Chapter 12, Problem 12.36QP

Interpretation Introduction

Interpretation:

The type of unit cell of Iodine lattice has to be identified from the given picture.

Concept Introduction:

The simplest and basic unit of a crystalline solid is known as unit cell.  It is cubic in shape.  It is the building block of crystalline solids.  The unit cells repeat themselves to build a lattice.  Crystalline solids consist of many of such lattices.  There are three types of unit cell – simple cubic unit cell, body – centered cubic unit cell and face – centered cubic unit cell.

A simple cubic unit cell is the simplest form of a cubic unit cell.  A cube has eight vertices, twelve edges and six faces.  Similarly a cubic unit cell has eight vertices, twelve edges and six faces.  If in a cubic unit cell, the components occupy only the eight vertices, then the unit cell is known as simple cubic unit cell.  So, each simple cubic unit cell has $\frac{\text{1}}{\text{8}}\text{th}$ of an atom at each vertex.  Thus the number of atoms per simple cubic unit cell is –

$\text{8}\text{vertices}\text{×}\frac{\text{1}}{\text{8}}\text{th}\text{of an}\text{atom}\text{=}\text{1}\text{atom}$

In a body – centered cubic unit cell is another type of unit cell in which atoms are arranged in all the eight vertices of the unit cell with one atom per vertex.  Further one atom occupies the center of the cube.  Thus the number of atoms per unit cell in BCC unit cell is,

$\text{8}\text{×}\frac{\text{1}}{\text{8}}\text{atoms}\text{in}\text{corners}\text{+}\text{1}\text{atom}\text{at}\text{the}\text{center}\text{=}\text{1}\text{+}\text{1}\text{=}\text{2}\text{atoms}$

In a face – centered cubic unit cell the atoms are arranged in all the eight vertices of the unit cell with one atom per vertex.  Further all the six faces of a cubic unit cell are occupied with one atom per face.  Thus the number of atoms per unit cell in FCC unit cell is,

$\text{8}\text{×}\frac{\text{1}}{\text{8}}\text{atoms}\text{in}\text{corners}\text{+}\text{6}\text{×}\frac{\text{1}}{\text{2}}\text{atoms}\text{in}\text{faces}\text{=}\text{1}\text{+}\text{3}\text{=}\text{4}\text{atoms}$