Classification of solids on the basis of their conductivities :

(i) Conductors : They have conductivity range from `10^4 - 10^7 ohm^(-1) m^(-1)`.

(ii) Insulators : They have conductivity range from `10^(-20) - 10^(-10) ohm^(-1) m^(-1)`.

(iii) Semiconductors : They have conductivity range from `10^(-6) -10^(4) ohm^(-1) m^(-1)`.

(i) A conductor conducts electricity through movement of electrons or ions.

(ii) Metallic conductors conduct electricity through movement of electrons.

(iii) Electrolytes conduct electricity through movement of ions.

(iv) Metals conduct electricity in solid as well as molten state.

(v) Conductivity of metal depends upon no. of valence electrons available per atom.

`=>` Atomic orbitals of metal form molecular orbitals which are very close in energy to each other and forms band.

`=>` If this band is partially filled or it overlaps with a higher energy unoccupied band then electrons flow easily under electric field and shows conductivity (fig 1.29a).

`=>` If the gap between filled valence band and empty conduction band is large, electrons are unable to jump to conduction band from valence band and behaves like insulator.

In semiconductors the gap between the V. B. and C.B. is small. So, only some electrons can jump to C.B. and show some conductivity. With increase in temperature, electrical conductivity increases. e.g. `Si` and `Ge` and these are also called intrinsic semiconductors.

Conductivity of intrinsic semiconductors is increased by adding an appropriate amount of suitable impurity. This process is called doping. It is done with an impurity which is electron rich or electron deficient as compared to intrinsic semiconductors.

Note : These impurities introduce electronic defects.

Electron-rich Impurities : `Si` and `Ge` `=>` `14` group elements

No. of valence electrons `= 4`

When `Si` and `Ge` are doped with a group `15` element like `P` or `As` (no. of valence electrons `= 5`) occupy some of the lattice sites in `Si` or `Ge` (fig 1.30b). Four electrons are used in making four covalent bonds with four neighbouring `Si` atoms leaving one extra electron. This extra fifth electron becomes delocalised and increases the conductivity of doped `Si` or `Ge`. Here increase in conductivity is due to the presence of negatively charged electron and is also called `n`-type semiconductor.

Electron–deficit Impurities : When `Si` or `Ge` are doped with a group `13` element like `B`, `Al` or `Ga` (no. of valance electrons `= 3`) occupy some of the lattice sites in `Si` or `Ge`. These electrons are used in making three covalent bands with four neighbouring `Si` atoms. The place where the fourth valence electron is missing is called electron hole or electron vacancy (fig 1.30c). An electron from a neighbouring atom can come and fill the electron holes and it appears as if hole is moving from its original position. Under the influence of electric field, electrons would move towards the positively charged plate through electronic holes, but it would appear as if electron holes are positively charged and are moving towards negatively charged plate. This type of semi conductors are called `p`-type semiconductors.

Various combinations of `n`-type and `p`-type semiconductors are used for making electronic components.

(i) Diode : A combination of `n`-type and `p`-type semiconductors and used as a rectifier.

(ii) Transistors : A combination made by sandwiching one type of semiconductor between two layers of other type of semiconductor. `npn` or `pnp` type of transistors are used for detecting or amplifying radio or audio signals.

(iii) Photo-diode : Used for conversion of light energy into electrical (solar cell) energy.

A large variety of solid state materials have been prepared by combination of groups `13` and `15` or `12` and `16` to make average valence of four as in `Ge` or `Si`.

Examples of group `13-15` compounds are `InSb`, `AlP` and `GaAs`

Examples of group `12-16` compounds are `ZnS`, `CdS`, `CdSe` and `HgTe`.

In these compounds, bonds are not perfectly covalent and ionic character depends on the electronegativities of the two elements.

Note : (i) Metal oxides show marked differences in electrical properties.

(ii) `TiO`, `CrO_2` and `ReO_3` behave like metals.

(ii) `ReO_3` is like metallic copper in its conductivity and appearance.

(iv) `VO`, `VO_2`, `VO_3` and `TiO_3` show metallic or insulating properties depending on temperature.

The origin of magnetic properties lies in the electrons. Each electron in an atom behaves like a tiny magnet.

Magnetic moment originates from two types of motions :

(i) Its orbital motion around the nucleus

(ii) Its spin around its own axis (fig 1.31)

Electron is a charged particle and when undergoes these motions is considered as a small loop of current which possesses a magnetic moment. Thus each electron has a permanent spin and an orbital magnetic moment associated with it.

`1 mu_B = 9.27 xx 10^(-24) A m^2`

(i) `text(Paramagnetism)` : (a) Weekly attracted by a magnetic field.

(b) Magnetised in the magnetic field in same direction.

(c) Lose magnetism in the absence of magnetic field.

(d) It is due to the presence of one or more unpaired electrons.

(e) e.g. `O_2`, `Cu^(+2)`, `Fe^(+3)`, `Cr^(+3)` etc.


(ii) `text(Diamagnetism)` : (a) Weekly repelled by a magnetic field.

(b) Weekly magnetised in a magnetic field in opposite direction.

(c) Shown by substances in which all electrons are paired because pairing of electrons cancels their magnetic moments.

(d) e.g. `H_2O`, `NaCl` and `C_6H_6`


(iii) `text(Ferromagnetism)` : (a) Strongly attracted by magnetic field.

(b) Can be magnetised permanently.

(c) In solid state, the metal ions of ferromagnetic substance are grouped together into small regions called domains and each domain acts as a tiny magnet.

(d) In the absence of magnetic field, domains are randomly oriented and their magnetic moments get cancelled.

(e) In the presence of magnetic field, domains are oriented in the direction of magnetic field and a strong magnetic effect is produced (Fig 1.32 a).

(f) e.g. `Fe`, `Co`, `Ni`, `CrO_2`, gadolinium.

(iv) `text(Anti ferromagnetism)` : (a) In this, domains are oppositely oriented and cancel out each others magnetic moment (Fig 1.32 b).

(b) e.g `MnO`


(v) `text(Ferrimagnetism)` : (a) In this, magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers (fig 1.32 c).

(b) Weekly attracted by magnetic field as compared to ferromagnetic substances.

(c) Loses ferrimagnetism on heating and become paramagnetic.

(d) e.g. `Fe_3O_4` (magnetite) and ferrites like `MgFe_2O_4` and `ZnFe_2O_4`.