces between them, other particles are able to move between the liquid particles. As liquid particles move, they collide with each other and other particles. These collisions push other particles through the liquid.

Other Types of Matter

Not all forms of matter can be readily described as solids, liquids or gases.

Liquid crystals are substances with particles that lose the fixed position of solid particles in only one or two dimensions when they are heated or an electric current is passed through them. The forces between the particles in a liquid crystal are weak, and their arrangement is easily disrupted. “LCDs”or liquid crystal displays are used in watches, thermometers, calculators, laptops and televisions.

Amorphous (Greek for “without form”) materials have an irregular arrangement of particles. These substances do not have a definite melting point and include many materials some examples of which are: peanut butter, candles, cotton candy, glass, rubber, plastic and asphalt. Amorphous carbon is produced from the decomposition of carbon compounds. When animal bones decompose, bone black is produced. It is used as a pigment and in the refining of sugars. The decomposition of coal produces the coke that is used in dry cell batteries. The figures below show the regular crystal structure of quartz and the amorphous structure of glass.

QuartzGlass (Amorphous)


In this lesson we have learned

  • The 4 states of matter are solid, liquid, gas and plasma.
    particles of a solid have strong intermolecular forces, very little space between the particles and the particles can only vibrate.
  • crystalline solids have particles arranged in a regular pattern
  • particles in amorphous solids do not have a regular pattern
  • crystalline solids can be atomic, molecular or ionic.
  • particles of liquids have strong intermolecular forces (but weaker than solids), are further apart than solids, and are able to slide past each other.

ASSIGNMENT: Internet Reading assignment – Do one or the other

  1. Televisions and monitors can come in several forms. Two of these are plasma displays and liquid crystal display (LCD).

This assignment will require you to research both types of televisions and make a decision on the best television for you. A good place to start is Read about:

  • How a plasma display works.
  • How a liquid crystal display works.

Then decide:

  • Which type is a better television and reasons for choice.
  • Include a general description of how the technology works
  1. Describe 4 various uses for plasmas, and include a description of how the technology works in each

Aug 2006Module 11