Chemistry Class 12th

UNIT I: Solid StateClassification of solids based on different binding forces14 Topics

Molecular

ionic covalent and metallic solids

amorphous and crystalline solids (elementary idea)

unit cell in two dimensional and three dimensional lattices

calculation of density of unit cell

packing in solids

packing efficiency

voids

number of atoms per unit cell in a cubic unit cell

point defects

electrical and magnetic properties

Band theory of metals

conductors

semiconductors and insulators

Molecular
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packing efficiency
A crystal lattice is made up of a very large number of unit cells where every lattice point is occupied by one constituent particle. The unit cell can be seen as a three dimension structure containing one or more atoms. We always observe some void spaces in the unit cell irrespective of the type of packing. Percentage of spaces filled by the particles in the unit cell is known as the packing fraction of the unit cell. In this section we shall learn about packing efficiency. Packing fraction of different types of packing in unit cells is calculated below:
Packing efficiency in Hexagonal close packing and Cubic close packing structure:
Hexagonal close packing (hcp) and cubic close packing (ccp) have same packing efficiency. Let us take a unit cell of edge length “a”. Length of face diagonal, b can be calculated with the help of Pythagoras theorem,
From the figure, radius of the sphere, r = 4 × length of face diagonal, b
In ccp structures, each unit cell has four atoms,
Packing efficiency in body centered cubic structure:
In body centered cubic unit cell, one atom is located at body center apart from corners of the cube. Let us take a unit cell of edge length “a”. Length of body diagonal, c can be calculated with help of Pythagoras theorem,
Packing efficiency in simple cubic structure:
In simple cubic unit cell, atoms are located at the corners of cube. Let us take a unit cell of edge length “a”. Radius of atom can be given as,