Chemistry, Third edition

IONIC BONDING

Properties of ionic compounds

Ionic compounds have certain properties common to all of them:

1. High melting and boiling points (they are non-volatile)

They are solids at room temperature. When a substance melts, then boils, its

particles separate. A great deal of energy is needed to break down the lattice structure

because the ions are attracted together by strong forces. The structure must therefore

be heated to a high temperature before it melts.

2. Soluble in water

Water contains molecules that have one end positively charged and the other end

negatively charged (the charge separation is called a dipole). When ionic compounds

are put into water, the water molecules are able to break down the crystal lattice by

attracting the ions in the lattice and pulling them into the solution. In this way, the

lattice breaks up and the ions mix in with the water molecules – the substance dis-

solves. Water is a polar solventand tends to dissolve ionic compounds. The positive

end of a water molecule can attach itself to an anion in the unit cell and pull it away

from the structure. Similarly, the negative end of the water molecule can attract away

a cation. This is shown in Fig. 4.4.

3. Conduct electricity when molten or in aqueous solution

In order for a substance to conduct electricity, it must possess charged particlesthat

canmove(in a metal, mobile electrons can carry the electric current). Solid ionic

compoundsdo not conduct electricity– the ions are held firmly by strong electrostatic

forces and cannot move. When the substance is melted or dissolved in water, how-

ever, the ions move freely and can carry an electric current. An aqueous solution, or

melt, of an ionic substance that behaves in this way, is called an electrolyte.

4. Ionic crystals shatter easily

When a force is applied to the crystal, the layers of ions in the crystal structure can

‘slip’ so that similarly charged ions are next to one another. The like charges of the

ions repel one another and the crystal structure shatters, see Fig. 4.5.

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(b) Water molecules

breaking up the lattice

(a) Part of the unit cell
before dissolving

Negative end of

water molecule

Positive end of

water molecule

Fig. 4.4An ionic compound dissolving in water.

(a)

Ions with the same

charge repel each

other and the

structure shatters

Stress

(b)

Cl–

Na+

Fig. 4.5The shattering of an ionic crystal: (a) before, (b) after.