Chemistry, Third edition

GROUP 14 ELEMENTS 201

Table 12.6Oxidation states of Group 14 elements. Very unusual oxidation

states are in brackets, preferred oxidation states are in boxes

Element Oxidation states

Carbon C (2) □ 4 e.g. CCl 4
Silicon Si (2) □ 4 e.g. SiO 2
Germanium Ge 2 □ 4 e.g. GeCl 4
Tin Sn 2 □ 4 e.g. SnO 2
Lead Pb □ 2 e.g. PbO 4

Oxidation numbers (states) of Group 14 elements

(i) Using Lewis symbols, describe the bonding in SnCl 4 and SnCl 2.

(ii)PbO 2 is a strong oxidizing agent, whereas PbO is not. Can you offer an explanation?

(iii)Whereas CH 4 is a very stable compound, PbH 4 is not. Why do you think this is?

(iv)Can you think of a compound where carbon (very unusually) has oxidation state 2?

(v)Write the formulae for two oxides of germanium. Which would you expect to be the more

stable?

Exercise 12H

BOX 12.4

Inert pair effect

Carbon has the electronic structure 2.4 or

1s^2 2s^2 2p^2. If we use the ‘electrons in boxes’

model the electronic structure can be written

as

If an electron is promoted from a 2s orbital to

a 2p orbital, the electronic structure will be

Now the carbon can combine with a covalency

of four, because it has four unpaired

electrons which can combine with the

unpaired electrons of other elements, or

groups, to make four covalent bonds.

Remember that when covalent bonds are

formed, energy is released and the formation

of four covalent bonds will release more

energy than the formation of two covalent

bonds. Energy is required to promote an

electron from the 2s orbital to the 2p orbital,

but this is more than ‘paid back’ in the case

of carbon, when the element forms four

covalent bonds. Because carbon is a small

atom, it forms short and strong covalent

bonds – a great deal of energy is released

when they are formed. As Group 14 is

descended the covalent bonds formed by the

elements become longer and weaker

(because their atoms are larger) and the

energy release gained by the formation of four

covalent bonds does not become ‘worth’ the

energy of promotion of ns electrons into np

orbitals. This is why the oxidation state of  2

becomes more favourable as compared with

that of 4 and lead(IV) compounds are good

oxidizing agents.

1s 2s 2p

1s 2s 2p

Hydrolysis of tetrachlorides

The tetrachlorides of Group 14 (XCl 4 ) are liquids with a tetrahedral structure. All,

exceptCCl 4 , react with water (or hydrolyse) to form HCl and the oxides (XO 2 ).