Crystalline and Amorphous Solids

 Crystalline and Amorphous Solids

Introduction

Think of a diamond and some cotton candy. Do you think these substances are alike? They are different in every aspect, and yet they are both solids. What differentiates these is the structure and arrangement of their particles. Let us learn about the two types of solids, crystalline Solids and Amorphous Solids.

Crystalline Solids

These are the most common type of solids. Their characteristics are what we associate solids with. They are firm, hold a definite and fixed shape, are rigid and incompressible. They generally have geometric shapes and flat faces. And examples include diamonds, metals, salts etc.

Crystalline and Amorphous Solids


To understand crystals we must understand their structure. The arrangement of particles in a crystalline solid is in a very orderly fashion. These particles are arranged in a repeating pattern of a three-dimensional network. This network is known as a Crystal lattice and the smallest unit of a crystal is a Unit Cell. If you see the X-ray of a crystal this distinct arrangement of the unit cells will be clearly visible.

Crystal Structures and Crystal Chemistry

Solid state chemistry is concerned mainly with crystalline inorganic materials.

Crystal structure information includes.

  1. Unit cells
  2.  Dimensions of the Unit cells
  3.   Positions or atomic coordinates of atoms inside unit cell Crystal chemistry combines this basic structural information
  4.  Elements
  5.   Principal oxidation states
  6.   Ionic radii
  7.   Coordination requirements 
  8.  Preferences for ionic/covalent/metallic bonding.

 Unit Cells and Crystal Systems

The smallest repeating unit which shows the full symmetry of the crystal structure.

The seven crystal systems listed in the seven independent unit cell shapes that are possible in three dimensional(3D) crystal structures. Six of these unit cell shapes are closely inter-related and are either cubic or can be derived by distortingacubeinvariousways,asshowninFig.1.4. Thus, if one axis, c, is of different length to the others, the shape is tetragonal; if all three axes are different, the shape is orthorhombic. If, now, one of the angles,β,isnot90◦,the shape is monoclinic, where as if all three angles differ from 90◦,the shape is triclinic.

Finally, if the cube is stretched, or compressed, along a body diagonal so that all three angles remain equal, but different from 90◦, the shape is trigonal.

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