Chemistry, Third edition

GROUP 2 ELEMENTS

Group 2 elements – the alkaline earth

metals: Be, Mg, Ca, Sr, Ba, Ra

The chemistry of these elements (except Be) is dominated by their tendency to lose

two electrons and their compounds are generally ionic. The general equation for this

reaction is

MM^2 2e

The metals tend to be less reactive than their corresponding alkali metals because

removal of two electrons requires more energy. The alkaline earth metals also have

higher densities and melting points than the alkali metals. Reactivity of the Group 2

metals increases as the group is descended (Table 12.3).

Some chemical properties of Group 2 metals (with the exception of Be, which

forms mostly covalent compounds) are:

1.They burn in air to form normal oxides (barium forms some peroxide). The

general equation is

2M(s)O 2 (g)2MO(s)

2.The alkaline earth metals, when heated, also react with nitrogen to form ionic

nitrides:

3M(s)N 2 (g)M 3 N 2 (s)

3.Ca, Sr and Ba react with water to form the hydroxide and hydrogen gas:

M(s)2H 2 O(l)M(OH) 2 (aq)H 2 (g)

3.Magnesium reacts with cold water only slowly, but rapidly reacts to form the

oxide when heated in steam:

Mg(s)H 2 O(l)MgO(s)H 2 (g)

4.The metals react with aqueous acids, liberating hydrogen:

M(s)2HCl(aq)MgCl 2 (aq)H 2 (g)

5.Some alkaline earth metals have characteristic flame colours, shown in Table 12.4.

6.Their carbonates decompose on strong heating into carbon dioxide and the oxide:

MCO 3 (s)MO(s)CO 2 (g)

7.The Group 2 nitrates decompose into the oxide, brown gaseous nitrogen dioxide

and oxygen:

2M(NO 3 ) 2 (s)2MO(s)4NO 2 (g)O 2 (g)

8.Their hydroxides are not as thermally stable as those of the alkali metals. Group 2

hydroxides decompose on heating:

M(OH) 2 (s)MO(s)H 2 O(l)

12.3

197

Physical properties

of the alkaline

earth metals

Can you explain why the

melting points of the

alkaline earth metals are

higher than those of the

corresponding alkali

metals?

Exercise 12E

Table 12.4Flame

colours of the alkaline

earth metals and their

compounds

Element Flame colour

Calcium brick red

Strontium scarlet

Barium apple green

Table 12.3Trends in the properties of the alkaline earth metals and

their compounds

Reactivity with air, Solubility Solubility Basic strength Solubility of
Metal water and acids of sulfates of carbonates of hydroxides hydroxides

Be |poor | does not exist |amphoteric |insoluble

Mg ||soluble | |weak |insoluble

Ca |increases |

insoluble
|||
Sr ||increasingly |increasingly |increasingly |increasingly
Ba insoluble insoluble strong soluble