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.
Which of the following transition metals is not very hard?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Zn
Transition metals are hard and have high melting points because of strong metallic bonding. Both the ns electrons and the (n−1)d electrons take part in this bonding, and the more unpaired d electrons a metal has, the stronger the bonding.
Zinc, cadmium and mercury have a completely filled d10 subshell (Zn, Z = 30: [Ar]3d104s2). Their d electrons do not take part in metallic bonding, so only the two s electrons hold the atoms together. The bonding is weak, which makes these metals comparatively soft, with low melting points (Zn about 693 K; mercury is a liquid).
Option (1) Zn — Not very hard. Its d10 electrons take no part in metallic bonding, so the bonding is weak; it melts at about 693 K.
Option (2) Fe — Very hard. Iron ([Ar]3d64s2) has four unpaired d electrons for metallic bonding; it melts at about 1811 K.
Option (3) Cu — Very hard. Copper is a tough metal with a high melting point (about 1358 K).
Option (4) Ni — Very hard. Nickel ([Ar]3d84s2) has unpaired d electrons for bonding; it melts at about 1728 K.