1. solid state PPT GEOLOGY CLASS 12. BEST QUALITY 😉😉😉😉😉

Geology PPT By Madarchod Kamalnayan Bajaj Higher Secondary School.

1. UNIT 1: SOLID STATE

2. Introduction Physical states of matter- solid, liquid & gas Solid Liquid Gas Solids: solids have fixed chemical composition i.e. they have fixed mass, volume, shape and density. These change slightly with the change in temperature & pressure The smallest constituent particles of solids are atoms, ions or molecules. These smallest constituents will be called as 'particles’ in this chapter. Solids have strong inter particle forces of attraction. Heat Heat Cool Cool

3. Classification of Solids

4. Crystalline Solids Crystalline solids are homogeneous solids, in which constituent particles/atoms/ions/ molecules are arranged in a definite repeating pattern over a long range Crystalline solids have sharp melting points i.e. they melt at a definite temperature .E.g. Ice, salts like NaCl, metals like Cu, Au & materials like diamond, graphite, ceramics

5. All crystalline solids except those having a cubic structure are anisotropic in nature i.e. the magnitude of properties like refractive index, thermal & electrical conductivities etc. are different in different directions .

6. Amorphous Solids - Substances appear like solids, but do not have well developed, perfectly ordered crystalline structure are amorphous solids -They are not real solids, but truly supercooled liquids - Constituent particles are randomly arranged. - Particles do not have a long range ordered structure, but have a short range order Amorphous solids do not have ordered internal structure & do not have sharp M. P.

7. Amorphous Solids - On heating amorphous solids soften gradually & continuously & start to flow -The magnitude of physical properties like refractive index, conductivity do not change with change in direction. - Hence they isotropic. - ex. Tar, glass, rubber, plastic , metallic glass( metal & metalloid alloys)

8. Isomorphism- Two or more substance having same crystal structure [e.g.- NaF ( atoms in ratio 1:1), MgO (ratio 1:1) ] Chemical composition of substance have same atomic ratio The same atomic ratio, similar molecular formula or similar chemical properties of solid does not mean that they are isomorphous . E.g. NaCl & KCl. In these solids all properties are identical, but crystal structure are different.

9. Polymorphism: Single substance existing in 2 or more forms or crystalline structures is said to be polymorphous. →Polymorphs are formed in different conditions Eg. Calcite & orognite are the 2 forms of calcium carbonate α - quartz, β - quatrz , cristobalite are 3 of several forms of silica.

10. → Elements showing polymorphism exhibit allotropy. Eg. Diamond , graphite & fullerene are the 3 allotropic or polymorphic forms of carbon.

11. CLASSIFICATION OF CRYSTALLINE SOLIDS Ionic crystals Covalent Crystals Molecular Crystals Metallic crystals

12. Classification of Crystalline Solids 1.Ionic Crystals- Characteristics - Constituent particles of ionic crystals are charged ions( cations & anions) of different sizes. The oppositely charged ions are held together by electrostatic force of attraction ( coulomb force) They are hard & brittle with high melting points & can act as insulators at low temperature In solid state they are non conductors of electricity, but when melted or dissolved in water they become good conductors. Eg. NaCl , K 2 SO 4 , CaF 2 , KCl

13. Classification of Crystalline Solids 2. Covalent Network Crystals: Characteristics Constituent particles in covalent network solids are atoms The atoms are linked together by continuous covalent bonds, which gives rise to 3 dimensional network forming a giant molecule. The entire crystal is a single molecule Covalent network crystals are very hard & incompressible of all materials due to rigid & strongly bonded structure.

14. 2 . Covalent Network Crystals: Characteristics Covalent solids are poor conductors of heat & electricity as electrons are localised in covalent bonds & not mobile. They have high Melting & boiling points & can be used as insulators at low temperature. Eg. Diamond, silica ( quartz), boron nitride, Carborundum ( SiC ) Although Conductivity of covalent soilds is in general low & increases with temperature; there is no abrupt rise in conductivity.

15. 3. Molecular Solids: molecular solids like Cl 2 , CH 4 , H 2 CO 2 , O 2 on solidification give rise to molecular solids. Crystalline organic compounds are also molecular solids. Characteristics: 1. Constituent particles of molecular solids are molecules ( or unbonded single atoms) of same substance. 2. The bonds within the molecules are covalent. The molecules are held together by various intermolecular forces of attraction like: a) Weak dipole dipole interaction: it is seen in molecules like HCl, H 2 O, SO 2 which have permanent dipole moment

16. b) Very weak dispersion forces or London Forces: These are the forces present among non polar molecules like solid CH 4 , H 2 , & in mono atomic solids like Ar , Ne (gases at room temp.) hence called noble gases . c) Intermolecular hydrogen bonds: These are the bonds seen in H 2 O (ice) , NH 3 , HF & so on…. 3. Since the intermolecular attractive forces are weak, molecular solids are soft with low melting points. 4. Molecular solids are poor conductors of electricity & act as good insulators.

17. 4. Metallic Solids: They are crystalline solids formed by atoms of same metallic elements held together by metallic bond. Metallic Bond:→ In a solid metal, the valence electrons are delocalised over the entire crystal leaving behind positively charges ions. →Thus the metal crystals always seems like to be an array of positive ions in a sea of mobile electrons. →This interaction between cations & mobile electrons constitute metallic bond.

18. Characteristics: 1. metals are malleable & can be drawn into thin sheets. 2. Metals are ductile & can be drawn into wires. 3. Metals have good electrical & thermal conductivity. Eg. Metals like Na, K, Ca, Li, Fe, Au, Ag etc

19. CRYSTAL STRUCTURE -Crystal structure can be well explained by using 2 terms: 1. Crystal lattice 2. Basis - Lattice is the geometrical arrangement of points in a 3 dimensional periodic array -constituent particles (atoms/ions/molecules) attached to the lattice points forms basis of crystal lattice. -Crystal structure is obtained when you attach the basis to the lattice points.

20. Unit Cell Defn: The smallest repeating structural unit of a crystalline solid is called a unit cell. -The geometric shape of a unit cell will always be that of the macroscopic crystal i.e. If the crystal has cubic shape the unit cell will also be in the cubic form The dimensions ( edges) of unit cell along the 3 axes are denoted by a,b,c . The angles between these axes is denoted by α , β & γ

21. TYPES OF UNIT CELLS 1.Primitive/Simple unit cell: The constituent particles are present at its corners only 2. Body centered unit cell: In body centered unit cell one particle is present at the centre of the body in addition to the corner particles.

22. 3. Face centered unit cell: In this type one particle is present at the centre of each face of the unit cell in addition to the corner particles 4.Base centered unit cell: This unit cell consists of one particle at the centre of any two faces opposite to each other in addition to the corner particles.

23. CRYSTAL SYSTEMS Mathematical analysis shows that the lattice points can be arranged in 14 different ways in 3 dimensional order These 14 lattice describes the crystal structure & is called ‘ Bravais Lattice’ The spacing of the combinations of lattice points (a, b & c) along the 3 axes & the angles between these axes ( α , β & γ ) gives rise to 7 different crystal systems.

24. CUBIC SYSTEM 1) Simple Cubic Unit Cells/Primitive (SCC)- It has one particle at the 8 corners of a cube 2) Body centered unit cell: It has a particle at the 8 corners & a particle at the centre of the cube 3) Face centered cubic unit cell Fcc: It has a particle at the centre of each of its faces in addition to a particle at the 8 corners of the cube

25. Number of particles in Cubic Unit Cells 1)Simple Cubic Unit Cell( scc): →In scc there are constituent particles present at the 8 corners. → If these unit cells are stacked together, particles at each corner of the unit cell is shared by 7 other neighbouring unit cells. → Thus the corner particle contributes its ⅛ th part to the given unit cell Therefore scc has ⅛ X 8 = 1 particle per unit cell

26. 2) Body centered unit cell (bcc): → In a bcc unit cell there are 8 corner particles in addition to one more particle at its centre. → There is ⅛ th of a particle at the eight corners of the unit cell & one particle at the centre of a given cube which is not shared by any other unit cell. → Hence a bcc unit cell has one particle from eight corners & one particle in the centre of the cube → Thus bcc has ⅛ X 8 = 1 particle(corners) + 1 particle at centre = 2 particle/unit cell

27. 3) Face centred Cubic (FCC) Unit Cell- - There are 8 spheres at 8 corners of cube In addition there is one sphere at centre of each face. No. of atoms at 8 corners of unit cell of FCC are also 1/8 x 8 = 1 - Spheres at the centres of each face of unit cell are shared betn . 2 faces. Hence ½ sphere is present at face of each unit cell & there are total 6 faces of cube -Total spheres present in faces of unit Cell = ½ x 6 = 3 -Total spheres in FCC unit cell = 1+3 = 4

28. RELATION BETN. MOLAR MASS, DENSITY OF SUBSTANCE AND UNIT CELL EDGE LENGTH If unit cell edge length is ‘a’, then volume of unit cell = a 3 Let ‘m’ be the mass of a particle & Let ‘n’ be the no. Of particles Hence the Density of unit cell( ρ ) = Density of substance = = m X n / a 3 ....................(1) Molar mass of substance (M) is given as M = i.e. M = m X N A ( N A ( particle per mole) = Avogadro’s number) Hence, m = M / N A ................... (2) Combining eqn. 1 & 2 we have Mass of unit cell = m X n Mass of unit cell/volume of unit cell Mass of one particle X No. Of particle per mole ρ = M X n / a 3 X N A

29. PACKING OF PARTICLES IN CRYSTAL LATTICE When packing of particles are studied , individual particles are considered as spheres of equal size. → The number if spheres that touch any given sphere is its coordination number (CN) → Magnitude of the CN gives you the compactness of the spheres in closed pack structures. CLOSED PACKING IN ONE DIMENSION: This structure arises when the spheres are arranged in a row so that every sphere comes in contact with 2 neighbouring spheres.

30. 2 ) CLOSED PACKING IN TWO DIMENSIONS: A closed packing two dimensional structure results by stacking the one dimensional rows in such a manner that every row is in contact with each other. This gives rise to 2 types of packing: SQUARE CLOSE PACKING STRUCTURE: →One dimensional rows of closed packing spheres are stacked over each other such that the sphere allign with each other vertically & horizontally → If first row is ‘A’ , the second row being same as A is also labelled as ‘A’. This arrangement is called ‘AAAA’ type 2 dimensional arrangement. → CN in this arrangement is 4. → If the centres of all spheres are joined a square is obtained → Voids in square closed packing is square shaped large enough to hold one sphere

31. M b) HEXAGONAL CLOSED PACKING STRUCTURE: → In one dimensional close packed structure depressions can be observed between neighbouring spheres. → If second row is stacked in such a way that the spheres of second row fits in the depression of first row. A staggered arrangement is observed. → If the first row is ‘A’; second row being different is called ‘B’ → When a third row is placed in the staggered second row ; the sphere of the third row alligns with the sphere of the first row, hence the third row is ‘A’ type. → similarly spheres of the fourth row alligns with the spheres of the second row & so it will be ‘B’ type. → The resulting 2 dimensional arrangement will be of ‘ABAB’ type

32. b) HEXAGONAL CLOSED PACKING STRUCTURE: → In this type of structure each sphere is in contact with 6 spheres , hence the CN is 6. → If the centres of these spheres are joined a regular hexagon is obtained. → Such a 2 dimensional close packing is called hexagonal close packing in 2 dimensions.

33. 3) CLOSED PACKING IN 3-DIMENSION 2- Dimensional structure when stacked gives rise to 3 dimensional structure i )Stacking of square closed packed layers: → When square close packed layers are stacked One above other, such that the spheres of second layer is exactly above the spheres of the first layer , it gives rise to 3 dimensional SIMPLE CUBIC STRUCTURE. → In this arrangement spheres of all layers are alligned vertically & horizontally & hence all layers are alike & labelled as ‘A’ layers. → This arrangement is called ‘AAAA’ type → In simple cubic structure CN is 6 as each sphere is in contact with 6 spheres ; 4 in its own layer & one in the lower layer & one in the above layer. → Eg . POLONIUM crystallizes in simple cubic closed packed structure

34. → 3 dimensional closed packed structure is obtained by stacking hexagonal close packed layers. → The second layer is placed in the depression of the first layer → The spheres of second layer do not align with the spheres of first layer hence the first layer is labelled as ‘A’ & the second layer as ‘B’ → Only half of the triangular voids are covered totally by second layer. → This gives rise to a new void called tetrahedral voids which is formed from 4 spheres ii) Stacking of 2 hexagonal closed packed layers:

35. → Remaining half of the triangular voids have above them the triangular voids of second layer. → These overlapping triangular voids of two layers forms another void called octahedral voids → these voids are formed by 6 spheres.

36. iii) Placing the third hexagonal close packed layer: → This can be done in 2 ways: a)First way is placing the spheres of third layer in the tetrahedral voids of second layer → The spheres of the third layer align with the spheres of first layer & hence will be of ‘A’ type. →Similarly spheres of fourth layer aligns with the second layer & hence called as ‘B’ type. → The resulting arrangement will be ‘ABAB’ type. → The structure so obtained is 3D Hexagonal closed packed structure →Eg. Mg, Zn, have hcp structure

37. b) In this type the third layer is placed in the octahedral voids of second layer → The spheres of the third layer do Not align with spheres of previous layers & is called the ‘C’ layer. → When the fourth layer is placed In the octahedral voids of third layer, spheres align with the first layer & hence the fourth layer will be of ‘A’ type. → Similarly fifth & sixth layer will be of ‘B’ & ‘C’ type respectively. →Such an arrangement is called ‘ABCABC’ type of arrangement & the structure is called cubic close packed structure( ccp ) similar to fcc structure. Eg. Cu, Ag have ccp structure

38. Coordination Number in hcp & ccp structure

39. PACKING EFFICIENCY: DEFN: It is the fraction or the percentage of the space occupied by the spheres (particles) Thus Packing Efficiency = Volume occupied by particles in unit cell X 100 Total volume of unit cell 1.Packing efficiency of simple cubic lattice: STEP 1: Radius of sphere: In Scc spheres at corner touching each other along the edge. In the above fig a = 2r or r = a/2...........(1) Where a = egde length & r = radius

40. Step 2: Volume of sphere: Volume of a sphere = 4/3 π r 3 Substitute r from eqn 1 Volume of a sphere = 4/3 π (a/2) 3 = π a 3 /6 Step 3: Total volume of particles: As scc unit cell consists of 1 particle Volume occupied by one particle = 1 X π a 3 /6 = π a 3 /6 Step4: Packing efficiency: Packing Efficiency = Volume occupied by particles in unit cell X 100 Total volume of unit cell = π a 3 /6 X100 = 100 π /6 =100 X 3.142/6 = 52.36% a 3 Hence in simple cubic unit cell 52.36% space is occupied & 47.64% is empty

41. 2) Packing efficiency in bcc unit cell: → In bcc particles are present at the 8 corners in addition to one at the centre. → Centre sphere touches the 2 corner particles diagonally. Step1: Radius of sphere: Pythagoras theorem In ∆FED FED = 90 hence (FD) 2 = (FE) 2 + (ED) 2 (FD) 2 = a 2 + a 2 = 2a 2 Consider ∆AFD ADF = 90 (AF) 2 = (AD) 2 + (FD) 2 (AF) 2 = a 2 + 2a 2 = 3a 2 (AF) = √3a But (AF) = 4r hence 4r = √3a Therefore r = √3a/4………………………………(1) Step 2: Volume of one sphere : Volume of a sphere = 4/3 π r 3 Substitute r from eqn. 1 Volume of a sphere = 4/3 π ( √3a/4 ) 3 = √3 π a 3 /16

42. Step 3: Total volume of particles: As bcc unit cell consists of 2 particles Volume occupied by 2 particles = 2 X √3 π a 3 /16 = √3 π a 3 /8 Step4: Packing efficiency: Packing Efficiency = Volume occupied by particles in unit cell X 100 Total volume of unit cell = √3 π a 3 X 100 =100 X 3.142 X 1.732/8 = 68% 8Xa 3 Hence in body centered cubic unit cell 68% space is occupied & 32% is empty.

43. 3 ) Packing efficiency in fcc unit cell ( hcp or ccp ) → In fcc particles at the corner are assumed to touch the particle at the centre of the face ABCD Step1: Radius of sphere: By Pythagoras theorem In ∆ABC ʟABC = 90 hence (AC) 2 = (AB) 2 + (BC) 2 (AC) 2 = a 2 + a 2 = 2a 2 (AC) = √2a But (AC) = 4r hence 4r = √2a Therefore r = √2a/4 = a/2 √2 ………(1) Step 2: Volume of one sphere: Volume of a sphere = 4/3 π r 3 Substitute r from eqn. 1 Volume of a sphere = 4/3 π ( a/2√2 ) 3 = π a 3 /12√2

44. Step 3: Total volume of particles: As fcc unit cell consists of 4 particles Volume occupied by 4 particles = 4X π a 3 /12√2 = π a 3 /3√2 Step4: Packing efficiency: Packing Efficiency = Volume occupied by particles in unit cell X 100 Total volume of unit cell = π a 3 X 100 =100 X 3.142 /3 X 1.414 = 74% 3√2 X a 3 Hence in face centered cubic unit cell 74% space is occupied & 26% is empty or void

45. CRYSTAL DEFECTS OR IMPERFECTIONS → Real & naturally occurring crystalline substances are not perfect, they have some disorders or irregularities in arrangement of constituent particles (spheres) → These irregularities are known as defects or imperfections. → Defects are mostly caused during crystal formation specially if crystallization takes place faster. → Ideal crystals without any defects are possible at absolute zero of temperature, above it no crystals are 100% pure . → Even if the crystals have defect in them , electrical neutrality of solids are always maintained. → There are 3 types of defects 1] point defect 2] line defect 3] plain defect Point Defect : Its the irregularities that are produced in the arrangement of basis at the lattice points. a) Stoichiometric b) impurity c) Non stoichiometric defects

46. a) STOICHIOMETRIC DEFECTS →In this defect the stoichiometry is unchanged i.e. the ratio of the number of cations & anions of a compound remains the same as is represented by the chemical formula. Types: 1] VACANCY DEFECT: During crystallization a particle goes missing from its regular site in crystal lattice → This creates a vacant space at the lattice sites . → Crystal thus exhibits vacancy defect. → Due this defect the mass of the substance decreases with no appreciable change in volume , thus the density of the substance decreases. → Vacancy defect can also be developed when the substance is heated.

47. a) STOICHIOMETRIC DEFECTS 2] Self interstitial defect in chemical solids: → Space or voids in between the constituent particles at lattice points are called interstitial points. → When a particle of crystalline elemental solid occupy interstitial space in crystal structure , it is called self interstitial defect. It Can be caused in 2 ways An extra particle same as the existing particle occupies empty interstitial space at the lattice points This extra particle increases the mass of the substance without increase in volume & hence the density increases

48. a) STOICHIOMETRIC DEFECTS 2] Self interstitial defect in chemical solids: B] Particle leaves its original lattice point & occupies interstitial space in crystal. → Due to the displacement of the particle from its original site a vacancy is created at the regular site & interstitial defect results at a new position. → Here we find a combination of vacancy & interstitial defect. → There being no increase or decrease in mass of the substance the density of the substance remains the same

49. a) STOICHIOMETRIC DEFECTS 3] Schottky Defect: This defect occurs in ionic solids → Equal number of cations & anions disappear from their regular positions thereby creating vacancies. → a vacancy caused by the loss of cation is always accompanied by a vacancy formed by the loss of Anion. → two voids are created by the loss of a pair of ions. This vacancy is called Schottky defect. Conditions for Schottky defects: → This defect is found in ionic comps. Having high degree of ionic character. → High Coordination number of anion → Small difference between the size of cation & anion.

50. a) STOICHIOMETRIC DEFECTS Consequences of Schottky Defect: → Number of ions decreases causing a decrease in mass with no appreciable change in volume & hence density of substance decreases. → Since the number of Cations & anion missing are the same , electrical neutrality of the substance is maintained. Eg. NaCl , AgBr , KCl show Schottky defect

51. a) STOICHIOMETRIC DEFECTS 4] FRENKEL DEFECT: In this defect in an ionic compound, an ion (cation) moves away from its regular lattice site & occupy interstitial positions between lattice points. → Frenkel defect is a combination of vacancy & interstitial defect. Conditions for Frenkel defect: 1.can be seen in ionic compounds having large difference of size between cation & anion. 2. Ions of ionic compound may be having low coordination number. Consequences of Frenkel defect: 1.Since no ions are missing from crystal structure , the density & chemical properties remain unchanged. 2. Crystal remains electrically neutral due to equal number of cations & anions Eg. Zns , AgCl , AgBr , AgI , etc. Shows this defect.

52. b) IMPURITY DEFECT This defect is due to the presence of foreign atoms in the crystal lattice. There are 2 types of impurity defect: i )Substitutional Impurity Defect: →Atoms other than the host atoms are found at the lattice sites. → The regular atoms are replaced from their lattice sites by the impurity atoms. Eg. Solid solutions of metals ( alloys) → Brass is an alloy of Cu & Zn Here the host copper atom is replaced by an impurity i.e. Zinc atom Regular site of copper atoms are replaced by zinc atoms

53. Vacancy created by an Aliovalent Impurity → Vacancies can be created by addition of aliovalent ions as impurities ( ions having oxidation state different from host ions) → Eg. Suppose small amount of SrCl 2 (impurity) is added to NaCl during crystallization . → The Sr 2+ ( ox.no . +2) occupy some of the regular sites of Na + (ox.no.+1) → To maintain electrical neutrality every Sr 2+ ions remove 2 Na + ions. → Sr 2+ ions occupies one of the vacant space caused by the removal of 2 Na + ions keeping the other Na + ion site vacant

54. b) IMPURITY DEFECT ii) Interstitial Impurity Defect: In this defect the impurity atoms occupy interstitial spaces of lattice structure. → Eg. In steel normal sites are occupied by Fe atoms & carbon atoms are present in the interstitial spaces.

55. c) NON STOICHIOMETRIC DEFECTS In non Stoichiometric defects the ratio of number of cations to anions is different from those indicated by its chemical formula. → Change in stoichiometry does not change the crystal structure. Its of 2 types: i ) Metal deficiency defect: This defect is seen in compounds of metals that show variable oxidation states. → In some crystals the positive metal ion go missing from their original sites creating an extra negative charge. → This charge is balanced by the presence of cation of the same metal but with a higher oxidation state than that of the missing cation

56. c) NON STOICHIOMETRIC DEFECTS i.Metal deficiency defect contd... Eg. Let us consider a compound NiO , where one Ni 2+ is missing & creates a vacancy. This deficiency of 2 positive charges is made up by two Ni 3+ ions at the lattice sites . Composition of NiO becomes Ni 0.97 O 1.0

57. c) NON STOICHIOMETRIC DEFECTS ii) Metal excess defect: metal excess defects are of 2 types a)A neutral atom or extra positive ion occupies interstitial position. → This defect is shown by ZnO in 2 ways: → 1. Neutral Zn atom is present in interstitial space

58. → 2. When ZnO is heated Zn 2+ ions are trapped in interstitial space & electrons also diffuse in the crystal to occupy interstitial sites

59. b) By anion vacancies ( Colour or F - centres): → This defect imparts colour to the colourless crystal. Eg. NaCl when heated in the atmosphere of sodium vapours Na atoms are deposited on crystal surface. → Chloride ion diffuse to the crystal surface creating vacancy at its original site. → This chloride ion combines with Na on the surface to form NaCl with the release of electrons from sodium atom. Na + Cl Θ → NaCl + e – → electrons released diffuse into the crystal & occupy the vacant space of anion. → These vacant sites of anion occupied by the electrons are called F-centres or colour centres Nacl thus shows yellow colour due to the formation of F- centre Non stoichiometric formula of NaCl becomes Na 1+x Cl 1.0

60. Number of particles and unit cells in x g of metallic crystal : The number of particles and the number of unit cells in given mass of a metal can be calculated from number of particles 'n' per unit cell and volume 'a 3 ' of unit cell. Density (ρ) and molar mass (M) of a metal are related to each other through unit cell parameters as: ρ = mass/volume = number of particles in unit cell x M volume of unit cell x N A ∴ ρ = n × M a 3 x N A ∴ M = ρxa 3 xN A n where 'n' is the number of particles in unit cell and 'a 3 ' is the volume of unit cell.

61. Number of particles in 'x' g metal : Since Molar mass, M, contains N A particles ∴ x g of metal contains xN A particles. M Since M = ρxa 3 xN A n substitution of M gives Number of particles in 'x' g = xN A ρ a 3 N A n = xn ρ a 3

62. Number of unit cells in 'x' g metal : Since 'n' particles Ξ to 1 unit cell ∴ xn particle Ξ xn ρ a 3 ρ a 3 /n = x ρ a 3

63. Electrical properties of solids: solids are classified into the following three categories : conductors insulators semiconductors i. Conductors : Solids having electrical conductivities in the range 10 4 to 10 7 Ohm- 1 m- 1 are called conductors. Metals and electrolytes (ionic solids) are examples of electrical conductors. Metals conduct electricity by movement of electrons Electrolytes conduct electricity by movement of ions.

64. ii. Insulators : Solids having low electrical conductivities in the range 10- 20 to 10- 10 Ohm- 1 m- 1 are called insulators. Most nonmetals and molecular solids belong to this category. iii. Semiconductors : Solids having electrical conductivities in the range 10- 6 to 10 4 Ohm- 1 m- 1 are semiconductors. This range is intermediate between conductors and insulators. Metalloids like silicon, germanium belong to this category.

65. Band theory : Conductivities of metals, non metals & metalloids can be explained by Band Theory. A band is made of closely spaced electronic energy levels. Band formation can be correlated to formation of molecular orbitals (MOs) by interaction of atomic orbitals. According to MO theory interaction of atomic orbitals of combining atoms results in formation of equal number of MOs which spread over the entire molecule. Similar to this, interaction of energy levels of electrons in the closely spaced constituent atoms in solids result in formation of bands. Band theory considers formation of two types of bands, namely, conduction band and valence band .

66. i. Conduction band : The highest energy band containing electrons is the conduction band. It is formed by interaction of the outermost energy levels of closely spaced atoms in solids. Conduction band may be partially occupied or vacant. Electrons in conduction band are mobile and delocalized over the entire solid. They conduct electricity when electrical potential is applied. ii. Valence band : The band having lower energy than conduction band is the valence band. The electrons in valence band are not free to move because they are tightly bound to the respective nuclei.

67. iii. Band gap : The energy difference between valence band and conduction band is called band gap. Size of the band gap decides whether electrons from valence band can be promoted to vacant conduction band or not when band gap is too large to promote electrons from valence band to vacant conduction band by thermal energy, it is called forbidden zone. When band gap is small, electrons from higher energy levels in valence band can be promoted to conduction band by absorption of energy (such as thermal, electromagnetic).

68. Metals : Metals are good conductors of electricity. The outermost electrons of all the atoms in the metallic crystal occupy conduction band. The number of electrons in conduction band of metals is large. Hence metals are good conductors of electricity. The conduction bands in metals can be further labelled as 's' band , overlapping s and p bands and so on. This depends on the atomic orbitals involved in band formation.

69. Band formation in metallic conductors, thus, results in delocalization of the outermost electrons of all the metal atoms leaving behind metal ions. This is described as ‘cations of metal are immersed in the sea of electrons'. The cations of metal atoms occupying lattice sites vibrate about their mean positions. At higher temperatures, metal cations undergo increased vibrational motion about their lattice sites. The flow of electrons is interrupted by increased vibrational motion. As a result conductivity of metals decreases with increase in temperature.

70. Insulators : In insulators the valence band is completely filled with electrons and the conduction band is empty. The valence band and conduction band in insulators are separated by a large energy gap called forbidden zone . Here, thermal energy is insufficient to promote electrons from valence band to conduction band. As a result the conduction band remains vacant. The material is, therefore, an insulator.

71. Semiconductors : Electrical conductivity of a semiconductor material is intermediate between that of metals and insulators. The metalloids Si and Ge are semiconductors. Like insulators, the valence band in semiconductor is completely filled with electrons and conduction band is empty. However, the energy gap between the two bands is smaller than that in an insulator. At a temperature above absolute zero a few electrons in the valence band have enough thermal energy to jump through the small band gap and occupy higher energy conduction band.

72. Semiconductors : The conduction band, thus, becomes partially filled and the valence band becomes partially empty. The electrons in conduction band are free to move. When electric potential is applied to a semiconductor, it conducts a small amount of electricity . Such a pure semiconductor material which has a very low but finite electrical conductivity is called intrinsic semiconductor.

73. The electrical conductivity of a semiconductor increases with increasing temperature. This is because, the number of electrons with sufficient energy so as to get promoted to the conduction band increases as temperature rises. Thus, at higher temperatures, there are more mobile electrons in the conduction band and more vacancies in the valence band than at lower temperature. semiconductors are insulators at low temperatures and conductors at high temperatures.

74. Extrinsic semiconductors and doping : The conductivity of a semiconductor can be increased by doping. Defn: The process of addition of minute quantity of impurities to a semiconductor to increase its conductivity is called doping. The added impurity is called dopant . A doped semiconductor, having higher conductivity than pure intrinsic semiconductor, is an extrinsic semiconductor. There are two types of extrinsic semiconductors, namely, n- type semiconductor p-type semiconductor

75. n-type semiconductor : n-type semiconductor contains increased number of electrons in the conduction bond. An n-type semiconductor is obtained by adding group 15 element to intrinsic semiconductor which belongs to group 14. For example, consider, doping of Si with phosphorus. Si has a crystal structure in which each Si atom is linked tetrahedrally to four other Si atoms. When small quantity of phosphorous is added to pure Si, the P atoms occupy some vacant sites in the lattice in place of Si atoms. The overall crystal structure of Si remains unchanged. Four of the five valence electrons of P are utilized in bonding the closest of four Si atoms. Thus, P has one extra electron than needed for bonding. Therefore, Si doped with P has more number of electrons in the conduction band than those in the conduction band in pure Si.

76. Thus the conductivity of Si doped with P is higher than that of pure Si. The electrons in conduction band move under the influence of an applied potential and conduct electricity. Since the charge carriers are the increased number of electrons, Si or Ge doped with group 15 elements such as P, As, Sb or Bi is an n-type semiconductor.

77. p-type semiconductor : A p-type semiconductor is produced by doping a pure semiconductor material (Si or Ge ) with an impurity of group 13 elements. These elements contain less number of valence electrons than that of the pure semiconductor. Consider, for example, pure Si doped with boron. The B atoms occupy normal positions of some of the Si atoms in the lattice. Boron atom has only three valence electrons. It does not have enough electrons to form bonds with its four Si neighbours . B atom forms bonds with three Si atoms only. The missing fourth electron creates an electron vacancy. It is called a hole.

78. A hole has a tendency to accept electron from its close vicinity. Thus, a hole behaves as if it has a positive charge. The electrons in partially filled valence band move under the influence of an applied potential. The holes move in the opposite direction. Since the charge carriers are holes which behave like positive charge, the Si or Ge doped with group 13 elements like B, Ga or In (indium), is a p-type semiconductor.

79. MAGNETIC PROPERTIES OF SOLIDS 1.Diamagnetic solids : The substance in which all electrons are paired weakly repel in magnetic field . Such substances are called diamagnetic substances. Spinning of paired electrons balances the spins & cancels the magnetic moment. Eg. Nitrogen , Flourine , NaCl , Water & benzene are diamagnetic 2.Paramagnetic Solids: Substance with unpaired electrons are weakly attracted by the magnetic field. These are called paramagnetic substances. Spinning of unpaired electrons gives rise to magnetic moment . These substance exhibit magnetism in magnetic field but loses magnetic properties once the external magnetic field is removed Eg. Oxygen, Cu 2+, Fe 3+ ,Cr 3+ are paramagnetic in nature.

80. MAGNETIC PROPERTIES OF SOLIDS 3) Ferromagnetism: Substances having large number of unpaired electrons are attracted strongly by magnetic field. These substances are called ferromagnetic substances. These substances can be permanently magnetised as they retain magnetic properties even after the removal of external magnetic field. Eg. Fe, Co, Ni, CrO 2 , Gd ( gadolium )

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