Read the following passage and answer the next five questions based on it.
| Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
| Y | Zr | Nb | Mo | Tc | Ru | Rh | Pd | Ag | Cd |
| La | Hf | Ta | W | Re | Os | Ir | Pt | Au | Hg |
In any transition series, as we move from left to right the d-orbitals are progressively filled and their properties vary accordingly.
| Ce | Pr | Nd | Pm | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu |
| Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No | Lr |
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.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Zr and Hf share almost identical nuclear properties.
The lanthanoid contraction (the steady decrease in size across the 4f series) cancels the increase in size expected from the second to the third transition series. So zirconium (160 pm) and hafnium (159 pm) have almost identical radii, and therefore almost identical chemical properties: they occur together in nature and are very hard to separate.
Nuclear properties depend on the nucleus, and the two nuclei are completely different: Zr has Z = 40 and Hf has Z = 72. For example, hafnium absorbs neutrons strongly, while zirconium hardly absorbs them, which is why zirconium is used to hold the fuel in nuclear reactors and hafnium in control rods. So the statement about nuclear properties is incorrect; it is their chemical properties that are almost identical.
Option (2) Zr and Hf share almost identical nuclear properties. — The incorrect statement.
Option (1) Second ionisation enthalpy of Ag is greater than second ionisation enthalpy of Pd. — A correct statement. Ag (Z = 47) is [Kr]4d105s1, so \( \ce{Ag+} \) is 4d10, and the second electron must come out of a completely filled 4d subshell. Pd (Z = 46) is [Kr]4d10, so \( \ce{Pd+} \) is 4d9, which loses its next electron more easily.
Option (3) Melting point of Mn is lower than that of Cr. — A correct statement. Melting points rise to a maximum near the middle of the series, but Mn is an exception with an unusually low value (Cr about 2180 K, Mn about 1519 K).
Option (4) Interstitial compounds are non-stoichiometric and neither ionic nor covalent in nature. — A correct statement. They form when small atoms such as H, C or N are trapped in the holes of the metal lattice (TiC, \( \ce{Mn4N} \), \( \ce{Fe3H} \)); their formulas do not follow normal valencies.
NCERT (2026-27 edition):
Atomic and Ionic Sizes – …
