Maxwell Boltzmann speed distribution curve
The y axis of the graph shows the probablity that the speed of a molecule in a gas lies between x metres per second and and x + 1 metres per second. The curve shown corresponds to a temperature of about 80 K

• The red graph shows the curve for Helium gas
• The blue graph shows the curve for Argon.

Notice that:

• The area under each graph is the same.
• The area under each graph is equal to 1.
• At any chosen temperature, helium atoms tend to move faster than argon atoms.
• There is a wide spread of molecular speeds in a gas.
• Even at low temperatures, some molecules move very fast: many hundreds of metres per second

The quicktime animation below shows the effect on the MB curve of heating from around 80 K to 500 K.

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The graphs above are based on the formula shown below.

k is the value of the Boltzmann constant, m is the mass of a helium atom and n is the mass of an argon atom