CHEMISTRY TEXTBOOK

Can you recall?

• What are structures of
  diamond and graphite?


• What are the types of covalent bonds
  those link carbon atoms in diamond
  and graphite?


• Are all the valence electrons of
  carbon atoms in graphite localized to
  specific covalent bonds?

Try this...

Graphite is a covalent solid

yet soft and good conductor of

electricity. Explain.

1.3 Classification of crystalline solids :
Crystalline solids are further classified into
four categories : ionic solids, covalent network
solids, molecular solids and metallic solids.

1.3.1 Ionic crystals : Ionic crystals have the
following characteristics :

i. The constituent particles of ionic crystals
are charged ions. The cations and anions
may differ in size.

ii. Each ion of a given sign of charge is
bonded to ions of opposite charge around
it by coulomb force. In other words, the
particles of ionic crystals are held by
electrostatic force of attraction between
oppositely charged ions.

iii. Ionic crystals are hard and brittle. They
have high melting points.

iv. These are nonconductors of electricity
in solid state. However, they are good
conductors when melted or dissolved in
water.

For example : NaCl, K 2 SO 4 , CaF 2 , KCl are
ionic crystals.

1.3.2 Covalent network crystals

Characteristics of covalent network crystals

are as follows :

i. The constituent particles in covalent

network solids are atoms.

ii. The atoms in these crystals are linked by a

continous system of covalent bonds. The

result is a rigid three dimensional network

that forms a giant molecule. The entire

crystal is a single molecule.

iii. As a result of rigid and strongly bonded

structure, covalent network crystals are

very hard. In fact they are the hardest and

most incompressible of all the materials.

These crystals have high melting and

boiling points.

iv. The electrons are localised in covalent

bonds and hence are not mobile. As a

result, covalent solids are poor conductors

of heat and electricity.

For example : diamond, quartz (SiO 2 ), boron

nitride, carborandum are covalent network

solids.

Do you know?

Many crystalline forms of

silica (SiO 2 ) are found in nature.

Three of them are α-quartz, b-quartz and

cristobalite

cristobalite α-quartz

b-quartz

Do you know?

Diamond is the hardest known

material.