The Solid State

Question 121

Give reasons for Non-stoichiometric sodium chloride is a yellow solid.


When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal.
The Cl– ions diffuse to the surface of the crystal and
combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result the crystal now has an excess of sodium. The anionic sites occupied byunpaired electrons are called F-centre. They impart yellow colour to the crystals of NaCl.

The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.

Question 122

What is a semi-conductor? Describe the two main types of semi-conductors and contrast their conduction mechanism.



Semiconductor: - Solids having intermediate range of conductivity, i.e. from 10–6 to 10ohm–1 m–1 are called semiconductors. Semiconductors are of following two types:

(i) n – type of semiconductors

(ii) p – type of semiconductors

(i) n – type semiconductors – Semiconductors formed after doping with electron rich impurities to increase their conductivity are called n-type of semiconductors.

Example –

Silicon and germanium, each has four valence electrons as they belong to 14th group of periodic table. Arsenic and phosphorous belong to 15th group of periodic table and they have valence electrons equal to 5. When silicon or germanium is doped with phosphorous or arsenic, four electrons of phosphorous or arsenic out of five; make covalent bonds with four electrons of silicon or germanium leaving one electron free; which increases the electrical conductivity of silicon or germanium.

n-type semi-conductor.

Since the electrical conductivity of silicon or phosphorous is increased because of negatively charged particle (electron), thus this is known as n-type of semiconductor.

(ii) p – type of semiconductor - Semiconductors formed by the doping with electron deficient impurities; to increase their conductivity; are called p-type semiconductors. In p - type of semiconductors, conductivity increase because of formation of electron holes.

Example - Electrical conductivity of silicon or germanium is doped with elements, such as Boron, Aluminium or Gallium having valence electrons equal to 3. Three valence electrons present in these elements make covalent bonds with three electrons present in valence shell out of four of silicon or germanium leaving one electron delocalized. The place from where one electron is missing is called electron hole or electron vacancy.

When the silicon or germanium is placed under electrical field, electron from neighbouring atom fill the electron hole, but in doing so another electron hole is created at the place of movement of electron. In the influence of electrical filed electron moves toward positively charge plate through electron hole as appearing the electron hole as positively charged and are moving towards negatively charged plate.

 p-type semi-conductor.

Question 123

A cubic solid is made of two elements P and Q. Atoms of Q are at the corners of the cube and P at the body centre. What is the formula of the compound? What are the coordination numbers of P and Q?

Formula of a compound is same as the formula of unit cell. An atom at the corner of cube contributes only l/8th to the unit cell and there are 8 corners in a cube.
∴  No. of atoms of Q in the unit cell
                  =8×18 = 1.
An atom at centre of cube belongs only to this unit cell and there is only one body centre in the unit cell.
∴   No. of atoms of P in the unit cell = 1 x 1 = 1
Thus, the formula of compound is PQ or QP.
For body centred cubic unit cell, the coordination number is 8:8.
∴ Co-ordination number P = 8 and also coordination number of Q = 8.

Question 124

What is the difference between Schottky defects and Frenkel defects?


Schottky defects

Frenkel defects

1. It arises due to vacancy at a cation site and at a nearby anion site in an ionic solid.

2. Overall density of the crystal decreases.

3. It arises in ionic solids having high co-ordination number.

4.Example: NaCl, KCl, CsCl etc.

1. It arises when a cation leaves its lattice site and enters in an interstitial site in an ionic solid.

2. There is no change in the overall density.

3. It arieses in ionic solids having large anions and small cations.

Example: ZnS, AgCl, AgI etc

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