Chemistry Class 12 NCERT Solutions: Chapter 1 The Solid State Part 10

Q: 21. Gold (atomic radius Equation ) crystallises in a face-centred unit cell. What is the length of a side of the cell?

Ans:

For a face – centred unit cell:

Equation

It is given that the atomic radius, Equation

So, Equation

Equation

Hence, length of a side of the cell Equation

Q: 22. In terms of band theory, what is the difference

(I) Between a conductor and an insulator

(II) Between a conductor and a semiconductor

Ans:

(i) The valence band of a conductor is partially filled or it overlaps with a higher energy, unoccupied conduction band.

On the other hand, in the case of an insulator, the valence band is fully- filled and there is a large gap between the valence band and the conduction band.

(ii) In the case of a conductor, the valence band is partially filled or it overlaps with a higher energy, unoccupied conduction band. So, the electrons can flow easily under an applied electric field.

On the other hand, the valence band of a semiconductor is filled and there is a small gap between the valence band and the next higher conduction band. Therefore, some electrons can jump from the valence band to the conduction band and conduct electricity.

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