Which one of the following is the correct order of ionic radii ?

Options

A. `Pr^(3+) gt Gd^(3+) gt Tm^(3+)`
B. `Pr^(3+) lt Gd^(3+) lt Tm^(3+)`
C. `Pr^(3+) gt Tm^(3+) gt Gd^(3+)`
D. `Pr^(3+) lt Tm^(3+) lt Gd^(3+)`

Answer

A

Explanation

In 4f- series as the atomic number increase from La to Lu the atomic and ionic radil decreases gradually .This decrease in atomic and ionic size from lanthnum to lutetium is due to lanthanide contraction
hence the correct order of lonic radil of given element is
`underset(101 pm)(Pr^(3+)) gt underset(94 pm)( Gd^(3+)) gt underset(87 pm)(Tm^(3+))`