Question

Assertion (A) : Separation of Zr and Hf is difficult. Reason (R) : Zr and Hf have almost identical atomic and ionic radii due to lanthanoid contraction.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Zr and Hf are one of the most important examples of lanthanoid contraction. Their almost identical sizes make their chemical properties nearly indistinguishable.

Answer 1

The Correct Option is A

Concept: Zirconium (Zr) and Hafnium (Hf) belong to Group 4 of the periodic table. These elements exhibit remarkably similar physical and chemical properties. The primary reason for this similarity is the phenomenon known as

lanthanoid contraction. Lanthanoid contraction refers to the gradual decrease in atomic and ionic radii of the lanthanoids from La to Lu due to ineffective shielding of nuclear charge by 4f-electrons. This effect influences the size of elements that follow the lanthanoids in the periodic table.

Step 1: Examine the Assertion. The assertion states that separation of zirconium and hafnium is difficult. Zirconium and hafnium occur together in nature and possess very similar chemical properties. Since most separation techniques depend on differences in chemical behavior, the close similarity between these two elements makes their separation extremely difficult. Therefore, the Assertion is

true.

Step 2: Examine the Reason. The reason states that Zr and Hf have almost identical atomic and ionic radii due to lanthanoid contraction. Atomic radius of Zr: \[ \approx 160 \, pm \] Atomic radius of Hf: \[ \approx 159 \, pm \] Despite being in different periods, their sizes are nearly identical because the lanthanoid contraction offsets the expected increase in size. Therefore, the Reason is also

true.

Step 3: Determine whether the Reason explains the Assertion. Because of their nearly identical radii:

• Their charge density is similar.

• Their ionic behavior is similar.

• Their complex formation tendencies are similar.

• Their chemical reactivity is nearly the same.
As a result, conventional chemical methods fail to separate them efficiently. Thus, the reason directly explains why the separation of Zr and Hf is difficult.

Final Conclusion: Both Assertion and Reason are true, and the Reason correctly explains the Assertion. \[ \boxed{\text{Option (A)}} \]