Question

what is lanthanoids contraction

Answer 1

The lanthanoids contruction is the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to 71, lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements starting with 72, hafnium.

Answer 2

lanthanide contraction is due to screening (or) scolding effect due to 4f orbital enters into the penultimate shell