IONIC SOLIDS IONIC SOLIDS T.Y. B.Sc. Dr. R. K. Jadhav Dept. of Chemistry, S. M. Joshi College, Hadapsar, Pune.
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.
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
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.
Crystal Structure = Space Lattice + Basis Crystalline solids Crystal Structure = Space Lattice + Basis
General characteristics of Ionic compounds Ionic bond Hardness M.P.s & B.P.s Electrical conductivity Solubility
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.
Crystal Lattice Crystal Lattice The definite and ordered arrangement of the constituents extend over a large distance in a crystal called crystal lattice.
Solids Fixed, immobile (so to speak) Symmetry Crystals So what’s the inner order?
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
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
Cubic Unit Cell 3 types: 1) Primitive or Simple Cubic (SC) 2) Body-Centered Cubic (BCC) 3) Face-Centered Cubic (FCC)
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
What do they look like? BCC: FCC SC:
CCP and HCP: Efficiency in Stacking CCP = Cubic Close-Packing (it’s FCC) HCP = Hexagonal Close-Packing 74% packing efficiency
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
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
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
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
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
ZnS
ZnS
Other Types of Solids: Network Solids Array of covalently bonded atoms Graphite, diamond, and silicon The latter two sturdy, hard, & high m.p.’s
Graphite and diamond
Crystalline Structure of NaCl
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
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
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
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
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
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
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
Rocks are … Naturally consolidated mixtures of minerals or mineral-like substances. Examples of Building Stones at Rice 7
Note - labels are incorrect in the book Ionic Bonding Note - labels are incorrect in the book Fig 3.4 Cation (+) Anion (-) 8
Predictable Interface Angles Crystal Structure Perfect Crystals: Predictable Interface Angles 33 9
- Crystal systems and Bravais Lattices 10
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
Ionic Compounds Fig.3.4 Fig. 3.6 NaCl Halite Table Salt 12
CLOSE-PACKING OF SPHERES SINGLE LAYER PACKING CLOSE PACKING SQUARE PACKING Close-packing-HEXAGONAL coordination of each sphere
TWO LAYERS PACKING
s THREE LAYERS PACKING
Hexagonal close packing Cubic close packing
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
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
TYPE OF HOLES IN CLOSE PACKING TETRAHEDRAL HOLES OCTAHEDRAL HOLES
LOCATION OF TETRAHEDRAL HOLES IN CLOSE PACKING
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.
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
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---.
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 0.934 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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