`=>` From our earlier studies we came to know that liquids and gases are called fluids as they have tendency to flow(molecules are free to move).
`=>` The constituent particles in solids have fixed positions and therefore can only oscillate about their mean positions. That's why solids are rigid.

`=>` `color{green}("The stability of solid state depends on two factors :")`

(i) Intermolecular attractive forces.

(ii) Thermal energy which tends to keep the molecules apart.

At low temperature, the thermal energy is low and intermolecular attractive forces are strong and that's why they cling to one another and occupy fixed positions.

`=>` `color{green}("Solids have following characteristic properties :")`

(i) They have definite mass volume and shape.

(ii) Intermolecular distances are short.

(iii) Intermolecular forces are strong.

(iv) Their constituent particles (atoms, molecules or ions) have fixed positions and oscillate about their mean position.

(v) They are incompressible and rigid.

This classification is based on the nature of order present in the arrangement of their constituent particles.

(A) `color{green}("Crystalline Solids :")`

(i) Consists of a large number of small crystals.

(ii) These crystals have definite geometrical shape.

(iii) The arrangement of constituent particles is ordered.

(iv) It has long range order which means arrangement of particles repeats itself over the entire crystal.

(v) Examples are sodium chloride (`color{red}(NaCl)`) and quartz.


(B) `color{green}("Amorphous Solids :")` (Greek amorphos=no form)

(i) It consists of particles of irregular shape.

(ii) It has short range order which means regular and periodically repeating pattern is over short distances only (such patterns are scattered and in between the arrangement is disordered.

(iii) Example are quartz glass, rubber and plastics.

See Table 1.1

`=>` Pseudo Solids or Super Cooled Liquids : Since amorphous solids like liquids have tendency to flow that's why they are called pseudo solids or super cooled liquids.
`color{purple}(✓✓)color{purple} " DEFINITION ALERT"`
The solid which loses shapes on long standing, flows under its own weight and easily ditsorted by even mild distortion forces are called pseudo solids.


`=>` `color{green}("Anisotropy :")` It is the property shown by crystalline solids in which their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals. This is due to the different arrangement of particles in different directions.

`=>` `color{green}("Isotropy :")` This is the characteristic property of amorphous solids which show same physical property along any direction.

`color{red}("Note :")` Amorphous silicon is used for conversion of sunlight into electricity.