An ideal gas is a gas with no fixed volume or pressure. It expands to fill the container, travelling in random straight-line motion. The volume of the molecules themselves is negligible. These gas molecules do not exert long-range forces on others (no intermolecular force). Most gases at room temperature behave (approximately) like an ideal gas.
of ideal gas has a volume of .
The internal energy of ideal gas depends only on kinetic energy, i.e. temperature, since they don’t interact with another (no intermolecular forces and thus no potential energy). For example, the internal energy of monatomic ideal gases is .