Chemistry, Third edition

GROUP 1 ELEMENTS

Most of the compounds of the alkali metals are therefore ionic, and the alkali

metals have an oxidation state of 1 in their compounds.

Reactions of the alkali metals

1.They are all soft metals which tarnish in air, and for this reason they are generally

stored under oil. The products obtained on burning the metals with oxygen, in

the air, depend upon the metal:

4Li(s)O 2 (g)2Li 2 O(s) the normal oxideis formed

2Na(s)O 2 (g)Na 2 O 2 (s) a peroxideis formed

K(s)O 2 (g)KO 2 (s) for K, Rb and Cs a superoxideis formed, M,O 2 

2.They react readily with water, to form the corresponding alkali and hydrogen.

The reaction becomes very violent as the group is descended. A general equation

for this reaction is

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

2.The ionic equation is

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

2.All the Group 1 hydroxides are very soluble in water.

3.The metals and their compounds display characteristic flame colours. If a moist-

ened platinum wire is dipped into a substance containing the alkali metal and

introduced into a Bunsen flame, the flame becomes coloured. The colours are

shown in Table 12.1.

4.They form water-soluble, thermally stable carbonates (but note that Li 2 CO 3 can

be decomposed with strong heating in a Bunsen flame).

5.With the exception of LiHCO 3 , which exists only in solution, they form solid

hydrogencarbonates. Sodium hydrogencarbonate (baking powder) decomposes

on heating:

2NaHCO 3 (s)Na 2 CO 3 (s)H 2 O(l)CO 2 (g)

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Ionization energies of the alkali metals

Whereas the first ionization energies (the energy to remove one electron) of the alkali

metals are relatively low, the second ionization energies are very high.

Metal First ionization energy/kJ mol^1 Second ionization energy/kJ mol^1

Li 520 7298

Na 494 4563

K 419 3051

Rb 403 2632

Cs 376 2420

(i)Would you expect Na to form an Na^2 ion?

(ii)Why do you think Kis much more stable than K^2 ?

(iii)Suggest reasons why the second ionization energy of the metals is a great deal higher than

the first.

Exercise 12B

Table 12.1Flame

colours of the alkali

metals and their

compounds

Alkali Flame

metals colour

Lithium red

Sodium yellow

Potassium lilac (crimson

through blue

glass)

Rubidium red

Caesium blue