1. IONIC SOLIDS IONIC SOLIDS T.Y. B.Sc. Dr. R. K. Jadhav
Dept. of Chemistry,
S. M. Joshi College, Hadapsar, Pune.

2. Characteristic property of a crystal
Highly ordered periodic arrangement of the constituent atoms, ions or molecules. & sharp melting points; solid------liquids.
Eg. NaCl
Glass is not a crystal because it not having ordered arrangement of constituent atoms therefore glass is super cooled liquid.

3. Anisotropy & isotropy
Anisotropy: Mechanical, electrical and many other properties of crystals which depends upon direction along which they are measured.
Eg. NaCl
Isotropy: The properties of amorphous materials do not depends upon the direction of the measurement.
Eg. Glass, talc

4. Difference between crystalline solids and amorphous solids
Definite geometric shape
Definite and orderly arrangement of atoms, ions, or molecules in three dimensional space.
Long range order.
Sharp melting point.
Crystals are bound by plane faces.
Anisotropic.
It has unit cell.
egs. NaCl, S, sugar, etc.
Amorphous solids
No definite geometrical shape
Not having orderly arrangement of constituent.
Short range order.
No sharp M.P. & B.P.
No plane faces.
Isotropic.
No unit cells.
Egs. Rubber, plastic, glass, etc.

5. Crystal Structure = Space Lattice + Basis
Crystalline solids
Crystal Structure = Space Lattice + Basis

6. General characteristics of Ionic compounds
Ionic bond
Hardness
M.P.s & B.P.s
Electrical conductivity
Solubility

7. Formation of Ionic solids
Ionic solids are the substances, that are formed at the lower temp. by the condensation of ion pair molecules.
Ionic solids are formed by a process in which the gaseous ions that are far apart at the higher temp. are condensed upon cooling into an orderly three dimensional array in an ionic crystalline solid.

8. Crystal Lattice Crystal Lattice
The definite and ordered arrangement of the constituents extend over a large distance in a crystal called crystal lattice.

9. Solids Fixed, immobile (so to speak) Symmetry Crystals
So what’s the inner order?

10. Unit Cells
Unit cell = smallest repeating unit containing all symmetry characteristics. Unit cell reflects stoichiometry of solid
Several unit cell types possible, but atoms or ions placed at lattice points or corners of geometric object

11. Crystal Lattices 3D unit cells built like logos 
Crystal Lattice = arrangement of units cells
seven 3D units cells found
Simplest = Cubic Unit Cell (equal length edges meeting at 90° angles)
Each face part of 2 cubes
Each edge part of 4 cubes
Each corner part of 8 cubes
3D unit cells built like legos 
Crystal Lattice = arrangement of units cells
seven 3D units cells found
Simplest = Cubic Unit Cell (equal length edges meeting at 90° angles)
Each face part of 2 cubes
Each edge part of 4 cubes
Each corner part of 8 cubes

12. Cubic Unit Cell 3 types: 1) Primitive or Simple Cubic (SC)
2) Body-Centered Cubic (BCC)
3) Face-Centered Cubic (FCC)

13. Ionic Solids
Ionic crystals consist of the negative and positive ions, attracted to each other
Electron from one of the atoms removed and transferred to another: NaCl, AgBr, KCl
When the crystal is formed excess heat is generated

14. What do they look like?
BCC:
FCC
SC:

15. CCP and HCP: Efficiency in Stacking
CCP = Cubic Close-Packing (it’s FCC)
HCP = Hexagonal Close-Packing
74% packing efficiency

16. Solid Models: Close-Packed Spheres
Most atoms or ions forming solids have spherical symmetry
Considering the atoms or ions as solid spheres we can imagine crystals as closely packed spheres

17. How many atoms per unit cell? (cont.)
BCC: 2 net atoms w/in unit cell (SC + 1 in center)
FCC: 6 faces of cube  ½ atom w/in unit cell = 3 atoms + 1 atom (SC) = 4 net

18. CsCl SC unit cell Cs+ in center of cube  Cubic hole
Surrounded by 1 Cl- (in 8 parts)
1 Cs+ : 1 Cl-
Coordination # = 8
Why SC and not BCC?
Because ion in center different from lattice pt ions

19. NaCl FCC Lattice has net 4 Cl-/unit cell 1 Na+ in center of unit cell
(8x1/8)+(6x1/2) = 4
1 Na+ in center of unit cell
3 Na+ along edges of unit cell
(12x1/4) = 3
Thus, net total of 4 Na+ ions
Total 4 Cl- : 4 Na+  1:1

20. Tetrahedral holes
Each ion surrounded by 4 other oppositely-charged ions
Unit cell: 4 of each ion  total 8 ions
Coordination # = 4
8 tetrahedral holes in FCC unit cell
4 by Zn2+ and 4 by S2-
Zn2+ occupies ½ of tetrahedral holes and surrounded by 4 S2-
S2- forms FCC unit cell

21. ZnS

22. ZnS

23. Other Types of Solids: Network Solids
Array of covalently bonded atoms
Graphite, diamond, and silicon
The latter two  sturdy, hard, & high m.p.’s

24. Graphite and diamond

25. Crystalline Structure of NaCl

26. FAJAN’S RULES INTRODUCTION
Observations Not all ionic compounds have high melting
points.
Some covalently bonded compounds have higher than
expected boiling points due to dipoles in their structure
Reason in many substances bonding is not 100% ionic
or covalent
Ideal ionic
compound completely separate, spherical ions
electron densities are apart from each other
However, if the positive ion has a high charge density it can distort the negative ion by attracting the outer shell electrons to give an area of electron density between the two species ... a bit like a covalent bond 1

27. A compound is more likely to be covalent if the ...
The ion deformation depends upon 1) size of cation & anion 2) charge on cation & anion 3) electron configuration of the cation. These factors given by Fajan’s rule.
The feasibility of having some covalent character can be predicted using Fajan’s Rules.
A compound is more likely to be covalent if the ...
CATION SMALL SIZE it is “highly polarising” and attracts electrons in the anion
HIGH CHARGE
ANION LARGE SIZE it is “highly polarisable” and will be easily distorted 2 2

28. A compound is more likely to be covalent if the ...
Concept of deformation or polarization: The distortion of electron cloud of one ion by electron cloud of other ion is called polarization.
The feasibility of having some covalent character can be predicted using Fajan’s Rules.
A compound is more likely to be covalent if the ...
CATION SMALL SIZE it is “highly polarising” and attracts electrons in the anion
HIGH CHARGE
ANION LARGE SIZE it is “highly polarisable” and will be easily distorted
N.B. Just because a substance is less likely to be covalent according to
Fajan’s Rules doesn’t mean it will be ionic; it will remain covalent
but have some ionic character (or vice versa). 3

29. IONIC BONDING EXTREMES OF CHEMICAL BONDING
• 3-DIMENSIONAL GIANT IONIC LATTICE
• ALTERNATE POSITIVE AND NEGATIVE IONS
• HIGH MELTING POINT
• SOLUBLE IN WATER
• MOLTEN STATE CONDUCTS ELECTRICITY
ELECTRON DENSITY IS SEPARATED AND AROUND EACH SPECIES
The ideal ionic compound has completely separate, spherical ions and the electron densities are apart from each other. 4

30. EXTREMES OF CHEMICAL BONDING ELECTRON DENSITY IS BETWEEN EACH SPECIES
COVALENT BONDING
• MOLECULAR (SIMPLE OR MACRO)
• SIMPLE MOLECULES HAVE LOW MELTING PTS - WEAK INTERMOLECULAR FORCES
• USUALLY INSOLUBLE IN WATER BUT SOME ARE HYDROLYSED
• MOLECULES DON’T CONDUCT ELECTRICITY IN THE MOLTEN STATE
ELECTRON DENSITY IS BETWEEN EACH SPECIES
H : H
H H
The ideal covalent compound has the electron density exactly in between the species 5

31. FAJAN’S RULES A COMPOUND IS MORE LIKELY TO HAVE
SOME COVALENT CHARACTER IF...
• THE CATION IS SMALL AND/OR HAS A HIGH CHARGE - HIGHLY POLARISING
• THE ANION IS LARGE AND/OR HAS A HIGH CHARGE - HIGHLY POLARISABLE
MORE COVALENT CHARACTER
MORE COVALENT CHARACTER 6

32. r+ and r_ are very different
when
r+ and r_ are very different
The crystal adopts the Zince blende (ZnS) structure with coordination
number of 4 6

33. Rocks are …
Naturally consolidated mixtures of minerals or mineral-like substances.
Examples of Building Stones at Rice 7

34. Note - labels are incorrect in the book
Ionic Bonding
Note - labels are incorrect in the book
Fig 3.4
Cation (+)
Anion (-) 8

35. Predictable Interface Angles
Crystal Structure
Perfect Crystals:
Predictable Interface Angles 33 9

36. - Crystal systems and Bravais Lattices10

37. Electrons orbit in discrete shells
Atomic Structure
Negatively charged electrons surround the nucleus.
Nucleus contains ~mass of the atom
Protons -positive charge
Neutrons - no charge, i.e., neutral
Quarks, and more…
Electrons orbit in discrete shells
- outer shell is most reactive
Fig. 3.2 11

38. Ionic Compounds
Fig.3.4
Fig. 3.6
NaCl
Halite
Table Salt 12

40. CLOSE-PACKING OF SPHERES
SINGLE LAYER PACKING
CLOSE PACKING
SQUARE PACKING
Close-packing-HEXAGONAL coordination of each sphere

41. TWO LAYERS PACKING

42. s
THREE LAYERS PACKING

44. Hexagonal close packing
Cubic close packing

45. sss Cubic close packing
4 atoms in the unit cell (0, 0, 0) (0, 1 /2, 1 /2) (1 /2, 0, 1 /2) (1 /2, 1 /2, 0)
Hexagonal close packing
2 atoms in the unit cell (0, 0, 0) (2/3, 1 /3, 1 /2)
74% Space is occupied
Coordination number = 12

46. Hexagonal close packed
ABCABC… 12
Cubic close packed
ABABAB…
Hexagonal close packed 8
Body-centered Cubic
AAAAA…
Primitive Cubic
Stacking pattern
Coordination number
Structure
Non-close packing
Close packing 6

47. TYPE OF HOLES IN CLOSE PACKING
TETRAHEDRAL HOLES
OCTAHEDRAL HOLES

48. LOCATION OF TETRAHEDRAL HOLES IN CLOSE PACKING

49. IONIC CRYSTAL STRUCTURES
Ionic structures may be derived from the occupation of holes by oppositely charged ions (interstitial sites) in the close-packed arrangements of ions.

50. Holes in which positive ions pack
Hole Occupation - RADIUS RATIO RULE
Radius of the positive ion
Radius ratio =
Radius of the negative ion
Radius ratio
Coordinate number
Holes in which positive ions pack
0.225 – 0.414 4
Tetrahedral holes
0.414 – 0.732 6
Octahedral holes
0.732 – 1 8
Cubic holes

51. Radius Ratio
The ratio of radius of a cation to that of anion is called a radius ratio.
rr= rc/rA
usually rc/rA < 1
( therefore size of cation < size of an anion)
Radius ratio effect: The effect of this ratio in determining the coordination number and shape of an ionic solid is known as radius ratio effect.
i.e. if C+ is octahedrally surrounded by six A- ions, then its coordination no. is six an so on---.

52. RR effect of solids Example - The Cubic Case
Cesium chloride forms a lattice in which the chloride anions adopt a simple cubic packing arrangement, with each cesium cation occupying the center of a cube. CsCl has a radius ratio of which indicates that the cations are large enough to prevent the anions from contacting one another. Each unit cell contains one cesium cation and 8(1/8) chloride ions. Thus, each unit cell contains one formula unit. EIf the salt does not have a 1:1 stoichiometry, the less common ion occupies a certain proportion of the spaces. In calcium fluoride the cation to anion stoichiometry is 1:2. In the lattice each clacium ion is surrounded by eight chloride ion as in the CsCl lattice. To preserve the 1:2 cation to anion ratio, each alternate interstitial space is empty. This array is termed the fluorite structure. In a compound such as Li2O, the cation to anion ratio is 2:1. The structure is based on the CaF2 lattice, but each alternate anion site is empty. This lattice is named using the prefix anti-. In this case, the array is called an antifluorite structure.

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