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 Actinide Series (5f block elements)
In 1923 Neils Bohr postulated the existence of an actinide series analogous to the lanthanide series.
The fifteen elements from actinium to lawrencium constitute the actinide series of the periodic table.
General Properties of Actinide Series
1. The general electronic configuration of actinides is [Rn] 5f0,1-14 6d0,1-2 7s2 where Rn stands for radon core.
2. Oxidation states
These elements shows the oxidation states of +2, +3, +4, +5 and +6. Out of these, +4 oxidation state is most common state.
3. Radii of M3+ and M4+ ions
The ionic radii of actinide elements decrease gradually as we move along the actinide series. The steady decrease in the ionic radii with increase in nuclear charge is called actinide contraction and is analogous to lanthanide contraction.
Cause of actinide contraction
Cause of actinide contraction is the imperfect shielding by 5f-electrons. As we proceed from one element to the next one in actinide series, the nuclear charge increases by +1 at each next element which is not compensated due to poor shielding effect of 5f orbitals due to their more diffuse shape. Hence as the atomic number increases, the inward pull experienced by 5f-electrons increase. Consequently steady decrease in size occurs in the actinide series.
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