UNIT 1 SOLID STATE- (TYPE OF VOIDS) XII CHEMISTRY PART V KEY NOTES CBSE

 

• When spheres (atoms/ions) are packed, empty spaces are left — called voids.

A. Tetrahedral Voids

• Formed when a sphere of one layer rests in the depression of three spheres of another layer.

• Number of tetrahedral voids: 2 × number of atoms.

• Radius ratio:

​rsmall/rlarge=0.225

B. Octahedral Voids

• Formed at the centre of six spheres — 3 in one layer and 3 in the adjacent one.

• Number of octahedral voids: Equal to number of atoms.

• Radius ratio:

• rsmall/rlarge=0.414

C. Relationship Between Voids and Atoms

​

Type of Void	No. of Voids per Atom	Radius Ratio	Coordination No.

Tetrahedral

2

0.225

4

Octahedral

1

0.414

6

4. Examples

Structure Type	Example Compounds
SC	Polonium (Po)
BCC	α-Iron, Na, K
FCC	Cu, Al, Ag, Au
HCP	Zn, Mg, Ti

​5. Important Points

• Close packing = maximum efficiency = 74% (in FCC & HCP).

• Sequence of layers:

  • HCP: ABAB...

  • CCP: ABCABC...

• Tetrahedral voids are smaller than octahedral voids.

• Ionic crystals: smaller ions occupy these voids depending on their radius ratio.