Energy Bands in an Atom

Orbiting electrons contain energy bands and are confined to definite energy levels. The various shells in an atom represent these levels. Therefore, to move an electron from a lower shell to a higher shell a certain amount of energy is required.

This energy can be in the form of electric fields, heat,light, and even bombardment by other particles. Failure to provide enough energy to the electron, even if the energy supplied is just short of the required amount, will cause it to remain at its present energy level.

Supplying more energy than is needed will only cause the electron to move to the next higher shell and the remaining energy will be wasted. In simple terms, energy is required in definite units to move electrons from one shell to the next higher shell. These units are called QUANTA (for example 1, 2, or 3 quanta).

Electrons can also lose energy as well as receive it. When an electron loses energy, it moves to a lower shell. The lost energy, in some cases, appears as heat. If a sufficient amount of energy is absorbed by an electron, it is possible for that electron to be completely removed from the influence of the atom. This is called IONIZATION.

When an atom loses electrons or gains electrons in this process of electron exchange, it is said to be ionized. For ionization totake place, there must be a transfer of energy that results in a change in the internal energy of the atom.

An atom having more than its normal amount of electrons acquires a negative charge, and is called a NEGATIVE ION. The atom that gives up some of its normal electrons is left with fewer negative charges than positive charges and is called a POSITIVE ION. Thus, we can define ionization as the process bywhich an atom loses or gains electrons.

When atoms are spaced far enough apart, as in a gas, they have very little influence upon each other, and are very much like lone atoms. But atoms within a solid have a marked effect upon each other. The forces that bind these atoms together greatly modify the behavior of the other electrons.

One consequence of this close proximity of atoms is to cause the individual energy levels of an atom to break up and form energy bands . Discrete (separate and complete) energy levels still exist within these energy bands, but there are many moreenergy levels than there were with the isolated atom. In some cases, energy levels will have disappeared.

The illustration below shows the difference in the energy arrangement between an isolated atom and the atom in a solid. Notice that the isolated atom (such as in gas) has energy levels, whereas the atom in a solid has energy levels grouped into ENERGY BANDS.

The energy arrangement in atoms.

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