Chemistry Class 12 NCERT Solutions: Chapter 1 The Solid State Part 10
Q: 21. Gold (atomic radius ) crystallises in a face-centred unit cell. What is the length of a side of the cell?
For a face – centred unit cell:
It is given that the atomic radius,
Hence, length of a side of the cell
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
(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.