CBSE Class 12 Chemistry Notes: d and f Block Elements – f Block Introduction

f – BLOCK ELEMENTS

The elements in which the differentiating electron enters the anti penultimate energy level i.e. (n – 2)f, are called f – block elements. These are often called as inner transition elements or rare earth elements. The differentiating electron in transition elements may enter either 4f or 5f orbitals based upon which they are differentiated into lanthanides and actinides.

Lanthanides

In lanthanides the differentiating electron enters 4f orbital. These are cerium to lutetium. The name lanthanide is because they come immediately after lanthanum.

Actinides

In actinides the differentiating electron enters 5f orbitals. These are thorium to lawrencium. These elements come immediately after actinium.

Electronic configuration

General electronic configuration of f – block elements is

Lanthanides                 

Actinides                     

CHARACTERISTICS OF LANTHANIDES

1. Oxidation states

Lanthanides show only one stable oxidation state, which is not in the case of actinides. The typical oxidation state of lanthanides is +3. Some elements show +2 and +4 also, when they lead to

(a) a noble gas configuration e.g.

(b) a half filled f shell e.g.

(c) a completely filled f shell e.g. 

2. Lanthanide contraction

In lanthanide series with increasing atomic number there is a progressive decrease in the atomic as well as ionic radii. This regular decrease is known as lanthanide contraction. This is due to the poor shielding of f orbitals, which are unable to counter balance the effect of increasing nuclear charge. Net result is contraction in size.

Consequences of lanthanide contraction

Since the change in ionic radii in lanthanides is very small (only 15 pm from Ce³⁺ to Cu³⁺), their chemical properties are similar. This makes the separation of the elements in pure state very difficult.

Due to lanthanide contraction, the difference in size between second (4d) and third (5d) is very small.

As the size of the lanthanide ions decreases from La⁺³ to Lu⁺³, the covalent character of the hydroxides increases and hence the basic strength decreases. Thus La(OH)₃ is most basic whereas Lu(OH)₃ is least basic.

3. Complex formation

 The lanthanides do not have much tendency to form complexes due to low charge density because of their size. However, the tendency to form complex and their stability increases with increasing atomic number.

4. Chemical Behaviour

The first few members of the series are quite reactive like calcium. However with increasing atomic number, their behaviour becomes similar to that of aluminum.

(a) They combine with H₂ on gentle heating. When heated with carbon, they form carbides. On burning in the presence of halogens, they form halides.

(b) They react with dilute acids to liberate H_­2.

(c) They form oxides and hydroxides of the type N₂O₃ and M(OH)₃ which are basic alkaline earth metal oxides and hydroxides.

GENERAL CHARACTERISTICS OF ACTINIDES

1. Oxidation states

 The dominant oxidation state of these elements is +3 (similar to lanthanides). Besides +3 state, they also exhibit +4 oxidation state. Some actinides show still higher oxidation states. The maximum oxidation state first increases upto the middle of the series and then decreases i.e. it increases from +4 for Th to +5, +6 and +7 for Pa, V and Np but decreases in the succeeding elements.

2.   Chemical behaviour

The ability of actinides to exist in different oxidation states has made their chemistry more complex. Moreover, most of these elements are radioactive and the study of their chemistry in the laboratory is difficult.

(a) They react with boiling water to give a mixture of oxide and hydride.

(b) The combine with most of the non – metals at moderate temperature.

(c) All these metals are attacked by HCl but the effect of HNO₃ is very small due to the formation of a protective oxide layer on their surface.

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