The Actinide Series (5f block elements):
In the year 1923 Neils Bohr postulated the survival of an actinide sequence analogous to the lanthanide series.
The fifteen elements from actinium to lawrencium comprise the actinide sequence of the periodic table.
General Properties of Actinide Series:
The general electronic configuration of actinides is [Rn] 5f0,1-14 6d0,1-2 7s2, here Rn stands for radon core.
These elements display the oxidation states of +2, +3, +4, +5 and +6. Out of these, +4 oxidation state is most general state.
Radii of M3+ and M4+ ions:
The ionic radius of actinide elements reduce steadily as we move all along the actinide sequence. The steady reduction in the ionic radius with raise in nuclear charge is termed as actinide contraction and is analogous to the lanthanide contraction.
Cause of actinide contraction:
Cause of actinide contraction is the deficient shielding by 5f-electrons. Since we proceed from one element to the another one in actinide series, the nuclear charge raises by +1 at each next element that is not compensated due to the poor shielding effect of 5f orbitals due to their more diffuse shape. Therefore as the atomic number rises, the inward pull experienced by 5f-electrons raise. Consequently steady reduction in size takes place in the actinide sequence.
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