Atomic Structure Theory:

Atom:

Basic Constituent of all matter ( can be element or compound) is an atom. The atom consists of central nucleus surrounded by orbiting electrons.

Electrons:

Electrons each have a negative charge of 1.602 X 10^-19 C. Compared to the mass of the nucleus, electrons electrons are tiny particles are negligible.

Nucleus:

Nucleus of an atom consists of large cluster of two types of particles, protons and neutrons. Protons have a positive electrical charge and is equal to the charge of electron in magnitude (electrons have negative charge). A neutron has no charge at all. Protons and neutrons have a mass of about 1800 times higher than the mass of electrons. For a given atom, the number of protons in the nucleus is equal to the number of electrons that are orbiting around the nucleus. Thus an atom is electrically neutral. If an atom losses an electron, it losses some negative charge and becomes a positive ion. Similarly, if an atom gains electron, it becomes negative ion

Atomic Mass Number:

The number of protons in an atom is referred to atomic number (Z) of the atom. The atomic mass number is equal to the total number of protons and neutrons in the nucleus of an atom. The atomic weight is the actual weight of the atom and is slightly different from atomic mass number.

Energy Bands:

Within any given material there are two distinct energy bands in which electrons exist, the valence band and the conduction band. Separating these two energy bands, there exists an energy gap in which no electron can normally exist. This gap can be termed as forbidden energy gap.

In order to move an electron from conduction band, a small amount of energy is sufficient and to move an electron from valence band lot more energy is required. Electrons in the valence band are normally close to the nucleus and electrons in the conduction band are the electrons orbiting on the outer orbits. For a given type of material, the forbidden energy gap may be small, large or non existent. The major difference between the conductors, semiconductors and insulators is based on the relative width of the energy gaps.

Conductors:

In case of conductors there will not be any energy gap and valence band and conduction band overlaps. This is the reason very large electrons are available for conduction, even at extremely low temperatures. Small amount of energy is sufficient to rise the electron from valence band to conduction band.

Insulators:

Forbidden energy level in case of insulators will be very high

Semi Conductors:

Energy gap between the conduction band and valence band in case of semiconductors is moderate or small.

Flow of Electric Current:

When an external electrical field is applied, the free electrons in the conduction band will move to positive terminal of the battery and the holes moves towards the negative terminal of the battery.