Optical, Thermal and Other Properties
In Chapter 2 the build-up of electron shells was considered and it was stated that each electron shell and sub-shell within an atom represents a particular energy level (Section 2.6). In the normal state the electrons of an atom occupy the lowest permissible energy levels. This is called the ‘ground state’ for the atom. As mentioned Section 2.6 it is possible for the energy of an electron to be raised to a higher level. This is called the excited state. The excited state is an unstable state and excited electrons quickly revert to their ground state and in so doing cause energy to be emitted. This energy is emitted as a discrete packet or quantum. These quanta released by excited atoms are called photons and photons travel at the speed of light. The magnitude of energy quanta is of the order of 10−19 J and it is customary to use the electron volt as a unit of energy for these low values, rather than the SI unit of energy, the joule.
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