Crystalline and Amorphous Solids

How are Solids Classified?

A solid interface is defined as a small number of atomic layers that separate two solids in intimate contact with one another, where the properties differ significantly from those of the bulk material it separates.

Based on their crystal structures, solids can be classified into the following categories:

  1. Crystalline solids
  2. Amorphous solids

However, crystalline solids can be further classified into molecular, ionic, metallic, and covalent solids. A brief introduction to the classification of solids is provided in this article.

Classification of Solids – Crystalline and Amorphous Solids

An illustration detailing the classification of solids is provided below.

Classification of Solids

Classification of Solids – Crystalline and Amorphous Solids

What are Crystalline Solids?

The solids featuring highly ordered arrangements of their particles (atoms, ions, and molecules) in microscopic structures are called crystalline solids.

These ordered microscopic structures make up a crystal lattice that accounts for the structure of the solid at any given point. Examples of crystalline solids include salt (sodium chloride), diamond, and sodium nitrate.

What are Amorphous Solids?

The solids in which the particles are not arranged in any specific order or the solids that lack the overall order of a crystal lattice are called amorphous solids.

The term ‘amorphous’, when broken down into its Greek roots, can be roughly translated to “without form”. Many polymers are amorphous solids. Other examples of such solids include glass, gels, and nanostructured materials.

An ideal crystal is defined as an atomic arrangement that has infinite translational symmetry in all the three dimensions, whereas such a definite definition is not possible for an ideal amorphous solid (a-solid).

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Key Features of Crystalline and Amorphous Solids

  • Nature:
    Crystalline Solids – True Solids
    Amorphous Solids – Pseudo – Solids or super-cooled liquids
  • Geometry:
    Crystalline Solids – Particles are arranged in a repeating pattern. They have a regular and ordered arrangement resulting in a definite shape.
    Amorphous Solids – Particles are arranged randomly. They do not have an ordered arrangement resulting in irregular shapes
  • Melting Points
    Crystalline Solids – They have a sharp melting point
    Amorphous Solids – They do not have sharp melting points. The solid tends to soften gradually over a temperature range
  • Heat of Fusion: (The change in enthalpy when a substance is heated to change its state from solid to liquid.)
    Crystalline Solids – They have definite heat of fusion.
    Amorphous Solids – They do not have definite heat of fusion
  • Isotropism:
    Crystalline Solids – Anisotropic in nature. i.e., the magnitude of physical properties (such as refractive index, electrical conductivity, thermal conductivity etc) is different along with different directions of the crystal.
    Amorphous Solids – Isotropic in nature. i.e., the magnitude of the physical properties is the same along with all directions of the solid.
  • Cleavage Property
    Crystalline Solids – When cutting with a sharp edge, the two new halves will have smooth surfaces
    Amorphous Solids – When cutting with a sharp edge, the two resulting halves will have irregular surfaces
  • Rigidity:
    Crystalline Solids – They are rigid solids and applying mild forces will not distort its shape.
    Amorphous Solids – They are not rigid, so mild effects may change the shape.

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