1. Ideal gas molecules have no volume. Real gas molecules do have a finite size. This means that in a real gas, the molecules themselves occupy some of the space in the container, and this volume cannot be ignored at high pressures.
2. Ideal gas molecules have no intermolecular forces. Real gas molecules experience attractive forces between them (like van der Waals forces). These forces cause the molecules to deviate from the ideal gas behavior, especially at low temperatures and high pressures, where the molecules are closer together and the forces become more significant.
