Define lanthanoid contraction.
What is lanthanoid contraction?
With an increasing atomic number in the lanthanoid series, there is a progressive decrease in the atomic as well as ionic radii of trivalent ions from La3+ to Lu3+.
This regular decrease (contraction) in the size of the atoms and ions with increasing atomic number is known as lanthanoid contraction.
Lanthanoid Contraction : In the lanthanoids , the electrons are getting filled in the 4f-subshell. On moving from left to right, the nuclear charge increases and this increase is expected to be compensated by the increase in the magnitude of shielding effect by the 4 f- electrons However,
the f-electrons have very poor shielding effect. Consequently, the atomic and ionic radii decrease from left to right and this is knwon as lanthanoid contraction.
Compare the chemistry of actinoids with that of the lanthanoids with special reference to atomic and ionic sizes