Describe Cause of Actinide Contraction - QS Study
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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.

The gradual decrease in the ionic radii with the increase in atomic no. is called actinoid contraction. Consequently steady decrease in size occurs in the actinide series. This type of contraction also occurs in actinides. The jump in contraction between the consecutive elements in the actinides is greater than lanthanides. This is due to the lesser shielding of 5f−electrons which are therefore pulled more strongly by the nucleus.