Comprehension:
TRANSITION METALS
All the transition elements display typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre. With the exceptions of Zn, Cd, Hg and Mn, they have one or more typical metallic structures at normal temperatures.
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. The high melting points of these metals are attributed to the involvement of greater number of electrons from (n–1)d in addition to the ns electrons in the interatomic metallic bonding. In any row the melting points of these metals rise to a maximum at d5 except for anomalous values of Mn and Tc and fall regularly as the atomic number increases.
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. Since the enthalpy of atomisation is an important factor in determining the standard electrode potential of a metal, metals with very high enthalpy of atomisation (i.e., very high boiling point) tend to be noble in their reactions.
Why do transition elements exhibit higher enthalpies of atomisation?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Greater number of valence electrons
The enthalpy of atomisation is the energy needed to break the metal lattice into separate gaseous atoms. It measures the strength of the metallic bonding.
In transition metals, not only the ns electrons but also the (n−1)d electrons take part in metallic bonding. With a greater number of valence electrons (and unpaired d electrons) available, the interatomic bonding is strong, so the enthalpies of atomisation are high. In a series they rise to a maximum near the middle, where there is about one unpaired electron per d orbital, and then fall:
\( \displaystyle \text{V}: 515\ \mathrm{kJ\,mol^{-1}}\quad \text{Zn}: 126\ \mathrm{kJ\,mol^{-1}} \)
Zinc (Z = 30, [Ar]3d104s2), whose d10 electrons do not take part in bonding, has a very low value. So transition elements have high enthalpies of atomisation because of the greater number of valence electrons.
Why the other options are incorrect
Option (1) Large Surface Area — Incorrect. Surface area is not a property of an atom and does not decide the strength of metallic bonding.
Option (2) Lanthanoid contraction — Incorrect. It explains why the 4d and 5d elements of a group (e.g. Zr and Hf) have almost the same radii, not why all transition metals have high enthalpies of atomisation.
Option (4) d-d transition — Incorrect. It explains the colour of transition metal ions, not the strength of metallic bonding.