Read the passage carefully and answer the questions.
Nearly all the transition elements display typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre. The transition metals (with the exception of Zn, Cd and Hg) are very hard and have low volatility. Their melting and boiling points are high. They have high enthalpies of atomization. Ions of the same charge in a given series show progressive decrease in radius with increasing atomic number. However, the variation within a series is quite small. The filling of 4f before 5d orbital results in a regular decrease in atomic radii called Lanthanoid contraction which essentially compensates for the expected increase in atomic size with increasing atomic number. The net result of the lanthanoid contraction is that the second and the third d series exhibit similar radii (e.g., Zr 160 pm, Hf 159 pm) and have very similar physical and chemical properties. There is an increase in ionisation enthalpy along each series of the transition elements from left to right. The first ionisation enthalpy, in general, increases, but the magnitude of the increase in the second and third ionisation enthalpies for the successive elements, is much higher along a series. One of the notable features of a transition elements is the great variety of oxidation states these may show in their compounds.
In the first transition series of elements (Sc to Zn), which element show least enthalpy of atomization?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Zn
The enthalpy of atomisation is the energy needed to break the metal into free atoms, so it measures the strength of metallic bonding. In transition metals both the 4s electrons and the unpaired 3d electrons take part in the bonding: the more unpaired electrons, the stronger the interatomic attraction and the higher the enthalpy of atomisation. The values rise to a maximum near the middle of the series and then fall.
Sc (Z = 21): \( \mathrm{[Ar]\,3d^{1}\,4s^{2}} \), one unpaired 3d electron: 326 kJ mol−1.
V (Z = 23): \( \mathrm{[Ar]\,3d^{3}\,4s^{2}} \), three unpaired 3d electrons: 515 kJ mol−1.
Mn (Z = 25): \( \mathrm{[Ar]\,3d^{5}\,4s^{2}} \): 281 kJ mol−1, lower than its neighbours because the stable half-filled 3d5 set takes little part in bonding.
Zn (Z = 30): \( \mathrm{[Ar]\,3d^{10}\,4s^{2}} \), no unpaired electrons; only the 4s electrons bond, weakly: 126 kJ mol−1.
So the least enthalpy of atomisation in the series Sc to Zn is shown by Zn. The weak bonding also explains why the passage leaves out Zn, Cd and Hg when it calls transition metals very hard and of low volatility.
NCERT (2026-27 edition):
Physical Properties – “… They have high enthalpies of atomisation which are shown in Fig. 4.2. The maxima at about the middle of each series indicate that one unpaired electron per d orbital is particularly favourable for strong interatomic interaction. In general, greater the number of valence electrons, stronger is the resultant bonding.”
