Chemistry, Third edition

GROUP 14 ELEMENTS

Group 14 elements: C, Si, Ge, Sn, Pb

All these elements have four electrons in the outermost shell of their atoms (or the

general outer electronic structure ns^2 np^2 , where nis a whole number greater than

one). The elements form compounds with similar formulae but they show marked

changes in properties, as the group is descended, from carbon to lead.

About the elements

The element carbon exists mainly in two allotropic forms, diamond and graphite,

and has a very large branch of chemistry (organic chemistry) concerned with the

compounds that it forms because of its ability to form long chains by bonding with

atoms of itself. The ability of the atoms of an element to covalently bond with them-

selves is called catenation.

Silicon and germanium are metalloids, whereas tin and lead are metals. Silicon

and germanium have structures similar to that of diamond, tin exists in allotropic

forms, but lead exists in only one metallic form.

12.4

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BOX 12.2

Allotropic forms of tin

Tin has three crystalline forms:

13 °C 161 °C

-tin \=====\-tin \=====\-tin

(grey tin) (white tin) metallic

has a structure metallic

like diamond

Below 13 °C, powdery grey -tin forms on the white, metallic -tin allotrope and the metal

crumbles. Napoleon’s soldiers had tin buttons fastening their jackets and they used tin pots

and pans to cook with. During the winter invasion of Russia, in 1812, their buttons and pots

crumbled and it was said that this contributed to their defeat.

Catenation and multiple bonds

Apart from carbon, catenation does not occur to any great degree in the chemistry of

the other elements. Si and Ge do form hydrides that might be compared with the

lighter hydrocarbons, but they are not as stable. Catenation in these elements

involves the formation of Si–Si and Ge–Ge covalent bonds, which are longer and

therefore weaker than C–C bonds.

Carbon is also the only member of the group which is able to form multiple

bonds with itself or other elements, such as oxygen. Although the dioxides of carbon

and silicon have similar formulae (CO 2 and SiO 2 ), they have very different physical

and chemical properties, as shown in Table 12.5.

Carbon dioxide molecules are simple, linear, non-polar molecules with the structure

O=C=O

Silica does not have the same structure even though it has the formula SiO 2. Silicon

cannot form multiple bonds to oxygen and forms single covalent bonds instead.

Silica occurs in several forms, but in all of them silicon has covalent single bonds

to four oxygen atoms in a tetrahedral arrangement. Many units are bonded this