Basic Atomic Theory
7.3 Energy Shells
All electrons are arranged around the nucleus of an atom in a pre-determined manner as mentioned earlier but we should however have a closer look at these arrangements.
Together with figure 7.1 we should also refer to figure 7.2 to find out how the arrangement of the electrons, protons and neutrons in an atom is accomplished.
Protons (+) and neutrons
(no charge)
Negatively charged orbiting electrons
K - shell L - shell Nucleus
L
In the representation of figure 7.2 the centre circle will represent the nucleus consisting of the protons and neutrons. The outer circle or circles will indicate the shells for the orbiting electrons. Although these representations are two dimensional, the shape of a tennis ball can almost represent an atom in that the major shells of an atom are also sub-divided into sub-shells. This concept is illustrated in figure 7.2.
The major shells of an atom are designated the K, L M and N shells and the sub-shells are designated the S, P, D and F shells. Each shell, major or sub-shell, may only have a pre-determined number of electrons per shell and the number of electrons in the major shells is determined by the expression 2.n2 where n will represent the orbit number counting from the inner circle toward the outer circle. The number of electrons per major shell can thus be calculated in the following manner.
K shell: 2.n2 = 2 ×12 = 2 Thus, the K shell may only contain 2 electrons.
L shell: 2.n2 = 2 × 22 = 8 Thus, the L shell may only contain 8 electrons.
M shell: 2.n2 = 2 × 32 = 18 Thus, the M shell may only contain 18 electrons.
N shell: 2.n2 = 2 × 42 = 32 Thus, the N shell may only contain 32 electrons.
Since the number of electrons per major shell can be determined and predicted it follows that the number of electrons per sub-shell can also be determined and predicted. The number of electrons per sub-shell is given as follows:
S shell = 2 electrons P shell = 6 electrons D shell = 10 electrons F shell = 14 electrons
Electrons can be made to jump to other orbits by means of heat, light energy or an electric field at or near an atom. Whatever the form of energy used, it is referred to as ionization potential. This arrangement of electrons orbiting an atom will now allow one to determine the conducting properties of the element and especially the outer orbit or valence shell of an atom.
7.4 Valency
The number of electrons in the outer shell of an atom, called the valence shell, will determine the valency of that element and may be defined as an indication of the ability of an atom to gain or loose electrons and will determine the electrical properties of that element. Should an atom not have the full complement of electrons in the valence shell, that element is deemed to be a conductor since that particular atom will always be on the lookout for electrons to fill the vacancies, which exist. Should an atom however have the full compliment of electrons in the valence shell it will not need to look for electrons to fill up vacancies since no vacancies exist and is then deemed to be an insulator.
The above statements are not always true. A conductor may also be an insulator under certain specific conditions. Two or more atoms may combine to form a covalent bond.
The best manner in which to explain this concept is to look at two hydrogen atoms as illustrated in figure 7.3. This is not a compound that will be formed but is the sharing of electrons by two or more nuclei of atoms. In this instance the only shell of electrons will contain two electrons, which make that particular shell complete. This phenomenon is particularly important in the semi-conductor field and will be explained in more detail when semi-conductors are discussed. Therefore, we will find that there are no free electrons available since each atom shares its electrons with one another and that they are firmly attached in a crystal lattice structure by this covalent bond.
Figure 7.3
7.5 Conduction
A very important phenomena concerning conduction should however also be explained. In the incomplete valence shell of an element we refer to the spaces, which exist where, electrons should be, as holes. Strange as it may sound we will find that these holes also appear to be moving. This concept is illustrated in figure 7.4.
Orbiting electrons
Hydrogen atom 1 Hydrogen atom 2
Movement of electrons or conduction can and will take place in any given conducting material, in a desired direction, should a source of power be applied across such material. This conduction process can be by one or both of the following processes namely hole flow or transfer or by electron motion.
Free electrons in the conduction band will, under the influence of an applied potential, move around. The reason for this action is that since the electron has a negative charge it will be repelled by the negative charge of the applied potential but at the same time be attracted by the positive charge of the applied potential. In this manner we will find that electrons will always move from a negative charge toward a positive charge. This action is termed electron flow. Should we now observe figure 7.4 in more detail, it will be seen that as the electron moves from the negative charge of the applied potential toward the positive charge of the applied potential, that the hole moves from the positive charge of the applied potential toward the negative charge of the applied potential. This action is termed hole flow and both these concepts are of great importance and we will deal with these principles again when we discuss semi-conductors.
Figure 7.4
-+
+
+
Applied potential
Electron motion Hole motion