Types of Solids , Unit Cells ?

Types of Solids

(a) Crystalline solids: In a single crystal the regularity of arrangement of the pattern extends throughout the solid and all points are completely equivalent.

(b) Amorphous solids: An amorphous solid differs from a crystalline substance in being without any shape of its own and has a completely random particle arrangements, i.e. no regular arrangement. Example: Glass, Plastic

Crystalline solid	Amorphous solid
1. The constituent particles are arranged in a regular fashion containing short range as well as long range order.	1. The constituent particles are not arranged in any regular fashion. There may be at the most some short range order only.
2. They have sharp melting point	2. They melt over a range of temperature.
3. They are anisotropic i.e., properties like electrical conductivity, thermal expansion, etc have different values is different direction.	3. They are isotropic i.e., properties like electrical conductivity, thermal expansion, etc have same value in different directions
4. They undergo a cleavage.	4. They undergo an irregular cut

What is a crystal ?

A crystalline solid consist of a large number of small units, called crystals, each of which possesses a definite geometric shape bounded by plane faces.

The crystals of a given substance produced under a definite set of conditions are always of the same shape.

Unit Cells :

In this topic we would be studying certain properties of a solid which depend only on the constituents of the solid and the pattern of arrangement of these constituents.

To study these properties, it would be convenient to take up as small amount of the solid as possible because this would ensure that we deal with only the minimum number of atoms or ions.
This smallest amount of the solid whose properties resemble the properties of the entire solid irrespective of the amount taken is called a unit cell.

It is the smallest repeating unit of the solid. Any amount of the solid can be constructed by simply putting as many unit cells as required.

External Features of Crystals :

Crystals are bounded by surfaces which are usually planar and referred as faces. Edge is formed by the intersection of our adjacent faces. The angle between any two faces is called an interfacial angle.

Although the size of the faces or even shapes of the crystals of one and the same substance may vary widely with conditions of formation etc., but the interfacial angles between any two corresponding faces of the crystal remain invariably the same throughout.

Crystal Systems:

From external appearance we come across a variety of crystal forms in many shapes. But on the basis of length of axes and the axial angles, it has been possible to classify the various crystal forms into only seven fundamental systems. These are:

1. Cubic

2. Orthorhombic

3. Tetragonal

4. Monoclinic

5. Triclinic

6. Hexagonal

7. Rhombohedral

The crystals belonging to any one system may differ in shape, in size, etc., but their axial ratios as also the axial angles will always be the same. The intercepts on the three axes are denoted by a, b and c and the axial angles by α , β and γ.

Seven crystal systems : The seven crystal systems are given below

Crystal System	Bravais Lattices	Parameters of Unit Cell
		Intercepts	Crystal angle	Example
1. Cubic	Primitive, Face Centered, Body Centered = 3	a = b = c	α = β = γ = 90^o	Pb,Hg,Ag,Au Diamond, NaCl, ZnS
2. Orthorhombic	Primitive, Face Centered, Body Centered, End Centered = 4	a ≠ b ≠ c	α = β = γ = 90^o	KNO₂, K₂SO₄
3. Tetragonal	Primitive, Body Centered =2	a = b ≠ c	α = β = γ = 90^o	TiO₂,SnO₂
4. Monoclinic	Primitive, End Centered = 2	a ≠ b ≠ c	α = γ = 90^o, b ≠ 90^o	CaSO₄,2H₂O
5. Triclinic	Primitive = 1	a ≠ b ≠ c	α≠ β ≠ γ ≠ 90⁰	K₂Cr₂O₇, CaSO₄5H₂O
6. Hexagonal	Primitive = 1	a = b ≠ c	α = β = 90⁰, γ = 120^o	Mg, SiO₂, Zn, Cd
7. Rhombohedral	Primitive = 1	a = b = c	α = γ = 90^o, β ≠ 90^o	As, Sb, Bi, CaCO₃
Total = 14