Read the following passage and answer the next five questions based on it.
The transition metals are very hard and have low volatility. Their melting and boiling points are high. In any row, the melting points of these metals rise to a maximum at d5 and fall regularly as atomic number increases. The high melting points of these metals are attributed to the involvement of greater number of electrons from (n – 1)d in addition to ns electrons in the interatomic metallic bonding.
Which is the hardest metal ?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Cu
As the passage says, the hardness and high melting points of transition metals come from metallic bonding that uses (n – 1)d electrons as well as ns electrons. Zn, Cd and Hg have completely filled d10 subshells and paired ns2 electrons, so their d electrons do not take part in metallic bonding; they are soft metals with low melting points (Hg is even a liquid). Copper (Z = 29, [Ar]3d104s1) has much stronger metallic bonding: its enthalpy of atomisation is 339 kJ mol−1, against 130 kJ mol−1 for zinc (Table 4.4).
So of the metals given, the hardest is Cu.