During the late nineteenth century, scientists developed the kinetic molecular theory of matter to account for the behavior of atoms and molecules that make up matter.  The kinetic molecular theory is based on the idea that particles of matter are always in motion.  In other words, they are always on the go!  This theory can be used to explain the properties of all three states of matter in terms of the energy of particles and the forces that act between them.  So how does the kinetic- molecular theory apply to gases?  Well in order to simplify the study of gases, the theory provides of model of what is called an ideal gas.  An ideal gas is an imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory.

The kinetic-molecular theory of gases is based on the following 5 assumptions:

      Assumption # 1:  Gases consist of large numbers of tiny particles that are far apart relative to their own size.

These tiny particles usually occupy a volume that is about 1000 times greater than the volume occupied by particles in the liquid or solid state.  As a result, molecules of gases are much father apart than those of liquids or solids.  In fact, most of the volume occupied by a gas is empty space!  This idea accounts for the lower density of gases compared with that of liquids and solids.  It also explains why gases are so easily compressed.

      Assumption # 2:  Collisions between gas particles and between particles and container walls are elastic collisions.

What is an elastic collision?  Well, it is the idea of two things bumping together without losing any kinetic energy as long as the temperature is constant.  In other words, after the impact, both things do not slow down or gain any speed.

One way of imagining this is pretending that you are running in a room with the door closed.  According to this assumption, If you hit any wall, you automatically bounce back with the same initial speed.  You do not slow down or speed up.  In fact, you continue to bounce back and forth to all walls of the room.

        Assumption # 3:  Gas particles are in continuous, rapid, random motion.

As a result, they have kinetic energy which is the energy of motion.  In fact, these particles move in all directions.  Therefore, if you were running in the room described in assumption #2, you will hit all walls of the room.

       Assumption # 4:  There are no forces of attraction or repulsion between gas particles. 

Therefore, we can imagine ideal gas molecules as small billiard balls.  When two or more collide, they do not stick together but immediately bounce apart.

Now, using the analogy of you running in a room, pretend that you are running in this room with another person.  Since you are bound to run into each other because your motion is random, you will eventually make contact and bounce right off!  In other words, you continue moving without an increase or decrease in energy or speed.

       Assumption # 5:  The average kinetic energy of gas particles depends on the temperature of the gas.

Since all the particles of a specific gas have the same mass, their kinetic energies of gas particles increase with an increase in temperature and decrease with a decrease in temperature.

To take a closer look at this assumption, please click here: Temperature
 

Demo# 3:  Kinetic Theory Made Visible is a mental model of the kinetic-molecular theory.

Expansion

Gases do not have a definite volume or a definite shape.  They completely fill up any container that they are enclosed in and take its shape.  If we place a gas in a small container and then transfer it to a bigger container, the gas will expand to fill up the bigger container.  Assumptions 3 and 4 account for this idea because gas particles are always on the move and move such that they are not attracted or repelled by each other when they bump into each other.
 

Fluidity

Since the attractive forces between gas particles are practically non-existent (assumption # 4), gas particles glide easily past one another.  That is why they have the ability to flow just like liquids.

Compressibility

When a gas is compressed, the gas particles, which are initially far apart (according to assumption # 1), are now very close together.  As a result, the volume of the gas is decreased.  To learn more about compressibility and to see a Demo #1.

Other physical properties that the kinetic-molecular theory assumptions account for are:

Density and Diffusion & Effusion

These are called real gases.