Primitive unit cell

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chemistry 12th class cbse notes

Primitive unit cell:

In a Primitive unit cell, atoms are present at the corner only. An atom is shared by 8 unit cell. Hence the only 1/8th of the atom belonging to the particular unit cell. There are total 8 corners in each unit cell, therefore,1/8 part of 8 atoms would give the value of the number of atoms per unit cell.

In brief 8 corners, 1 atom at each corner, 1/8 of each atom in unit cell

In a simple cubic unit cell, each corner atom is shared between 8 unit cells
In a simple cubic unit cell, each corner atom is shared between 8 unit cells
A primitive cubic unit cell (a) open structure (b) space-filling structure (c) actual portions of atoms belonging to one unit cell.
A primitive cubic unit cell (a) open structure (b) space-filling structure (c) actual portions of atoms belonging to one unit cell.

Centred unit cell (body- centred):

Body- centred unit cell:

The number of atoms present at the different position is as follows:

  • 8 corners, 1 atom at each corner (1/8 of each atom in the unit cell)
  • 1 atom at the centre of the body (1 atom completely present in the unit cell)
Body- centred unit cell
Body- centred unit cell
A body-centred cubic unit cell (a) open structure (b) space filling structure  (c) actual portions of atoms belonging to one unit cell.
A body-centred cubic unit cell (a) open structure (b) space filling structure  (c) actual portions of atoms belonging to one unit cell.

The total number of atom present in each unit cell:

= Contribution of atoms at corner + contribution of atom at the centre of the body

Face-centred unit cell:

The number of atoms present at the different position is as follows:

  • 8 corners, 1 atom at each corner (1/8 of each atom in unit cell)
  • 6 faces, 1 atom at each face of the unit cell (1/6; of each atom in the unit cell)
Face-centred unit cell
Face-centred unit cell
A face-centred cubic unit cell (a) open structure (b) space filling structure  (c) actual portions of atoms belonging to one unit cell.
A face-centred cubic unit cell (a) open structure (b) space filling structure  (c) actual portions of atoms belonging to one unit cell.

The total number of atom present in each unit cell:

= Contribution of atoms at corner + contribution of atom at the face of the body

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