Basic of Crystal structure
Crystallography is the experimental science of determining the arrangement of atoms in the crystalline solids. Many properties of materials can be found easily by studying its crystal structure.
Firstly, we will see what crystal structure is. Thus we will define Crystal structure as
A crystal is a solid whose constituent atoms, ions or molecules are closely packed in a regular fashion with repeating pattern in three dimensions.
So basis on the atomic arrangement the solids can be classified as
- Crystalline solids
- Amorphous solids
Initially, in crystalline solid, all the constituent atoms are arranged in a regular pattern in three dimensions. Thus, Crystalline solids need high temperatures to break the inter-molecular forces. As a result they have a definite heat of fusion and melting points. This is because of the uniform arrangement of their constituent particles. The local environment is also uniform.
However, the physical properties of crystals are different in different directions when cut the crystal in any direction. Thus, this property is called anisotropic.
When the crystal is rotated around the axis. However, its crystal structure remains the same. So this is termed as symmetrical arrangement of atoms, ions or molecules.
However, the crystalline solids have different values of refractive index, thermal conductivity, mechanical strength and electrical conductivity in different directions.
As the word amorphous means shapeless in Greek. Thus in a non-crystalline or amorphous solid the atomic arrangement is random or non-periodic in nature.
As this has no particular shape. That implies the constituent particles of a solid are arranged in irregular arrangement. As the spacing between atoms, ions or molecules is not the same. Thus inter-molecular forces are also not the same
Because of their irregular geometric shapes when cleaved amorphous solids result in curved surfaces. In case of amorphous solids the values of refractive index, thermal conductivity, mechanical strength and electrical conductivity are the same in all directions. So amorphous solids are isotropic in nature.
In case of amorphous solids we don’t observe sharp melting points or a definite heat of fusion. Because an ordered array of components are absent a wide range of temperatures needs to be applied so that it can melt.
For examples : Polymers, plastics, rubbers and glass.
Difference between Crystalline solids and amorphous solids
Furthermore, Let us see the points of differences between Crystalline and Amorphous solids.
|Crystalline solids||Amorphous solids|
|1. They have a definite characteristic geometrical shape.||1. They have irregular shapes.|
|2. In addition there exists a long range order of regular pattern of arrangement of constituent particles.||2. In addition there exists a short range order of regular pattern of arrangement of constituent particles.|
|3. They are true solids.||3. They are pseudo solids or super cooled|
|4. They also have sharp melting points.||4. However, their melting points are not sharp.|
|5. They are anisotropic in nature.||5. However, they are isotropic in nature.|
|6. They have definite heat of fusion.||6. These crystals do not show definite heat of fusion.|
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