Quantum Theory

EM wave theory successfully explains “macroscopic” phenomena but fails to explain phenomena at the atomic and molecular levels. According to EM theory, electrons orbiting in an atom should constantly radiate EM waves, lose energy in the process, and ultimately crash into the nucleus— which, of course, does not happen. Thus, a new theory was required that would be capable of explaining the behavior of electrons in atoms and molecules.

The new theory—quantum theory—states that electrons in atoms (or molecules) exist in nonradiating states and that each state is associated with a specific energy level. The energy states possible for an atom differ from element to element, and those for a molecule differ from compound to compound. Each element or compound has a unique distribution of energy states (Fig 8-14).

Figure 8-14 Energy levels for the electrons in a hypothetical atom. The vertical axis indicates the increasing energy associated with each successive energy state for an atom’s electrons. Each element or compound has a unique

distribution of energy levels. (Illustration b y Edmond H. Thall, MD.)

When an electron “drops” from a higher energy state to a lower one, the difference in energy (E) is radiated as a “packet,” called a photon, with a characteristic frequency (ν) given by

E = hν