Chemistry Class 12 NCERT Solutions: Chapter 1 The Solid State Part 5
Q: 8. How many lattice points are there in one unit cell of each of the following lattice?
(I) Face-centred cubic
(II) Face-centred tetragonal
(I) There are (8 from the corners + 6 from the faces) lattice points in face-centred cubic.
(II) There are (8 from the corners + 6 from the faces) lattice points in face-centred tetragonal.
(III) There are (1 from the centre + 8 from the corners) lattice points in body-centred cubic.
Q: 9. Explain
(i) The basis of similarities and differences between metallic and ionic crystals.
(ii) Ionic solids are hard and brittle.
(i) The basis of similarities between metallic and ionic crystals is that both these crystal types are held by the electrostatic force of attraction. In metallic crystals, the electrostatic force acts between the positive ions and the electrons. In ionic crystals, it acts between the oppositely-charged ions. Hence, both have high melting points. The basis of differences between metallic and ionic crystals is that in metallic crystals, the electrons are free to move and so, metallic crystals can conduct electricity. However, in ionic crystals, the ions are not free to move. As a result, they cannot conduct electricity. However, in molten state or in aqueous solution, they do conduct electricity.
(ii) The constituent particles of ionic crystals are ions. These ions are held together in three-dimensional arrangements by the electrostatic force of attraction. Since the electrostatic force of attraction is very strong, the charged ions are held in fixed positions. This is the reason why ionic crystals are hard and brittle.