Solid State

Question 1

Packing Fraction

Answer 1

Simple Cubic cell: 52.8%
Body centered: 68%
Face centered: 74%

Question 2

Tetrahedral Void

Answer 2

2 on each body diagonal

Question 3

Octahedral void

Answer 3

At body center and at edge centres

Question 4

Close cubic packing (ccp) (same as fcc)

Answer 4

ABCABC… type arrangement
Coodination No: 12
Z=4
Packing efficiency:74%

Question 5

Hexagonal close packing

Answer 5

ABAB… type arrangement
Coodination No: 12
Z=6
packing efficency:74%

Question 6

Rock salt structure

Answer 6

Cl- —Fcc/ccp
Na+ —-Octahedral voids
No of formula units in one unit cell: 4

6:6 CN compound
Halides of Na,Li,K,Rb and NH4Cl

Question 7

Radius Ratio (cation:anion)

Answer 7

R.R.<0.155 -Linear void CN=2
0.155<=R.R<0.225 -Triangular void CN=3
0.225<=R.R.<0.414 - Tetrahedral void CN=4
0.414<=R.R<0.732 -Octahedral void CN=6
0.732<=R.R<1 -Body centre(Cubic void) CN=8
R.R=1 -Hcp CN=12

Question 8

CsCl Structure

Answer 8

Cs+ -Body center (Cubic void)
Cl- -Lattice sites (corner points)
8:8 CN compound
1 formula unit per unit cell
CsCl,CsBr, CsI

Question 9

ZnS structure

Answer 9

Zn2+ -Half of tetrahedral voids
S2- -Fcc
4 formula units per unit cell
4:4 CN compound
same structure as diamond
r+ +r-=0.25 * body diagonal

CuCl, BeS, CdS, HgS, CaO

Question 10

Fluonite Structure (CaF2)

Answer 10

r+>r-
Ca2+-fcc sites
F- -Tetrahedral voids
4 formula units per unit cell
CN: Ca-8 F-4
BaCl2,BaF2, SrCl2, SrF2, CaCl2

In anti-fluonite structure (Na2O), the cations and anions exchange their places.

Question 11

Defects in Crystals

Answer 11

This happens when crystallisation process occurs at fast or moderate rate.
Point defects are the irregularities or deviations from ideal arrangement around a point or an atom in a crystalline substance, whereas the line defects are the irregularities or deviations from ideal arrangement in entire rows of lattice points.

Question 12

Stoichiometric Defects

Answer 12

For non-ionic solids.

Vacancy Defect: When some of the lattice sites are vacant. This results in decrease in density of the substance. This defect can also develop when a substance is heated.

Interstitial Defect: When some constituent particles occupy an interstitial site. This defect increases the density of the substance. Same particles move from the lattice sites to the interstitial sites.

Question 12

Defects in ionic solids

Answer 12

Ratio between cations and anions remain same. For ionic solids

Frenkel defect: The smaller ion (usually cation) is dislocated from its normal site to an interstitial site. It creates a vacancy defect at its original site and an interstitial defect at its new location. It does not change the density. Found in solids where the difference in the size of the cation and anion is large. For example, ZnS, AgCl, AgBr and AgI.

Schottky Defect: Equal number of cations and anions go missing fromm the crystal lattice. Electrical neutrality is maintained and density decreases. Found in solids where the difference in the size of the cation and anion is small. For example, NaCl, KCl, CsCl and AgBr.

Question 13

Non-Stoichiometric Effects

Answer 13

Metal excess defect:
* A negative ion moves out of the lattice but leaves behind an electron to maintain neutrality. These electrons are trapped in F-centers and impart color to the solid.Eg: ZnO
* Due to the presence of extra cations in the interstitial sites. Electrical neutrality is maintained due to an electron is another interstitial site.
Metal deficiency defect:
Occurs in solids with transition metals with variable valence. This occurs due to a missing cation in a lattice site and presence of another cation having higher charge at a different lattice site. Eg: FeO, FeS, NiO, etc.

Question 14

Impurity defects

Answer 14

Due to the presence of impurity atoms in lattices sites (substitutional) or interstitial sites. Eg Na+ in NaCl is substituted by Sr from SrCl2; similar example is the solid solution of CdCl2 and AgCl.

Question 15

Ferromagnetic Substances

Answer 15

iron, cobalt, nickel,
gadolinium and CrO2

Question 15

Anti-ferromagnetic substances

Answer 15

Substances like MnO showing antiferromagnetism have domain structure similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each other’s magnetic moment

Question 16

Ferrimagnetic Substances

Answer 16

Ferrimagnetism is observed when the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers. They are weakly attracted by magnetic field as compared to ferromagnetic substances. Eg Fe3O4 (magnetite) and ferrites like MgFe2O4 and ZnFe2O4.

Question 17

In case of Schottky defect effective number of atoms in unit cell

Answer 17

=Z (1- % of schottky defect)