Chemistry, Third edition

12 · CHEMICAL FAMILIES

The halogens also react by sharing electrons to form covalent bonds. For example, in

the reaction of hydrogen gas with chlorine gas,

H 2 (g)Cl 2 (g)2HCl(g)

4. Character of chlorides and oxides

The fact that the elements become less metallic, from left to right across a period, is

shown by the characters of their chlorides and oxides. The chlorides of metals on the

left-hand side of a period are ionicanddissolve in waterto form neutral solutions

whereas the chlorides of non-metals on the right-hand side of the table are covalent

and react with water. There is a gradual change between these two extremes across the

period. Consider the chlorides of period 3:

NaCl(s) MgCl 2 (s) AlCl 3 (s) SiCl 4 (l) PCl 3 (l) S 2 Cl 2 (l) Cl 2 (g)

white ionic —covalent compounds, hydrolyse to —

solids, dissolve produce acidic solutions of HCl(aq), e.g.

in water SiCl 4 (l)4H 2 O(l)SiO 2 · 2H 2 O(s)4HCl(aq)

The oxides of period 3 change from ionic to covalentacross the period. Oxides of

the left-hand side metals are basic, whereas non-metallic oxides of the right-hand

side elements are acidic. There is a gradual change from basic to acidiccharacter of

the oxides across the period:

Na 2 O(s) MgO(s) Al 2 O 3 (s) SiO 2 (s) P 4 O 6 (s) SO 2 (g) Cl 2 O(g)

SO 3 (l)

—ionic solids————white————covalent———————

—basic—————amphoteric———acidic—————————

For example MgO(s) is basic, it reacts with acids to give a salt and water:

MgO(s)2HCl(aq)MgCl 2 (aq)H 2 O(l)

The oxide Al 2 O 3 (s) is amphoteric, it reacts with strong acids to produce salts:

Al 2 O 3 (s)3H 2 SO 4 (aq)Al 2 (SO 4 ) 3 (aq)3H 2 O(l)

It will also react with a strong alkali to produce a salt:

Al 2 O 3 (s)2NaOH(aq)3H 2 O(l)2Na[Al(OH) 4 ](aq)

sodium aluminate

The sulfur oxides are acidic; they react with water to form acids:

SO 2 (g)H 2 O(l)H 2 SO 3 (aq)

SO 3 (l)H 2 O(l)H 2 SO 4 (aq)

212

BOX 12.10

Distribution of metals and non-metals within the periodic table

The trends in metallic and non-metallic character of the elements within groups and periods

within the periodic table may be summarized in the following diagram:

B

Si

GeAs

SbTe

Po

Metalloids

along

this line

Increasingly

metallic

Less

metallic