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4 LIQUIDS AND SOLIDS eLearn.Punjab

When the oppositely charged ions are brought, close to each other energy is released. So
the lattice energy is the energy released when one mole of the ionic crystal is formed from
the gaseous ions. It is also defined as the energy required to break one mole of solid into
isolated ions in the gas phase. It is expressed in kj mole-1.
Na+(g) + Cl−(g) → NaCl(s) H = −787kJmole−1

Tables (4.8) Lattice energies

of ionic compounds

Ionic Lattice energy

compound (kJ/mol-1)

LiCl -833

NaF -895

NaCl -787

KCl -690

NaBr -728

KBr -665

Nal -690

Table (4.8) shows the lattice energies of many ionic compounds. It is clear from the table that
lattice energy decreases with the increase in the size of the cation keeping the anion same. It also
decreases with the increase in the size of anion. The reason in both cases is the same. With the
increase in the size of either cation or anion, the packing of oppositely charged ions becomes less
and less tight. The calculations related to the measurement of lattice energy will be discussed in
chapter seven.

4.7.2. Covalent Solids

Covalent solids are also called atomic solids, because they are composed of neutral atoms of
the same or of different elements. These atoms are held together by covalent bonds.
Covalent solids are of two types.
(i) Whenthecovalentbondsjointoformgiantmoleculeslikediamond,siliconcarbideoraluminiumnitride.
(ii) When atoms join to form the covalent bonds and separate layers are produced like that of
graphite, cadmium iodide and boron nitride.

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