Question

The atomic radii of cerium and promethium are quite similar because

• A. they belong to the same period in periodic table
• B. they have similar electronic configuration
• C. f-orbitals have poor shielding effect
• D. nuclear charge is higher on cerium than promethium

Answer 1

The correct option is C f-orbitals have poor shielding effect

The most significant characteristic of lanthanides is the contraction of the atomic size with increase in atomic number called as lanthanide contraction. While moving along the lanthanides series from ${}^{58}Ce$ to ${}^{71}Lu$, a regular decrease in the size of the atom/ion and an increase in atomic number is observed. This decrease in size is called the lanthanide contraction. The decrease in size, though continuous, is not regular. Lanthanide contraction takes place due to the imperfect shielding of a $4f$- electron which increases by one more in the same sub-shell.
So, due to this poor shielding effect, $Ce$ and $Pm$ have similar radii.