The energy of a molecule, atom, or subatomic particle that is confined to a region ofspace is quantized, or restricted to certain discrete values. These permitted energiesare called energy levels. The values of the permitted energies depend on the characteristics of the particle (for instance, its mass) and the extent of the region to which it isconfined. The quantization of energy is most important—in the sense that the allowedenergies are widest apart—for particles of small mass confined to small regions of space.Consequently, quantization is very important for electrons in atoms and molecules,but usually unimportant for macroscopic bodies. For particles in containers ofmacroscopic dimensions the separation of energy levels is so small that for all practicalpurposes the motion of the particles through space—their translational motion—isunquantized and can be varied virtually continuously. As we shall see in detail in Chapter 7, quantization becomes increasingly important as we change focus from rotationalto vibrational and then to electronic motion. The separation of rotational energy levels(in small molecules, about 10−23 J or 0.01 zJ, corresponding to about 0.01 kJ mol −1) issmaller than that of vibrational energy levels (about 10 kJ mol−1), which itself issmaller than that of electronic energy levels (about 10−18 J or 1 aJ, corresponding toabout 103 kJ mol−1). Figure F.4 depicts these typical energy level separations.