Q.(i) Write the name and atomic number of fourth element of 3d-series.

The key to this question lies in understanding why being in the same group makes separation difficult for some elements but not others. Many elements in the same group (e.g., Na and K) are easily separated. The real culprit here is a subtle but powerful effect: the lanthanide contraction.

1. The Assertion is true. Zirconium (Zr) and Hafnium (Hf) are notoriously difficult to separate. Their chemical properties are so similar that traditional methods like fractional crystallisation or precipitation are extremely inefficient. This is a well-known fact in inorganic chemistry.

2. The Reason is true, but incomplete. It is a fact that Zr and Hf lie in Group 4 of the periodic table. However, this statement alone does not explain why their separation is difficult. Many elements in the same group (e.g., Oxygen and Sulfur) have vastly different properties and are easily separated.

3. The missing link: Lanthanide Contraction. The real explanation is the lanthanide contraction. As we move across the lanthanide series (elements 57–71), the 4f orbitals are filled. These orbitals have a poor shielding effect, so the increasing nuclear charge pulls the outer electrons inwards, causing a steady decrease in atomic and ionic radii.

The key to this question is understanding lanthanide contraction — the gradual decrease in ionic radii from $La^{3+}$ to $Lu^{3+}$ as you move across the lanthanoid series. This happens because the 4f electrons are poor at shielding the nuclear charge, so the effective nuclear pull on the outer electrons increases steadily.

Let’s examine each statement one by one.

1. Statement (A): La is actually an element of transition series.

   Lanthanum (La, atomic number 57) is the first member of the lanthanoid series, but it is also placed in the d-block of the periodic table (Group 3). Its electronic configuration is $[Xe]\,5d^1\,6s^2$, which makes it a d-block element. So yes, La is indeed a transition element. This statement is correct.

2. Statement (B): Zr and Hf have almost identical atomic radii because of lanthanoid contraction.

   Zirconium (Zr, period 5) and hafnium (Hf, period 6) belong to Group 4. Normally, atomic radius increases down a group. But because the 4f orbitals (filled across the lanthanoids) are inserted between Hf and the element above it, the lanthanide contraction causes Hf’s radius to be nearly the same as Zr’s. This is a classic consequence. Statement (B) is correct.

3. Statement (C): Ionic radius decreases from $La^{3+}$ to $Lu^{3+}$ ion. …