The Solid State

• Question 9

What type of solids are electrical conductors, malleable and ductile?

Solution
Metallic solids are electrical conductors, malleable and ductile.
Question 10

Give the significance of a 'lattice point'.

Solution

(i) Each point in a lattice is called lattice point or lattice side.
(ii) Each point in a crystal lattice represents one constituent particle which may be an atom, a molecule or an ion.
(iii) Every lattice point has identical surroundings except those which are on the surface or corner of a crystal.
(iv) If a line is drawn by joining two lattice points, it would pass through a series of similar points at regular intervals.
(v) Lattice points are joined by straight lines to bring out the geometry of the lattice. These lines do not represent chemical bond.

Question 11

Name the parameters that characterize a unit cell.

Solution

Unit cell is the smallest portion of a crystal lattice.
A unit cell is characterized by:
(i) its dimensions along the three edges a, b and c. These edges may or may not be mutually perpendicular.
(ii) angles between the edges, α (between b and c), β (between a and c),γ (between a, and b). Thus, a unit cell is characterised by six parameters, a, b, c, α, β and γ.

Question 12

Distinguish between Hexagonal and monoclinic unit cells.

Solution

Ans. (i)                                        Hexagonal Unit cell                      Monoclinic unit cell

(1) Axial distances or edge
lengths                                              a = b ≠ c                                       a ≠ b ≠ c
(2)
Axial angles                                α = β = 90°                                    α = γ=90° β ≠ 90°

(3) Examples:                                   Graphite, ZnO, CdS                       Monoclinic Sulphur, PbCrO2
Na2SO4.10H2O
2 4 2