What is Lanthanoid contraction?
1) Definition of Lanthanoid Contraction: The Lanthanoid contraction may be defined as gradual
(or small) the decrease in atomic and ionic radii of lanthanoids with the increase in atomic number.
2) In Lanthanides, after Lanthanum (`""_57La`), the electrons are added to anti-penultimate shell i.e. 4f orbital.
3) There are 14 Lanthanoides from `""_58Ce` to `""_57Lu`
4) For each electron, one proton is also added to the nucleus of the atom of the element. Hence from `""_58Ce`to `""_71Lu` as atomic number increases, nuclear charge increases, therefore nuclear attraction increases.
5) Due to this as atomic number increases, atomic volume or radius decreases as observed with all the elements along the period.
6) But in case of Lanthanoids, this decreases in atomic volume or radius is very slow or comparatively small. This is explained in terms of Lanthanoid contraction