PHYSICAL CHEMISTRY - Solid State

Solid is the state of matter in which the molecules atoms or ions are closely packed. They are hold together by strong cohesive forces and cannot move at random. These substances are characterised by definite shape, definite volume, and definite mass, non-compressibility, rigidity, very slow diffusion, and mechanical strength. Example of solids are ; Sodium chloride , KCL , Lime stone , Diamond etc.

Solids Are Broadly classified into two type :-

1. Crystalline solids
2. Amorphous solids

<b>1. Crystalline solids :-</b> A solid is said to be a crystalline state , if the various units ( Ions, Atoms or Molecules) are arranged in definite geometrical pattern with in the solid. Examples are ; Sodium chloride , Sugar etc.
They are further classified as :-

* Ionic Solid :- The solid in which positively and negatively charged ions are arranged in a regular manner through out the crystal are known as Ionic Solid.
Ex. - NACL , KCL etc.

* Molecular Solid :- The solid in which the molecules are held together by weak van der waal's forces are called Molecular Solids.
Ex. - Solid carbon dioxide, Ice , Iodine etc.

* Covalent Solids :- The solids in which the constituents are held together by covalent bond are called covalent solids.
Ex. - Diamond, silicon, carbide, silica etc.

* Metallic Solids :- The solid in which the constituent particles are kernels immersed in a sea of mobile electrons. The forces which hold the constituent are metallic bond.
Ex. - Iron , Silver , Gold etc.

* Atomic solids :- The solids in which the constituent are held together by London  dispersion forces are called atomic solids.
Ex. - Noble Gases

<b>2. Amorphous Solids :-</b> A solid in which the constituent structure units (ions, atoms, or molecules) of which the solid made are not arranged in any regular fashion, although it possesses certain rigidity, a definite shape and definite volume is called Amorphous Solid.
Ex. - Glass , Rubber , Polymers , Plastic pitch etc.

~ ISOTROPY and ANISOTROPY :-  In amorphous solids the properties like conductivity refractive index thermal expansion etc are identical in all direction , this property is known as istropy. Liquid and gases also follow this property. These substances are called isotropic substances. The property as mentioned above for crystalline substance have different values in different directions. This type of behaviour is called Anistropy. And the substances which exhibit these properties are called Anisotropic substances.
Thus amorphous substancrs are isotropic and crystalline substances are Anisotropic in nature.