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The term quantum means, literally, amount. It acquired its special significance in physics when Bohr and others theorized that physical quantities such as energy and light could not have a range of values as on a continuum, but rather could have only discrete, step‐like values. The individual steps are so small that their existence escaped the notice of physicists until Bohr postulated them to explain his theory of the atom. We now refer to Bohr’s theory as quantum theory and the resulting explanations of motion in the atomic scale as quantum mechanics to distinguish it from the classical mechanics described by Isaac Newton, which is still needed for everyday engineering.

Figure 1.7 Electron orbitals and sub‐orbitals. (a) s orbital, (b) p suborbital, (c) p suborbitals, p_x, p_y, p_z.