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18 Cards in this Set
- Front
- Back
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Solids |
Only crysralline subustances are regarded as the true solids. These have a definite and regular network structure |
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Lattice energy |
Energy released when gaseous ions seperated by infinite distance combine to form the one mole of crystal is lattice energy. Denoted by U |
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Calculation of lattice energy |
Born Lande equation Given in pic Where M is madlung constant value dependent upon ions And n is born exponent |
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Lattice energy dependence |
Factors increasing the lattice energy 1. Small ion size 2. Higher ionic charge 3. Higher coordination number |
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Crystal groups/ crystal systems / bravias lattices |
There are 32 point groups or crystal groups which can be grouped into seven basic crystal systems viz 1. Cubic 2. Orthorhombic 3. Tetrogonal 4. Monoclinic 5. Triclinic 6. Hexagonal 7. Rhombohedral These crystal systems differ in the cell dimensions and interplanar angles |
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Lattic energy experimental measurement |
Only few compounds it can be measured directly Born Haber cycle used example of the born haber cycle for calculation of lattice energy to be attached in pic |
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Born Haber cycle |
Based on hesses law of constant heat summation Used to calculate lattice energy Energy of formation Electron affinity The energy of protonation (example attached) |
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Crystal systems/point groups/ bravais lattices |
There are 32 crystal systems divided into 7 types of crystal systems These seven crystal systems can be arranged into 14types of lattices called bravias latices In toto 32 crystal groups 7 crystal systems 14 bravias lattices seven crystal systems can be remembered by Ist alphabets of namea as COTMTHR |
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Packing types packing fraction and voids |
Close packing Cubic close packing and hexagonal close packing are the most effecient packing arramgements in solids 2. Both ccp and hcp have only 26percent void volume whereas bcc has 32 percent and simple cubic arrangemnt has 48 percent void volume. The void vol can be calculated .3. Maxumum coordunation number 12 minimum 2 . 3. Maxumum coordunation number 12 minimum 2 |
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Calculating crystallographic density |
Crystallographic density is given by |
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Characteristic structure of ionic crystals |
9 common types viz 1. Rock salt structure Fcc structure of anions in which octahedral voids are filled by the cations and vice versa Coordination number of cation anion 6:6 compounds with rock salt structure are KCl and NaCl a few exMples. 2. Cesium chloride structure Each cation surrounded by eight anioons at the corners of the cube and vice versa Coordinatuon no: 8:8 3. Zinc Blend (sphalerite structure) Expanded fcc lattice in which tetrahedral voids are filled by the cations. Coordination 4:4 4. Wurtzite: structure similar to sphalerite type but based on the hcp packing of anions in which tetrahedral voids are occupied by the cations and vice versa Coordination number: 4:4 5. Flourite structure: based on mineral CaF2 this structure has the cations occupying half of the cubic voids between anions arranged in primitive cubic arrangement. The cations are themselves in the extended fcc arrangement in which all rhe tetrahedral voids are occupied by the anions Coordination number 8:4 6. Antiflourite K2O structure Same as flourite structure with roll of cations and anions reversed 7. Rutile structure TiO2 structure Coordination number: 6:3 8. Beta cryatallobite structure found in sio2 similar to zinc blende wurtzite structure 9. Other structures are nickel arsenide pervkosite and spinel structure. |
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Conductors semiconductors and insulators |
The difference between the three is based in conductivity at room temp Conductors have highest cnductivity 10^8 to 10^2 s.cm^-1 Semicinductors have conductivity 10^2 to 10^-9 Insulators less than 10^-9
In conductors the conduction band is partially filled with electrons and the energy gap between the valence band and conduction band is small In some metallic conductors the conduction band is overlaping with the valence band Semiconductors(intrinsic semiconductors) these semiconductors have empty conduction band at lower temperatures but occupied conduction band at higher temperatures. The energy gap is less so that the rise in temperature and application of small potential difference makes the electrons to surpass the energy gap and conduct electricity. Insulators : these have very high energy gap which cant be surpassed even at higer temperatures and highest potential difference Energy gaps in different materials is given in pic |
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Intrinsic and extrinsic semicinductors |
Intrinsic semicinductors are pure semicinductors whuch are undoped Extrinsic semiconductors are doped semicinductors If dopant is the pentavalent impurity belonging to group 15 such as nitrogen arsenic phosphorus then it is called n type semiconductor. In n type semiconductors the electrons are the carriers of the current a new donar band is introduced in the energy gap filled with electrons Trivalent impurity p type semiconductor Holes are carriers of current In p type semiconductors. The band energy levels of the p and n type semiconductors are shown in the pic. |
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Super-Conductors |
The conductors which have resistance approaching to zero are called the super-conductors. Super conductivity is of two types viz low temperature superconductivity High temperature superconductivity 1.Low temperature Superconductivity Discovered by Kammerlingh Onnes: The resistance of mercury approaches to zero at liquid helium temperature. 4.21K. The critical temperature range for the operation of superconductivity is less than 11K. low temperature Superconductivity. Explanation of low temp Superconductivity Messener Effect: Superconductors are diamagnetic the superconductivity is due to cooper pair which is the pair of electrons. 2. High Temperature Superconductivity Occurs beyond 10 K. The materials have perovskite structure. |
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Imperfections In solids/Defects |
1. An imperfection or defect is the deviation from the regular arrangment of the lattice points in the solid crystal. 2. The defects may be localised of extended Localised Defects These defects are also called point defects and pertain to be localised at particular lattice point rather than over whole crystal The localised defects may be Stoichiometric defects which dont change the formulla of the ionic compound. These are also called the intrinsic defects Non stoichiometric defects which change the formulla of the ionic compound. these defects are discussed one by one as under A. Stoichiometric Defects These Include Schottky Defect: when the oppositely charged ions are missing altogether from their places. Example in Rocksalt structure of the sodium chloride the sodium ions and equal number of the chloride ions are missing from the lattice points. These defects occur in ionic solids where the ionic size of cation and anion are similar the coordination number is high The number of Schotky defects increase with the increase in temperature. Schotky defects decrease density and increase the conductivity of the crystals. Frenkel Defect: An ion (mainly cation ) dislocated from the lattice position to the interstitial site. The formulla remains same but a cation hole is created which increases conductivity Density remains unchanged Frenkel defects are common in compounds where the difference in size of cations and anions is high. Coordination number is lower and the charge on the cations and anions is also higher. 2. Non-Stoichiometric Defects These defects change the Formulla of the compound and are of following types Metal Excess: the cations become more than the anions by either cations occupy the interstitial sites or the anions leave the lattice sites. an example when the Sodium metal is heated with the sodium chloride the chloride ions from the lattice difuse to surface leaving behind their electrons at their sites to maintain the electrical neutrality. These electrons are excited by the visible light and hence give colour to the crystal. These Sites of anion vacancy which contain electrons are called F-centres Fabre centres which means colour centres. metal deffciency: Occurs in the crystals of compounds wher the metal can show variable oxidation state to maintain the neutrality. Examples are that of FeO other nickel and copper oxides. extended defects include edge dislocations and screw dislocations. |
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Investigation of the crystal structures/ Braggs equation/ d spacing/ miller indices |
shown in pic |
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Systematic Absences in the various Cubic lattices |
shown in Pic |
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Laws of Crystallography |
Law of Constancy of Interfacial Angles: The law of the constancy of interfacial angles (or 'first law of crystallography') states that the angles between the crystal faces of a given species are constant, whatever the lateral extension of these faces and the origin of the crystal, and are characteristic of that species. It paved the way for Haüy's law of rational indices. Law of Rational Indices: The intercepts made by the plane on different axis bear a simple whole no ratio to each other. |
