Chemistry, Third edition

GIANT MOLECULES 73

3.in the solid state(in contrast to ionic compounds). There is no chemical change to

the metal when electricity is passed through it. If a potential difference is applied

to the metal, the negative cloud of electrons will be attracted to the positive

potential. These same electrons can transfer kinetic energy through the solid

quickly, explaining why metals are good conductors of heat.

3.The structure can be distorted without breaking into pieces, because it is held

together as a whole by the electron cloud. This is why metals are malleable and

ductile. The strength of the metallic bond depends upon ratio of the number of

free electrons in the electron cloud per ion. The more valence electrons an atom

of a metal releases into the electron cloud, the stronger the metallic bond.

Metallic bonding

(i) How many electrons are there in the valence shell of magnesium?

(ii) Draw a diagram, similar to Fig. 5.6, to illustrate metallic bonding in magnesium.

(iii)How many electrons are there in the valence shell of aluminium?

(iv)Arrange the following metals in order of increasing strength of their metallic bonds:

magnesium, aluminium and sodium.

(v) Which metal in (iv)would you expect to have the highest melting point and why?

Exercise 5H

BOX 5.3

Allotropes

Some other elements which exist as different allotropes are:

1.Oxygen consists of diatomic molecules in oxygen gas(O 2 ) and triatomic molecules in ozone

(O 3 ).

2.Phosphorus can exist as three allotropes: white phosphorus(P 4 , its structure is shown on

page 19); red phosphorus, which is used in matches and consists of chains of P 4 units (Fig. 5.7);

andblack phosphorus, obtained by heating white phosphorus under pressure, which consists of

sheets of phosphorus atoms.

3.Sulfur can exist in many forms including rhombic sulfurandmonoclinic sulfur. They both

contain S 8 molecules (page 19), but crystallize in different forms. Plastic sulfur, which consists

of chains of sulfur atoms, is a form of sulfur that can be ‘stretched’.

Giant molecules

Allotropes of carbon

The element carbon exists in three solid forms, called allotropes, which differ in the

way that the carbon atoms are bonded together. Allotropes are different forms of the

same element; in each form the atoms are arranged in different ways. Three

allotropes of carbon are: diamond, graphite and buckminsterfullerene.

5.6

Fig. 5.7The structure of red phosphorus.

For more about

crystal structures

see Appendix 5 on

the website.