Question

In spite of long-range order in the arrangement of particles why are the crystals usually not perfect?

Answer 1

To solve this question, we first need to know what crystals are. A solid substance in which its constituents like molecules, atoms, or ions from a crystal lattice, because they are arranged in a microscopic structure extending in all directions and are highly ordered, is known as a crystal or a crystalline solid.

Complete answer:
A crystalline solid has a long-range order of arrangement. It means that the arrangement of particles in a crystal lattice of the solid occurs in a regular pattern and repeats itself over the entire crystal periodically.
Now, the existence of an ideal crystal or a crystal without any defects is not possible (just like an ideal gas). Due to various factors, defects can occur in the crystal lattice of a crystalline solid and these defects are known as crystal defects.
The defects in the crystal lattice of a crystalline solid influence the behavior of the substance. The optical, electrical, and mechanical properties of the solid change.
Factors that result in defects in a crystalline solid are
- high energy radiations like X-rays or neutrons striking the solid
- quick cooling from high temperature
- deformation of solid
Hence despite long-range order in the arrangement of particles, crystals usually are not perfect.

Note:
It should be noted that three types of defects can occur in a crystalline solid.
The defect which involves the displacement of a single ion from its lattice point to a nearby space in the lattice is known as the Frenkel defect.
The defect which involves the removal of two ions that are oppositely charged from the crystal lattice is known as the Schottky defect.
And finally, when the atoms of the crystal are replaced by foreign atoms, or when an impurity is added in the spaces in the crystal lattice, it is known as the impurity defect.