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. Ce⁴⁺(f⁰)

(b) a half filled f shell e.g. Eu²⁺(f⁷)

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.