CBSE Class 12-science Answered

A regular arrangement of the constituent particles i.e. atoms, ions or molecules of a crystal in a three dimensional space is called crystal lattice or space lattice.

There are two types of crystal lattice:

Two dimensional lattices:

1. A two dimension lattice is a regular arrangement of points in a plane of the paper.
2. We choose any four points and connect them to obtain the unit cell.
3. Here, the unit cell may have the interior point. Such unit cells are called as centered unit cells.
4. The unit cell in which the interior point is absent is called primitive unit cell.
5. A complete lattice can be generated by repeatedly moving a unit cell in the direction of its edges by a distance equal to its edge.

The smallest three dimensional portion of a complete space lattice which when repeated over and again in different directions produces the complete space lattice is called as unit cell.

• We can give an example of a thick wall made by bricks where brick is similar to unit cell and wall is the complete space lattice.
• The unit cell is generally considers as a ‘fundamental building block’ of the crystal lattice because the lattice can be constructed by unit cells.