Chemistry, Third edition

12 · CHEMICAL FAMILIES

way to form a network solid (not unlike the diamond structure), resulting in a

high-melting-point solid. The formula of silica is SiO 2 because each silicon atom has

a ‘half-share’ of four oxygen atoms, as shown in Fig. 12.2.

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BOX 12.3

Dry ice

Solid carbon dioxide (dry ice) is obtained by

releasing CO 2 from cylinders so that it expands

and cools. A ‘snow’ is produced that is pressed

into blocks. The solid carbon dioxide is used to

refrigerate ice cream, meat and other

foodstuffs. Solid carbon dioxide sublimes

(changes directly from a solid to a gas) into

carbon dioxide vapour at normal atmospheric

pressure so that it is liquid-free when used as a

refrigerant – hence the name dry ice.

Silicon and germanium hydrides

(i)iWrite a structural formula for the covalent compound

Si 6 H 14.

(ii)Describe the bonding and shape of the molecule Ge 2 Cl 6.

Exercise 12G

Fig. 12.2The structure of
silica.

Oxidation states

The elements of Group 14 combine with oxidation number 2 and 4. As the group

is descended, there is an increased tendencyfor the elements to combine with other

elements using the oxidation number 2 instead of 4. Carbon generally exhibits

oxidation state four in its compounds, whereas the other elements in the group can

form tetravalent and divalent compounds. The most important oxidation states for

the elements are shown in Table 12.6. Lead(IV) compounds however, are oxidizing

agents, because lead(II) is the most stable oxidation state for the element. This grad-

ual preference of the valency (or oxidation number/state) of 2 instead of 4 can be

explained by the inert pair effect, described in Box 12.4.

Most of the compounds of Group 4 elements are covalent, with the exception of

compounds of tin and lead in oxidation state 2, which are normally described as ionic.

Table 12.5Properties of carbon and silicon dioxides

Compound Carbon dioxide (CO 2 ) Silicon dioxide or silica (SiO 2 )

Form under

normal conditions colourless gas sand or quartz

Chemical properties

weakly acidic gas, reacts with unreactive

bases to form salts (carbonates)

slightly soluble in water: the weakly insoluble in water

acid solution formed is carbonic acid

in soft drinks under pressure it

carbonates them or makes them fizzy

used in fire extinguishers because

it does not support combustion