Chapter 7 The Structure of Atoms and Periodic Trends - 7-6 Periodic Trends and Chemical Properties - Applying Chemical Principles - Questions - Page 280: 4

See the answer below.

a) f subshells, where the electrons are added in lanthanides, is not efficient at shielding the electrons from the outer s subshell from nucleus attraction, hence an increase in the atomic number translates into an increase in the effective nuclear charge and a decrease in radius from left to right among lanthanides. b) Since radius' increase with period number, it argues for the placement of La below Y.