# Crystal Lattices and Unit Cells, End Centred, Formation of Lattice Part 1 (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Crystal Lattices and Unit Cells

• The ‘crystal lattice’ is the pattern formed by the points and used to represent the positions of these repeating structural elements.
• The crystal lattice is the array of points at the corners of all the unit cells in the crystal structure.

## Crystal Lattice

The crystal lattice is the symmetrical 3-D structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.

### Characteristics of Crystal Lattice

• In a crystal lattice, each atom, molecule or ions (constituent particle) is represented by a single point.
• These points are called lattice site or lattice point.
• Lattice sites or points are together joined by a straight line in a crystal lattice.
• When we connect these straight lines, we can get a three-dimensional view of the structure. This 3D arrangement is called Crystal Lattice also known as Bravais Lattices.

## Unit Cell

Unit Cell is the smallest part (portion) of a crystal lattice. It is the simplest repeating unit in a crystal structure. The entire lattice is generated by the repetition of the unit cell in different directions.

### Characteristic of Unit Cell

The following characteristics define a unit cell:

• A unit cell has three edges a, b and c and three angles , and between the respective edges
• The a, b and c may or may not be mutually perpendicular
• The angle between edge b and c is , a and c are β and that of between a and b is

Unit cells are of two types namely:

• Primitive Unit Cells
• Centred Unit Cells

### Parameters of a Unit Cell

Unit Cell Parameters

There are six parameters of a unit cell. These are the 3 edges which are a, b, c and the angles between the edges which are α, β, γ. The edges of a unit cell may be or may not be perpendicular to each other.

### Types of Unit Cell

Types of Unit Cell

#### Primitive Unit Cells

When the constituent particles occupy only the corner positions, it is known as Primitive Unit Cells.

#### Centred Unit Cells

When the constituent particles occupy other positions in addition to those at corners, it is known as Centred Unit Cell. There are 3 types of Centered Unit Cells:

• Body Centred: When the constituent particle at the centre of the body, it is known as Body Centred Unit cell.
• Face Centred: When the constituent particle present at the centre of each face, it is known as Face Centered Unit cell.
• End Centred: when the Constituent Particle Present at the Centre of Two Opposite Faces, It is Known as an End Centered Unit Cell

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