Lanthanoid Contraction Why is the radius of Europium so unusually high and out of the general trend ? Moreover, at different sources I am getting different values of radii. In some (e.g., Wikipedia) the radii are following the regular trend and in some Europium has been shown to be bigger. (Source : NCERT Class 12 Chemistry Part-1)
Well spotted! I dare say, the book appears to have used a covalent radius for Europium, while using atomic radii for the other lanthanides.

See "atomic radius" vs "covalent radius" values: for Europium, Samarium, and Gadolinium.

Also see this article if you don't like my referencing Wikipedia :P