We noted earlier in the chapter that solids and liquids are nearly incompressible, an observation suggesting that atoms are fairly hard and cannot be pressed together once they come into contact with each other. Based on this observation, suppose we were to model atoms as “hard spheres” that do not interact except for occasional elastic collisions when two atoms come into contact and bounce apart. FIGURE 18.7 reminds you that such a gas completely and uniformly fills its container.

FIGURE 18.7 A gas completely and uniformly fills its container at all temperatures. Changing the temperature changes the speeds of the molecules but not their ability to fill the container.

This is a model of an atom—what we might call the ideal atom—because it ignores the weak attractive interactions that hold liquids and solids together. A gas of these noninteracting atoms is called an ideal gas. It is a gas of small, hard, randomly moving atoms that bounce off each other and the walls of their container but otherwise do not interact. The ideal gas is a somewhat simplified description of a real gas, but experiments show that the ideal-gas model is quite good for real gases if two conditions are met:

1. The density is low (i.e., the atoms occupy a volume much smaller than that of the container), and

2. The temperature is well above the condensation point.

If the density gets too high, or the temperature too low, then the attractive forces between the atoms begin to play an important role and our model, which ignores those attractive forces, fails. These are the forces that are responsible, under the right conditions, for the gas condensing into a liquid.

We’ve been using the term “atoms,” but many gases, as you know, consist of molecules rather than atoms. Only helium, neon, argon, and the other inert elements in the far-right column of the periodic table of the elements form monatomic gases. Hydrogen $(\text{H}{}_2)$, nitrogen $(\text{N}{}_2)$, and oxygen $(\text{O}{}_2)$ are diatomic gases. As far as translational motion is concerned, the ideal-gas model does not distinguish between a monatomic gas and a diatomic gas; both are considered as simply small, hard spheres. Hence the terms “atoms” and “molecules” can be used interchangeably to mean the basic constituents of the gas.