CLASS XII CHEMISTRY

Smallest repeating unit in a space lattice is called unit cell.
Thereare4 typesofunitcells,7crystalsystemsand14bravaislattices.
. Types of unit cell No.ofatomsperunitcell
 Simple cubicunitcell 8x1/8=1
 FCC(Facecenteredcubic) 8x1/8+6x1/2=4
BCC(Body centeredcubic) 8x1/8+1x1=2
 End centered 8x1/8+1/2 x 2 = 2 10.
Hexagonalclosepackingand cubicclosepackinghaveequal packing efficiencyi.e.74% 11. Packingefficiency= Volume occupied by the sphere*100/Volume of unit cell

Radius ratiois the ratio of radius of void to the radius of sphere .
For tetrahedral void radius ratio = 0.225
Foroctahedralvoidradiusratio = 0.414
.No.oftetrahedralvoid=2x N (N=No.ofparticles)
.No.ofoctahedral void=N

Simplecubicunitcell a=2r
. FCC a=4r/√ 2
 BCC a=4r/√ 3

Point defect or Atomic defect‐>it is the deviation from ideal arrangement of constituent atom
.Point defects are two types
(a)Vacancy defect
(b)Interstitial defect

Frenkel defect
 is the combination of vacancy and interstitial defects.Cations leave their  actual
lattice sites and come to occupy the interstitial spaces.Density remains the same
eg.AgCl.