All matter is made
from atoms. It is the smallest particle of a chemical element possessing
the chemical properties of the element.
In 1911, Rutherford established that an atom consists of a positively charged nucleus and a number of negatively charged electrons that revolve around the nucleus in various orbits.
The most important
properties of atomic and molecular structure may be exemplified using a
simplified picture of an atom, called the Bohr Model. This model
was proposed by Niels Bohr in 1915. It is not completely correct,
but it has many features that are approximately correct and it is sufficient
for much of our discussion.
Bohr’s Atomic Model
In 1913 Niels Bohr proposed the following postulates.
a) An atom consists of a positively
charged nucleus around which negatively charged electrons revolve in
different circular orbits.
b) The electrons can revolve
around the nucleus only in certain permitted orbits, i.e., orbits of
certain radii are allowed.
c) The electrons in each
permitted orbit have a certain fixed amount of energy. The larger the
orbit (i.e. larger radius), the greater the energy of the electron.
d) If an electron is given additional energy, it is lifted to a higher orbit. The atom is said to be in a state of excitation. This state does not last long, because the electron soon falls back to the original lower orbit. As it falls, it gives back the acquired energy in the form of heat, light, or other radiation.
In Bohr's model, the protons and neutrons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun.
The energy of the
electron in the Bohr atom is restricted to certain discrete values, i.e., the energy
is quantized. This means that only certain orbits with certain radii are
allowed; orbits in between simply do not exist.
According to this model,
the electrons in an atom revolve around the nucleus in different orbits. Each
orbit is called a Shell or an Energy level. The
following figure shows the shells around the nucleus and its designations.
These shells (Energy levels) are filled in sequence; K (or n = 1) is filled first, then L (or n = 2), M (or n = 3), N (or n = 4), and so on. The maximum number of electrons that each shell (Energy level) can accommodate is shown in the table given below.
The outermost orbit electrons in an atom are known as valence electrons and play an important role in determining the physical and chemical properties of a material.
The valence electrons of
different materials possess different energies. The higher the energy of a
valence electron, the lesser it is bound to the nucleus.
As we discussed in the previous
chapter, in certain materials, particularly metals, the valence electrons
possess very high energy that they are loosely attached to the nucleus. This
makes them good conductors of electricity.
These loosely attached
valence electrons that move randomly within the material are called free
electrons. The free electrons can be easily removed or detached by
applying a small amount of external energy. They serve as the charge carriers
in solids and determine the electrical conductivity of a material.
As we discussed so far, according to Bohr's model
of the atom, electrons revolve around the nucleus in different orbits with definite
energy values.
For an isolated atom, these energies can be represented as follows.
However, in solids large
number of atoms is closely packed and atoms are not free from the influence of
the neighbouring atoms. Therefore, energy levels are broadened and become bands
of energy called Energy Bands.
Each energy band consists
of a very large number of very closely spaced energy levels.
Adjacent bands are separated by an energy gap referred to as the Forbidden Energy Gap or Forbidden Gap, which represents a range of energies that no electron can possess.
The energy band occupied
by the valence electrons is called the Valence Band. As regards occupancy, it may be either completely filled or partially filled with
electrons but can never be empty.
The electrons which have left the valence band are called the conduction electrons. They practically leave the atom or are only weakly held to the nucleus. The band occupied by these electrons is called the Conduction Band. This band lies next to the valence band as shown below.
If an electron in the valence band gets sufficient energy (either through electrical or thermal), it can jump across the forbidden gap and enter into the conduction band as shown below.
Such electrons are then free to move from atom to atom and give rise to a current flow.
When a valence electron enters the conduction band, it leaves an empty energy level in the valence band. It is now deficient in an electron and behaves like a positive charge. Such positive vacancies are called holes.
Electrical Conductors, Semi-Conductors and Insulators in terms of Energy Bands
As we have discussed in the previous chapter on the basis of electrical conductivity, materials are generally classified into three types; Conductors, Insulators, and Semiconductors. Now let’s discuss how their behaviour can be explained using Energy Band diagrams.
Conductors
In terms of energy bands, the conduction band and valence band in the conductors are very close to one another or may overlap as shown above.
Due to thermal vibration, the conduction band is partially filled with valence electrons which move about randomly.
With an applied electric field, electrons at the top of the valence band acquire sufficient energy to raise them into the conduction band.
These electrons are readily available for conduction as a flow of current through the conductor.
Insulators
In terms of energy band
structure, the conduction band and the valance band of an insulator are separated
by a very large forbidden energy gap as shown above.
The gap is about 5.5
eV for an insulator like diamond.
The gap is a few eV
for other insulators at room temperature.
NOTE:
Electron Volt (eV) is a unit that is used to measure
energy. An electron Volt is the energy that an electron gains when it travels
through a potential of one volt.
(1 eV = 1.6 x 10^-19 J)
For these types of materials, the probability of valence electrons acquiring sufficient energy to overcome the energy gap is negligible at room temperature and applied electric fields.
Therefore, virtually no effective electrons are present in the conduction band and current cannot flow in insulators.
Semiconductors
Semiconductors have a
forbidden gap between the filled valence band and the unoccupied conduction band. This
gap is small compared to the gap in the case of insulators as shown above.
Examples:
- At room temperature, the width of the forbidden gap for Silicon (Si) is 1.2 eV.
- At room temperature, the width of the forbidden gap for Germanium (Ge) is 0.7 eV.
The narrow energy gaps in
Silicon and Germanium imply that some electrons from the
completely filled lower valence band can be thermally excited to the empty
conduction band where they are able to conduct a small current on the application
of an electric field.
In terms of energy bands,
at room temperature, Semiconductors have,
- A partially filled conduction band.
- A partially filled valence band.
- A narrow-forbidden gap (< 2 eV).
0 Comments