Question

Name the parameters that characterize a unit cell.

Answer 1

The unit cell is known as the smallest unit which has the full symmetry of the particular crystal structure. It shows the crystalline pattern in which the particular crystal structure or a solid is made up of. And when the particular unit cell is repeated and the thing formed is called a lattice structure.

Complete answer:
The parameters on which the unit cell is characterized is the following:
The dimension of the unit side along the three edges that is a, b and c. These edges are either perpendicular to each other or not.
The other parameter is the angles formed between the edges and that are $\alpha (between ‘c’ and ‘b')$,$\beta (between ‘a’ and ‘c')$, and $\gamma (between'a'and'b')$
So the unit cell is characterized by six parameters and that are a, b, c, $\alpha ,\beta ,and\,\gamma$.
The unit cells are being divided into two categories and they are:
(a)Primitive unit cell: This is the unit cell in which the constituent particles are present at the corners of the unit cell.
(b)Centred unit cell: In this unit cell the particles are present at other positions and the particles are mandatory to be present at the corners. This unit cell is subdivided into three parts
(i)Body centred unit cell: In this the particles are present at the body centre and at the corners.
(ii)Face centred unit cell In this the particles are present at each face and the corners.
(iii)End centred unit cell: In this the particles are present between the two opposite faces and at the corners.

Note:
The table below shows you the some crystal system with their axial distance and axial angles

Crystal system	Axial distance	Axial angles
Cubic	a=b=c	$\alpha =\beta =\gamma \ne {{90}^{o}}$ $\alpha =\beta =\gamma ={{90}^{o}}$
hexagonal	$a=b\ne c$	$\alpha =\beta ={{90}^{o}}\gamma ={{120}^{o}}$
tetragonal	$a=b\ne c$	$\alpha =\beta =\gamma ={{90}^{o}}$
trigonal	a=b=c	$\alpha =\beta =\gamma \ne {{90}^{o}}$

In monoclinic the axial lengths are not equal to each other and alpha and beta angles are equal to right angles. In triclinic the edges are unequal to each other and all the angles are unequal to the right angle.