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7/8/2013

Navrachana School, Sama. Zaid Mansuri 1

CHEMISTRY

11Navrachana School, Sama. Navrachana School, Sama.

....Zaid Mansuri....Zaid Mansuri

The correlation between structure and properties helps indiscovering new solid materials with desired propertieslike ……

high temperaturehigh temperature

superconductors,

magnetic materials,

biodegradable polymers for packaging,biodegradable polymers for packaging,

bio-compliant solids for surgical implants, etc.



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1 . 1 General Characteristics of Solid State1 . 1 General Characteristics of Solid State

(i) They have definite mass, volume and shape.

(ii) Intermolecular distances are short.

(iii) Intermolecular forces are strong.

(iv) Their constituent particles (atoms, molecules or ions)have fixed positions and can only oscillate about theirmean positions.

(v) They are incompressible and rigid.



1.2 Amorphous and Crystalline Solids1.2 Amorphous and Crystalline Solids

S o l i d s

Crystalline Amorphous(Greek amorphous = no form)

-long range order

eg : Sodium chloride and quartz are typical examples

-sharp MP

-short range order

eg : Glass, rubber and plasticsare typical examples

-range of MP

The structure of amorphous solids issimilar to that of liquids.similar to that of liquids.

On heating Amorphous crystalline at some temperature.

Eg : Some glass objects from ancient civilizations are found to become milkyin appearance because of some crystallization.



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Amorphous solids are also called as pseudo solids or super cooled liquids.b.coz they have tendency to flow like liquids.

Window panes are thicker at bottom !

Crystalline solids are Anisotropic

i.e. some of their physical properties like

electrical resistance orrefractive index

show different values when measured alongshow different values when measured alongdifferent directions in the same crystals.( b’coz of different arrangement of particlesin different directions)

Amorphous solids are Isotropic





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Suming up



1.3 Classification of Crystalline Solids1.3 Classification of Crystalline Solids

On the basis of nature of intermolecular forces

operating between them, solids are classified into

4 categories …..

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1.3 Classification of Crystalline Solids1.3 Classification of Crystalline Solids



Metallic bond



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1.4 Crystal Lattices and Unit Cells1.4 Crystal Lattices and Unit Cells

Crystal lattice : A regular three dimensional arrangementof points in space is called a crystallattice.

characteristic

lattice.

There are only 14 possible three dimensional lattices. These are called BravaisLattices

Characteristics of a crystal lattice:

(a) Each point in a lattice is called lattice point or lattice site.(b) Each point in a crystal lattice represents one constituent particle which may be

an atom, a molecule (group of atoms) or an ion.(c) Lattice points are joined by straight lines to bring out the geometry of the lattice.



Unit cell : is the smallest portion of a crystal lattice which, when repeated in different directions, generates the entire lattice.

Characteristics of a unit cell :

(i) a, b and c - sides - may or may not be mutually perpendicular.mutually perpendicular.

(ii) a, ß & - angles

Thus, a unit cell is characterized by six parameters, a, b, c, a, ß and .



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1.4.1 Primitive and Centered Unit Cells1.4.1 Primitive and Centered Unit Cells

Primitive Unit Cell

Unit cells

Centred Unit CellsPrimitive Unit Cell

particles are present only on the corner positions.

Centred Unit Cells

one or more constituent particles at positions other than corners in addition to those at corners, it is calleda centered unit cell.

(i) Body-Centred Unit Cells (bcc)

(ii) Face-Centred Unit Cells (fcc)(ii) Face-Centred Unit Cells (fcc)

(iii) End-Centred Unit Cells





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1.5 Number of Atoms in a Unit Cell1.5 Number of Atoms in a Unit Cell

1.5.1 Primitive Cubic Unit Cell

Shows contribution of each sphere

Corners 8 x 1/8 = 1 atomActu

al

Only centre of sphere and not

actual size

Corners 8 x 1/8 = 1 atomal size



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1.5.2 Body-CenteredCubic Unit Cell

8 corners × 1/8 = 1 atom+

1 at body centre = 1 atom

Total per unit cell = 2 atoms



1.5.3 Face-CentredCubic Unit Cell

8 corners x 1/8 = 1 atom

6 at faces x 1/2 = 3 atoms

Total per unit cell = 4 atomsTotal per unit cell = 4 atoms



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1.6 Close Packed Structures1.6 Close Packed Structures

(a) Close Packing in 1DC.N. = 2

Coordination number : is the number of nearest neighbors of a particle.

C.N. = 2

(b) Close Packing in 2D

C.N. = 4 C.N. = 6



(c) Close Packing in 3D

(i) Square close-packed in 3D

A A A A A A …



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(ii) hexagonal close packing in 3D

(a) Placing second layer (B) over the first layer (a)



Voids are formed…

For N spheres in closed packed spheres there are

N octahedral voids &

2N tetrahedral voids 2424Navrachana School, Sama. Navrachana School, Sama.


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(b) Placing third layer over the second layer

Covering Tetrahedral Voids:ABABAB…. (hcp)

Covering Octahedral Voids:ABCABCABC….(ccp or fcc)



Covering octahedral voids ABCABC…

ccp

Covering tetrahedral voids ABAB…

hcp



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1.6.1 Formula of a Compound and Number of Voids Filled

Tetrahedral voids in ccp / fcc

Total no. of tetrahedral voids = 1 x 8 = 82727

Navrachana School, Sama. Navrachana School, Sama. ....Zaid Mansuri....Zaid Mansuri

Octahedral voids in ccp / fcc

at edge centre : ¼ x 12 = 3at body centre : 1 x 1 = 1

Total no. of octahedral voids = 42828


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Summing up ….

Formula of fcc/ccp

If in fcc or ccp, atoms Aoccupy all octahedral voids and B occupy the

Lattice points , the unit cell contains Lattice points , the unit cell contains

4 Aand 4 A i.e. the formula of the crystal is AB

If in fcc or ccp A occupy all tetrahedral voids and B occupy the

Lattice points , the unit cell contains

8 Aand 4 Bi.e. the formula of the crystal is A 2B



Example 1.1 A compound is formed by two elements X and Y. Atoms of the element Y (as anions) make ccp and those of the element X (as cations) occupy all the octahedral voids. What is the formula of the compound?

Ans : XY

Example 1.2 Atoms of element B form hcp lattice and those of the element A occupy2/3rd of tetrahedral voids. What is the formula of the compound formedby the elements A and B?

Ans : A4B3



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1.7 Packing Efficiency1.7 Packing Efficiency

What is packing efficiency?

Packing efficiency is the percentage of total space filled by the particles.

So, If

total vol. of the unit cell 100%

Vol. occupied by the spheres in the unit cell ? (packing efficiency)

Packing efficiency = vol. occupied by the spheres in the unit cell x 100 %total vol. of the unit cell



1.7 Packing Efficiency1.7 Packing Efficiency

1.7.1 Packing Efficiency in hcp

and ccp Structures 74%

Packing efficiency = vol. occupied by 4 spheres in the unit cell x 100 %total vol. of the unit cell



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1.7.2 Efficiency of Packing in

bcc Structures 68%

Packing efficiency = vol. occupied by 2 spheres in the unit cell x 100 %Packing efficiency = vol. occupied by 2 spheres in the unit cell x 100 %total vol. of the unit cell



1.7.3 Packing Efficiency in Simple

simple cubic 52.4 %

Packing efficiency = vol. occupied by 1 sphere in the unit cell x 100 %total vol. of the unit cell



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1.8 Calculations Involving Unit Cell Dimensions1.8 Calculations Involving Unit Cell Dimensions

Density of the unit cell, d = mass of the unit cell volume of the unit cell (v)

= z . m , z = no. of atoms in unit cella3 a = edge length of unit cell

m = mass of an atom in unit cell

d = z . M ( b’coz, m = M / NA )a3 NA



An element has a body-centred cubic (bcc) structure with a cell edgeof 288 pm. The density of the element is 7.2 g/cm3. How many atomsare present in 208 g of the element?

Example 1.3



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1.9 Imperfections in Solids1.9 Imperfections in Solids• Crystals though have long range order ---- are not perfect

• Solids consists aggregate of large no. of small crystals.

• crystals have defects ---when crystallization takes place at moderate or fast rate

• even at extremely slow rate ---- crystals are not free from defects (imperfections)

Crystal defects : are basically irregularities in the arrangement of constituent particles.

Line defects : are the irregularities or deviation from ideal arrangement

Point defects : are the irregularities or deviation from ideal arrangement in entire row

Point defects : are the irregularities or deviation from ideal arrangement around a point or an atom



Crystal defects

Line defectsPoint defects

Stoichiometric defects

Impurity defects Non-Stiochiometicdefectsdefects

1. Vacancy defects

Metal excess defects Metal deficient defects

These are the point defectsthat do not disturb thestoichiometry of the solid.They are also called intrinsicor thermodynamic defects.

Due to anionicvacancies

2. Interstitial defects

3. Frenkel defects

4. Schottky defects

Due to extra cationsin the interstital



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1. Vacancy defects When some of the lattice sites are vacant, the crystal is said tohave vacancy defect (Fig. 1.23).- density of the substance decreases.-This defect can also develop when a substance is heated.- Shown by non-ionic solids



2. Interstitial defects When some constituent particles (atoms or molecules) occupyan interstitial site, the crystal is said to have interstitial defect.-density of the substance increases- shown by non-ionic solids



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Note : Ionic solids must always maintain electrical neutrality. Rather than simple vacancy or interstitial defects, they show these defects as Frenkel andSchottky defects.

3. Frenkel defects When the smaller ion (usually cation) is dislocated from its normalsite to an interstitial site it is called Frenkel defect.- It creates a vacancy defect at its original site and an interstitialdefect at its new location.defect at its new location.

- Frenkel defect is also called dislocation defect.- density of the solid does not change.- shown by ionic substance in which there is a large difference inthe size of ions,

- for example, ZnS, AgCl, AgBr and AgI due to

Vacancy

Interstitial



4. Schottky defects In this defect equal number of cations and anions are missing from their positions.- basically a vacancy defect.- Electrical neutrality is maintained.- density of crystal decreases- Eg : in NaCl there are approximately 106 Schottky pairs per cm3

at room temperature. In 1 cm3 there are about 1022 ions. Thus,there is one Schottky defect per 1016 ions.there is one Schottky defect per 10 ions.

- shown by ionic substances in which the cation and anion are ofalmost similar sizes.

- Eg : NaCl, KCl, CsCl and AgBr.- Note : AgBr shows both, Frenkel as well as Schottky defects.

Vacancy

Vacancy



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Impurity defects When some foreign impurities replace the ions of thecrystal, the crystal is said to have Impurity defect.- e.g : SrCl2 in NaCl. Each Sr+2 replaces two Na+ ions to maintain

the stoichiometry. One site is occupied while other remains vacant (cationic vacancy).hence equal no. of cationic vacancies are generated in this case.

- e.g : CdCl2 + AgCl solid solution

Foreign ion

Vacancy



Due to anionicvacancies

e.g : NaCl & KCl

When,NaCl ----- Na(vapours)

F-Centers

Metal excess defects

NaCl(crystal) ----- Na(vapours) F-Centers

- NaCl yellow- LiCl pink- KCl violet ( or liliac)

F-centreF-centre



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Metal excess defects

Due to extra cationsin the interstital

ZnO --- Zn+2 + ½ O2 + 2e-

White yellow

- O is lost as O2 reversibly.- excess of Zn+2 occupies the interstitial nearby.- thus, Zn1+xO- thus, Zn1+xO

Metal deficient defects

Many solids – difficult to prepare in stoichiometric composition- More anions than metals- Fe0.93O to Fe0.96O- some Fe+2 Fe+3 + e- to maintain the electrical neutrality of the crystal

0.93 0.96- some Fe+2 Fe+3 + e- to maintain the electrical neutrality of the crystal



1.10 Electrical properties1.10 Electrical properties

Solids

Conductors Semi-conductors Insulators

104 - 107 ohm-1m-1

10-20 - 107 ohm-1m-1

10-6 -104 ohm-1m-1 10-20 -10-10 ohm-1m-1104 - 107 ohm-1m-1 10-6 -104 ohm-1m-1

good

10-20 -10-10 ohm-1m-1



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Band : the atomic orbitals of metal atoms form molecular orbitals which are so close in energy to each other as to form a band.

Conductionband

Valence band

Conduction band : lowest unoccupied band

Valence band : highest occupied4747


Conductors Semi-conductors Insulators

Conductivity decreases with rise in temperature

Conductivity increases with rise in temperatureEg : Si & Ge- Also calledIntrinsic semi conductors

-

Cond. can be increasedCond. can be increasedby adding suitable Impurities ( Doping) Electronic defects

Electron rich impurities Electron deficient impurities

Si (gr.14) doped with P or As (gr.15) Si (gr.14) doped with B Al or Ga (gr.13)



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Applications of n-type and p-type semi conductors :

• diode p-type and n-type semiconductors & is used as a rectifier• transistors npn & pnp amplify radio or audio signals.• photo-diode solar cells

• Combination of different group atoms to give average of 4e-s• Combination of different group atoms to give average of 4e s

gr. 12 + gr. 16 e.g: ZnS, CdS, CdSe and HgTe

gr. 13 + gr. 15 e.g: InSb, AlPb and GaAs have very fastresponse

• In these compounds, the bonds are not perfectly covalent and the ioniccharacter depends on the electronegativities of the two elements.character depends on the electronegativities of the two elements.

• interesting ! transition metal oxides (TiO, CrO2 and ReO3) behave like metals!!ReO3 is like metallic copper in its conductivity and appearance.VO, VO2, VO3 and TiO3 show metallic or insulating properties depending on temperature.



1.11 Magnetic properties1.11 Magnetic properties

Every substance magnetic properties due to e-s behave like tiny magnets

Magnetic moment

Orbital motion Spin motionOrbital motion Spin motion

• Electron being a charged particle and undergoing these motions can be considered as a small loop of current which possesses a magnetic moment.

• Each electron has a permanent spin and an magnetic moment

Bohr magneton, µB = 9.27 × 10–24A m2.5050


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1. Paramagnetism : due to presence of unpaired electrons weakly attracted by the magnetic field magnetized in a magnetic field in the same direction lose magnetism in the absence of mag. field e.g : O2, Cu2+, Fe3+, Cr3+



Diamagnetism: • weakly repelled by a magnetic field. • eg: H2O, NaCl and C6H6• They are weakly magnetised in a magnetic field in opposite direction. • shown by those substances in which all the electrons are paired and there are no unpaired electrons. • Pairing of electrons cancels their magnetic moments and they lose their • Pairing of electrons cancels their magnetic moments and they lose their magnetic character.



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Ferromagnetism: • A few substances like iron, cobalt, nickel, gadolinium and CrO2 are attracted very strongly by a magnetic field. Such substances are called ferromagnetic substances.• can be permanently magnetised. • In solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny together into small regions called domains. Thus, each domain acts as a tiny magnet. • In an unmagnetised piece of a ferromagnetic substance the domains are randomly oriented and their magnetic moments get cancelled. When the substance is placed in a magnetic field all the domains get oriented in the direction of the magnetic field and a strong magnetic effect is produced. This ordering of domains persist even when the magnetic field is removed andthe ferromagnetic substance becomes a permanent magnet.



Antiferromagnetism: • 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



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Ferrimagnetism: • the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers. • weakly attracted by magnetic field as compared to ferromagnetic substances. • Examples: Fe3O4 (magnetite) and ferrites like MgFe2O4 and ZnFe2O4 are examples of such substances. • These substances also lose ferrimagnetism on heating and become paramagnetic.

End of chapter



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