Read the following passage and answer the next five questions based on it:

In any transition series, as we move from left to right the \(d-\)orbitals are progressively filled and their properties vary accordingly

The above are the two series of f-block elements in which the chemical properties won't change much. The 5f-series elements are radioactive in nature and mostly are artificially synthesized in laboratories and thus much is not known about their chemical properties.

Identify the incorrect statement.

Zr and Hf shares almost identical nuclear properties.

The correct answer is option (2) -Zr and Hf shares almost identical nuclear properties.

Statement-wise Evaluation

Option 1. Second ionization enthalpy of Ag is greater than second ionization enthalpy of Pd : Correct 

• Ag configuration: $[\text{Kr}] \, 4d^{10} 5s^1$
• $1^{\text{st}}$ IE removes $5s^1 \rightarrow \text{Ag}^+$ becomes $4d^{10}$ (very stable)
• $2^{\text{nd}}$ IE removes an electron from filled $4d^{10}$ shell $\rightarrow$ requires very high energy
• Pd configuration: $[\text{Kr}] \, 4d^{10} 5s^0$
• Removing two electrons does not involve breaking an already formed $\text{Ag}^+$-type extra stability.

Therefore, second ionization enthalpy of $\text{Ag}$ is indeed higher than $\text{Pd}$.

So Statement 1 is true.

2. Zr and Hf share almost identical nuclear properties: Incorrect statement

• $\text{Zr}$ and $\text{Hf}$ are similar due to lanthanide contraction, which affects atomic size and chemical/physical properties.
• Nuclear properties relate to isotopes, binding energy, radioactivity, etc. — these are not identical.
• Textbooks always say chemical properties, not nuclear.

Hence this statement is factually wrong.

3. Melting point of Mn is lower than that of Cr : Correct 

$\text{Mn}$ has relatively weaker metallic bonding compared to $\text{Cr}$, so its melting point is lower. This is a known irregular trend in the $3d$ series.

4. Interstitial compounds are non-stoichiometric and neither ionic nor covalent:  Correct 

They are metallic in character and usually non-stoichiometric. This is standard theory of transition metals.