Chemistry, Third edition

12 · CHEMICAL FAMILIES

6.They form nitrates that decompose to nitrites and oxygen on strong heating (but

note that LiNO 3 decomposes to the oxide). The general equation for decom-

position of the nitrates is

2MNO 3 (s)2MNO 2 (s)O 2 (g)

7.Their salts (usually colourless or white) are ionic and not hydrolysedin solution

(they do not react with water, they only dissolve in water).

The elements become more reactiveas the group is descended. The atomic radius

of the elements increases as the group is descended and the single electron in the

outer shell of the atoms becomes farther away and more shielded from the pull of the

nucleus by full shells of inner electrons. The outer electrons therefore become less

strongly bound down the group and the ionization energy of the elements decreases.

This is shown by the values given in Exercise 12B.

Trends in the chemical properties of the alkali metals and their compounds are

summarized in Table 12.2.

The chemistry of francium (Fr) is not as well studied as that for the rest of the

alkali metals, because it is radioactive and scarce. The longest-lived isotope of the

element has a half-life of about 20 min.

196

Trends in the physical properties of the alkali metals

The boiling points of the metals are as follows:

Metal Boiling point/K

Li 1615

Na 1156

K 1033

Rb –

Cs 942

(i) Explain the trend shown by the boiling point values as the group is descended. (Hint

consider the strength of the metallic bonding in the elements.)

(ii)Predict a boiling point value for Rb.

(iii)The alkali metals are so soft that they can all be cut with a pen-knife. They become softer

as the group is descended. Can you think of an explanation for this trend?

(iv)The metals have low densities (Li, Na and K float on water). Why do you think this is?

Exercise 12C

Chemistry of

francium

From your knowledge of

the alkali metals and the

trends in their properties,

predict:

(i) The reactivity of

francium towards air

and water. Write

balanced equations for

the reactions that

occur.

(ii)The formula, solubility

and thermal stability of

francium carbonate.

(iii)The effect of heat on

francium nitrate. Write

an equation for the

reaction.

Exercise 12D

Table 12.2Trends in the chemical properties of the alkali metals and

their compounds

Thermal

stability of

Reactivity carbonates Thermal Thermal Solubilities

with air in a bunsen stability stability of

Metal and water flame of hydroxides of nitrates carbonates

Li ||decomposes ||decomposes to oxide|

Na ||stable and |stable and |decompose to |soluble and

K |more |increasing |increasing |nitrites increasing |increasing

Rb |vigorous |stability |stability |stability |solubility

Cs