Question

Which one of the following statements about lanthanides is false?
(A) Lanthanides are separated from one another by ion-exchange method.
(B) the ionic radii of trivalent lanthanides steadily increases in atomic number
(C) All lanthanides are highly dense metals
(D) Most typical oxidation state of lanthanides is $ + 3$

Answer 1

Hint: Lanthanides are f block elements and are rare elements. f-block is placed at the bottom of the modern periodic table. They are rare earth metalsSo, naturally, they will have higher atomic weights.f-block consists of periods 6 and 7 and groups 3 to 17 from which the period 6 elements are lanthanides and period 7 elements are actinides.

Complete step by step answer:
Let’s consider each option in detail.

1.Lanthanides are separated by the ion-exchange method.
This method is used to separate dissolved ions of lanthanides from one another.
The ionic and soluble solution of lanthanides is passed through a long column containing resin. The lanthanide ions, depending upon their size, bind to the resin.
Lanthanum, being the smallest lanthanide binds most tightly here as lutetium, being the largest binds weakly.
Hence, the ion exchange method is quite reliable to separate lanthanides

2.The ionic radii of trivalent lanthanides steadily increase with an increase in atomic number
The ionic radii of trivalent lanthanides decrease with the increasing atomic number due to poor shielding effect with an increase in the number of electrons.
So the statement the ionic radii of trivalent lanthanides steadily increases with an increase in atomic number contradicts the fact.

3.All lanthanides are highly dense metals.
The lanthanides have a high atomic number as well as atomic mass. Lutetium, the last member of the lanthanide series is the densest lanthanide with the highest melting point.

4.The most typical oxidation state of lanthanides is $ + $3
It is the most stable and common oxidation state of lanthanides. The 2 electrons from the 6s subshell is removed as well as 1 electron from the 4f subshell to attain the $ + $3 oxidation state.
Lanthanides can also attain other oxidation states as well as $ + $2 or $ + $4

So, the correct answer is Option B .

Note: The general electronic configuration of lanthanides is $\left[ {Xe} \right]4{f^{1 - 14}}5{d^{0 - 1}}6{s^2}$ Lanthanides are known to have the property of lanthanide contraction

It is caused by the poor shielding effect of electrons which results in greater than expected decrease in atomic radii than expected. Promethium is the only radioactive element among lanthanides.