CHEMISTRY TEXTBOOK

Zn atom is present in the interstitial space

as shown in Fig. 1.21(a)

Cl ions diffuse to the crystal surface

creating vacancies at their regular sites. These

Cl ions combine with Na atoms on the surface

to form NaCl, by releasing electron from

sodium atom.

Na + Cl NaCl +e

The electrons released diffuse into the

crystal and occupy vacant sites of anions as

shown in Fig. 1.22. The anion vacant sites

occupied by electrons are F-centres or colour-

Fig. 1.21 (a) : Neutral Zn atom at interstitial centres.

site

Fig. 1.21 (b) : Zn2+ ions and electrons at

interestitial sites

Fig. 1.22 : An F-centre in a crystal

NaCl shows yellow colour due to the

formation of F-centre. The crystal of NaCl

has excess Na. The nonstoichiometric formula

of NaCl is the Na1+xCl1.0

1.9 Electrical properites of solids

Can you recall?

• What is electrical conductivity?


• What is meant by the terms electrical
  'insulator' and 'semiconductor'?

In the second case, when ZnO is heated it
decomposes as :

ZnO Zn^2 ⊕ + 1/2 O 2 +2e

The excess Zn^2 ⊕ ions are trapped in
interstitial site in the lattice. The electrons also
diffuse in the crystal to occupy interstitial sites
as shown in Fig. 1.21(b).

In both the cases, nonstoichiometric
formula of ZnO is Zn1+xO1.0

Can you think?

When ZnO is heated it turns

yellow and returns back to original

white colour on cooling. What

could be the reason?

• By anion vacancies (Colour or F-centres)
  This type of defect imparts colour
  to the colourless crystal. For example, when
  NaCl crystal is heated in the atmosphere of
  sodium vapour, sodium atoms are deposited
  on the crystal surface.

Solids show very wide range of electrical

conductivity. Accordingly solids are classified

into the following three categories : conductors,

insulators and semiconductors.

i. Conductors : Solids having electrical

conductivities in the range 10^4 to 10^7 Ohm-1m-1

are called conductors. Metals and electrolytes

(ionic solids) are examples of electrical

conductors. Metals conduct electricity by

movement of electrons while electrolytes

conduct electricity by movement of ions. In