Question

Which has melting point below ${{500}^{\circ }}C$?
(A)- $Ag$, $Cu$
(B)- $Zn$, $Cd$
(C)- $Cd$, $Cu$
(D)- $Ag$, $Zn$

Answer 1

Hint: The melting point of the transition metals is very high of the metallic bonds they form because of the vacant d-orbital present in their orbitals but the last element of each period has a very low melting point because of the completely filled orbital.

Complete step by step solution:
All the elements present in the above options belong to the transition element. $Zn$ is the zinc, $Cu$ is the copper, $Cd$ is the cadmium, and $Ag$ is the silver.
The melting point of the transition metals is very high in the metallic bonds they form because of the vacant d-orbital present in their orbitals. Due to the presence of vacant d-orbital and very high heat of atomization, they form a strong metallic bond, which requires high energy to break and they have a high melting point.
But the last element of each period has a very low melting point because of the completely filled orbital and due to this completely filled d-orbital, they are not available for making metallic bonds with other elements. So they have a low heat of atomization and low melting point.
So the melting point of zinc is ${{419}^{\circ }}C$, cadmium is ${{321.07}^{\circ }}C$, copper is ${{1084}^{\circ }}C$, and silver is ${{961}^{\circ }}C$. Since zinc is the last element of 3d series and cadmium is the last element of the 4d series.

Therefore, the correct answer is an option (B)- $Zn$, $Cd$.

Note: Enthalpy of heat atomization is the heat needed to break the lattice of the metal to get free atoms or ions. In the transition metals, tungsten ($W$) has the highest melting point and mercury ($Hg$) has the least.