The Solid State

Question 101

Why is potassium chloride sometimes violet instead of pure white.

when a crystal of KCl is heated in an atomsphere of potassiuim vapour the Katoms lose electron to form K+ ions. The released electron diffuse into the crystal and occupy anionic site (F- centre)
These electron impart violet colour to the KCl crystal.
Question 102

What is the coordination number of each type of ions in a rock-salt type crystal structure?

Rock salt or NaCl has face centred cubic structure ,here each ion (cation/anion) is surrounded by 6 ions of opposite charges, thus coordination number is 6.
Question 103

What is the total number of atoms per unit cell in a face-centred cubic (fcc) structure?


A face-centred cubic (fcc) unit cell contains atoms at all the corners and at the centre of all the faces of the cube. It can be seen that each atom located at the face-centre is shared between two adjacent
unit cells and only 1/2 of each atom belongs to a unit cell.

Thus, in a face-centred cubic
(fcc) unit cell:
(i) 8 corners atoms × 1/8 atom per unit cell=8×1/8
  = 1 atom
(ii) 6 face-centred atoms ×
1/2 atom per unit cell = 6 ×1/2 = 3 atoms
∴ Total number of atoms per unit cell = 4 atoms

Question 104

What type of substances exhibits anti-ferromagnetism?

A substance in which magnetic moments align in such a way that the net magnetic moment is zero, exhibits anti-ferromagnetism. exampe MnO, FeO, NiO etc.

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