Modern inorganic chemistry

172 GROUP IV

Silicon

Silicon burns when heated in air to red heat giving silicon dioxide,

SiO 2. Several crystalline forms of SiO 2 are known.

Si + O 2 ~> SiO 2 : AH = - 910 kJ mol"l (approximate)

Note the much larger enthalpy of formation of silicon dioxide as

compared with carbon dioxide; this arises in part because of greater

strength in the Si—O bonds and also because the Si—Si bond in

silicon is much weaker than the C—C bond (p. 162).

Tin

Ordinary white tin is not attacked by air at ordinary temperatures

but on heating in air it forms tin(IY) oxide, SnO 2.

Sn + O 2 -> SnO 2

Lead

Finely divided lead, when heated in air, forms first the lead(II) oxide,

litharge', PbO, and then on further heating in an ample supply of

air, dilead(II) lead(IV) oxide, 'red lead', Pb 3 O 4. Lead, in a very finely

divided state, when allowed to fall through air, ignites and a shower

of sparks is produced. Such finely divided powder is said to be

"pyrophoric'. It can be prepared by carefully heating lead tartrate.

COMPOUNDS OF GROUP IV ELEMENTS

HYDRIDES

Carbon hydrides are commonly called hydrocarbons. They are very

numerous and the study of these compounds is outside the scope of

this book. Reference will therefore be made only to the main groups.

Alkalies

Methane, CH 4 , is the first member of this series, all of which have the
general formula CnH2n + 2 Every carbon atom in any alkane mole-
cule has a tetrahedral configuration and is joined to four other
atoms. Alkanes are resistant to attack, at room temperature, by