Question

Which element does not show allotropy?
A. Pb
B. Si
C. C
D. Sn

Answer 1

Allotropy is the property of some chemical elements exhibiting two or more than two forms or structures within the same physical state. In simple words it is the structural modification by changing the bonding of the atoms of elements.

Complete answer:
Group $14$ in the periodic table consist of carbon, silicon germanium, tin and lead. Allotropic forms of group $14$ elements are as follows:
Carbon (C): It has a tendency to form many allotropes due to its property of catenation. It is structurally different from other elements of group $14$. There are three allotropes of carbon which are diamond, graphite and Buckminsterfullerene.
Silicon (Si): It exists in its two allotropic forms at the room temperature known as crystalline allotrope and amorphous allotrope. The crystalline allotrope is grey in colour with a metallic appearance whereas the amorphous allotrope is a brown coloured powder.
Germanium (Ge): It exists in its two allotropic forms known as $\alpha -$germanium and $\beta -$germanium. Under standard conditions, germanium metal exist as $\alpha -$germanium which has a structure similar to diamond whereas if we increase the pressure to $120{\rm{ kbar}}$, then the structure formed is termed as $\beta -$germanium.
Tin (Sn): There are two allotropes of tin which are $\alpha -$tin and $\beta -$tin. Generally, tin exists in its $\beta -$tin allotropic form, but at temperature below ${13.25^o}C$ under standard conditions, it is converted to its $\alpha -$tin allotropic form.
Lead (Pb): It is the only element in the carbon family i.e., group $14$ which exist in its pure state and does not show any allotropy because its orbitals are too diffused that it cannot form strong $Pb - Pb$ single bonds.
Hence, option (A) is the correct answer.

Note:
The allotropic forms of elements are widely used in the industry like diamond is used in rock drilling machines, graphite is used for making of lubricants, fullerene is used as a catalyst in water purification, allotropes of silicon are used for manufacturing of solid-state electronic devices like transistors, etc.