Answer
See explanation below.
Work Step by Step
The greater than expected decrease of radii of lanthanide elements from atomic number 57 to 71 is termed as lanthanide contraction.The size increases from period 4 to period 5, but remians almost constant from period 5 to period 6.
However, atomic mass increases from period 5 to period 6, but there is a little increase in volume, because of which period 6 elements are dense.
