Electrical Properties

Solids show a variety of range of electrical conductivities. Electrical conductivity is the reciprocal of resistivity. Resistivity is the property of solids to resist the flow of electricity, whereas  conductivity is the property of solids to conduct electricity.

The SI unit of resistivity is ohm meter. Since, conductivity is the reciprocal of resistivity, thus its unit is reciprocal of ohm meter, i.e. ohm^-1 m^-1.Conductivity is represented by Greek letter σ (sigma). On the basis of magnitude of range of conductivities, i.e. from 10^-20 to 10⁷ ohm^-1 m^-1, solids can be classified into three types:

(a) Conductor:- Those solids which can flow electricity are called conductors.Solids having range of conductivities in order 10⁴ to 10⁷ ohm^-1 m^-1 are classified as conductors. Metals are good conductor of electricity. Silver has conductivity in the order of 10⁷ ohm^-1 m^-1 is considered as very good conductor. Two types of conductors:

(a) Metallic conductors

(b) Electrolytic conductors

(a) Metallic Conductors:- Flow of electricity is due to flow of electrons without any chemical change in metal.

(b) Electrolytic Conductors:- Flow of electricity take place in the molten state or in aqueous  solution. The flow of electricity is due to flow of ions. eg:- NaCl , KCl.

In solid state, they conduct electricity only to a small extent which is due to presence of defect (holes and electrons etc.)

 

(b) Insulator: - Solids having range of conductivity in order 10^-20 to 10^-10 ohm^-1 m^-1 are considered as insulators. Solids which do not allow the electricity to pass through them are called insulators. eg:- S, P, plastic, wood, Rubber etc.

 

(c) Semiconductor: - Solids having intermediate range of conductivity, i.e. from 10^-6 to 10⁴ ohm^-1 m^-1 are considered as semiconductors. Those solid whose conductivity lies between metallic conductors and insulators Electrical conductivity is due to the presence of impurities and defects.

Conductivity increases with the increase in temperature as the defects pr holes increases with increase of temperature.

eg:- Monoxide of transition element show large variation in electrical properties.

TiO(M), Ti₂O₃(M – I), TiO₂(I)

ReO₃ has the conductivity and appear as that of Copper (Cu).

Conduction of Electricity in Metals:

Metals exhibit electrical conductivity due to of movement of electrons. Electrolytes exhibit electrical conductivity due to movement of ions. Metals exhibit electrical conductivity in solid and molten states both while electrolytes exhibit electrical conductivity only in molten state and aqueous solution.

Conductivity in metals depends on presence of unpaired electrons in their valence or the outermost shell per atom. Electrons present in valence shell of metals are free to move and allow flow of electricity in metals.

Energy level (atomic orbital) with electrons and vacant energy levels present in metals having   very small difference in energy they together are called energy band. The empty energy levels or unoccupied energy levels are known as conduction band also as they help in conduction of electricity.

When partially filled energy levels (atomic orbital) are too close or overlapped with unoccupied energy level or conduction band; electrons can easily flow between them under the electrical field. Because of flows of unpaired electrons from occupied energy level to conduction band metals conduct electricity.

Conduction of electricity in Insulators:

In insulators the difference in energy between occupied energy level and unoccupied energy level (conduction band) is much higher because of which electrons can not flow easily from occupied energy band to the next higher unoccupied energy band resulting insulators do not conduct electricity as electrons do not flow.

Conduction of electricity in Semiconductor:

In semiconductors like Si and Ge, the energy gap between outermost shell and conduction band is so smaller that electrons may jump from filled orbital to conduction band when put under electrical field. Thus, lower gap between valence band and conduction band semiconductor show the conduction of electricity.