The position of f block elements in the periodic table, is explained above.
The elements in which the extra electron enters ( n- 2 )f orbitals are called f- block elements. These elements are also called as inner transition elements because they form a transition series within the transition elements. The f-block elements are also known as rare earth elements. These are divided into two series.
i) The Lanthanide series (4f-block elements)
ii) The Actinide series (5f- block elements )
The Lanthanide Series
The Lanthanide series include fifteen elements i.e. lanthanum (57 La) to lutetium (71 Lu). Lanthanum and Lutetium have no partly filled 4f- subshell but have electrons in 5d-subshell. Thus these elements should not be included in this series. However, all these elements closely resemble lanthanum and hence are considered together.
General properties of Lanthanides
1. Electronic configuration
The electronic configuration of Lanthanides are listed in the table . The fourteen electrons are filled in Ce to Lu with configuration [54 Xe ]4f1-14 5d1 6s2
2. Oxidation states
The common oxidation state exhibited by all the lanthanides is +3 (Ln3+) in aqueous solutions and in their solid compounds. Some elements exhibit +2 and +4 states as uncommon oxidation states.
La - +3
Ce - +3, +4, +2
Pr - +3, +4
Nd - +3, +4, +2
3. Radii of tripositive lanthanide ions
The size of M3+ ions decreases as we move through the lanthanides from lanthanum to lutetium. This steady decrease in ionic radii of M3+ cations in the lanthanide series is called Lanthanide contraction.
Cause of lanthanide contraction
The lanthanide contraction is due to the imperfect shielding of one 4f electron by another in the same sub shell. As we move along the lanthanide series, the nuclear charge and the number of 4f electrons increase by one unit at each step. However, due to imperfect shielding, the effective nuclear charge increases causing a contraction in electron cloud of 4f-subshell.
Consequences of lanthanide contraction
Basicity of ions
i) Due to lanthanide contraction, the size of Ln3+ ions decreases regularly with increase in atomic number. According to Fajan's rule, decrease in size of Ln3+ ions increase the covalent character and decreases the basic character between Ln3+ and OH- ion in Ln(OH)3. Since the order of size of Ln3+ ions
La3+> Ce3+ ............... >Lu3+
ii) There is regular decrease in their ionic radii.
iii) Regular decrease in their tendency to act as reducing agent, with increase in atomic number.
iv) Due to lanthanide contraction, second and third rows of d-block transistion elements are quite close in properties.
v) Due to lanthanide contraction, these elements occur together in natural minerals and are difficult to separate.