Why there is a sudden increase in the radius of Europium?see the image....

As we know that s electrons sheilds more and the extent of sheilding decreases in the order:
s>p>d>f
Also the sheilding of exactely half filled or full filled orbitals are higher as compared to the normal sheilding of that particular orbital.Here, F-rbtal is exactely half-filled and hence having more shilding which accouns for its irregular trendi nsize................................................................................................................................................................