Question

Give reason: ${\text{Zn, Cd}}$ and ${\text{Hg}}$ are soft and have low melting point.

Answer 1

A metallic bond is formed when one or more unpaired electrons are present. With increase in the number of unpaired electrons, the strength of the metallic bond increases. Due to this, the hardness and melting point increases.

Complete Step by step answer: The element symbols ${\text{Zn, Cd}}$ and ${\text{Hg}}$ represent the elements zinc, cadmium and mercury. These are transition elements or d block elements in which the d-subshell is filled. No unpaired electrons are present as all the electrons are paired in the completely filled d subshell.
The atomic number of zinc is 30. It has 30 electrons. The electronic configuration of zinc is $\left[ {{\text{Ar}}} \right]{\text{3}}{{\text{d}}^{10}}{\text{4}}{{\text{s}}^2}$ .
The atomic number of cadmium is 30. It has 48 electrons. The electronic configuration of zinc is $\left[ {{\text{Kr}}} \right]{\text{4}}{{\text{d}}^{10}}{\text{5}}{{\text{s}}^2}$ .
The atomic number of mercury is 30. It has 80 electrons. The electronic configuration of zinc is $\left[ {{\text{Xe}}} \right]{\text{4}}{{\text{f}}^{14}}{\text{5}}{{\text{d}}^{10}}{\text{6}}{{\text{s}}^2}$ .
Due to the absence of unpaired electrons, the formation of the strong metallic bond is not possible. Due to this, the elements ${\text{Zn, Cd}}$ and ${\text{Hg}}$ are soft and have low melting point.
Transition metals other than zinc, cadmium and mercury are hard and have high melting points. This is because they have unpaired electrons and form strong metallic bonds.

Note: Transition metals have partially filled (n-1)d subshells. However, in case of zinc, cadmium and mercury, (n-1)d subshells are completely filled. Hence, zinc, cadmium and mercury are not regarded as d-block elements.