# 4: Gases

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• 4.1: The Perfect Gas
One way to begin to describe the nature of matter is to make a simplified, idealized model. The perfect gas is one such idealized model. In a perfect gas (or ideal gas), there are no interactions among the particles, which themselves have no volume. In this section it will be shown how a sample of idealized gas particles trapped in a container can be described by the perfect gas law, using the four variables of pressure, temperature, the number of gas particles, and the volume of the container.
• 4.2: Real Gases (Deviations From Ideal Behavior)
Real gas molecules have volume and experience intermolecular forces, and so are not accurately described by the perfect gas law. However, the perfect gas law can be modified to take into account the non-ideality of real gases. In this section we will describe how the characteristics of real gases cause their non-ideality, and then derive the modified gas laws.
• 4.3: The Repulsive Term in the Lennard-Jones Potential
Proposed by Sir John Edward Lennard-Jones, the Lennard-Jones potential describes the potential energy of interaction between two non-bonding atoms or molecules based on their distance of separation. The potential equation accounts for the difference between attractive forces (dipole-dipole, dipole-induced dipole, and London interactions) and repulsive forces.

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