Question

In the context of the lanthanides, which of the following statements is not correct?
(A)- There is a gradual decrease in the radii of the members with increasing atomic number in the series.
(B)- All the members exhibit a +3 oxidation state.
(C)- Because of similar properties of separation of lanthanide is not easy.
(D)- Availability of 4f electrons results in the formation of compounds in +4 state for all the members of the series.

Answer 1

Hint: -The series of chemical elements consisting of 15 metallic chemical elements with atomic number 57-71 are known as Lanthanide or Lanthanoid. Lanthanide elements begin from Lanthanum and end at Lutetium and with Scandium and Yttrium, they make up the rare-earth metals.

Complete step by step solution:
-Lanthanides have the extra electron (also called differentiating electron) which enters in $(n-2)f$orbital. Since $(n-2)f$orbital lies comparatively deep within the inner to the penultimate shell, therefore the elements are also called Inner transition elements.
-The lanthanoid have the general electronic configuration as $4{{f}^{1-14}}5{{d}^{0-1}}6{{s}^{2}}$, with some exception due to full fill or half-filled electronic configuration, wherein the 4f electrons of lanthanides did not participate in bond formation. Lanthanoid atoms have similar configurations and similar physical and chemical properties, and the most common valences are 3 and 4.
-We have so far learnt the general and major characteristics and features of lanthanides. Let us now consider the statements in each option one by one to find the incorrect statement.
-Option A says that there is a gradual decrease in the radii of the members with increasing atomic number in the correct series because the overall decrease in atomic and ionic radii from lanthanum to lutetium is a unique feature in the chemistry of lanthanoid and this is known as Lanthanide contraction.
-Option B says that all the members exhibit in +3 oxidation state. This is a correct statement because all the most stable valencies for all the elements are three. However, all elements do not compulsorily show the +3 oxidation state. Some elements also exhibit +1, +2 and +4 valency. For example, Gadolinium exhibits +1, Cerium, Praseodymium, Neodymium exhibits both +2 and +4 oxidation states.
-Option C says because of similar properties the separation of lanthanide is not eased. This is because of the consequences of lanthanide contraction, without lanthanide contraction, chemical separation of lanthanides would be extremely difficult. Due to lanthanoid contraction, Zr and Hf, Nb and Ta, Mo and W have the same size and hence similar properties due to which their separation is not easy. Therefore it is a correct statement.
-Option D says that the availability of 4f electrons results in the formation of the compound in +4 state for all the members of the series, which is incorrect. All compounds of the lanthanoid series do not exhibit +4 oxidation.

Therefore, the correct answer is option D.

Note: -The International Union of Pure and Applied Chemistry, which is the international body in charge of chemical nomenclature, prefers the term ‘Lanthanoid’, because ‘-ide’ ending is usually reserved for negatively charged ions.