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1. THE SOLID STATE, , HAIZEL G. ROY, H.S.S.T. (HG) CHEMISTRY, GOVT. H.S.S. KALAMASSERY, ERNAKULAM
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GENERAL CHARACTERISTICS OF SOLIDS, Have definite mass, volume and shape., Intermolecular distances are short., Intermolecular forces are strong., Constituent particles have fixed positions., They can only oscillate about their mean positions., Incompressible and rigid.
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CLASSIFICATION OF SOLIDS, Solids can be classified into two types. They are, Crystalline Solids, Amorphous Solids
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DIFFERENCE BETWEEN CRYSTALLINE AND AMORPHOUS SOLIDS
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ANISOTROPY, The, , physical, , properties, , like, , electrical, , resistance or refractive index show different, values when measured along different, directions in the same crystal., The arrangement of particles is different in, different directions.
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ISOTROPY, , The value of any physical property would, be same along any directions., No long range order and the arrangement is, irregular along all the directions.
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PSUEDO SOLIDS OR SUPER COOLED LIQUIDS, , Amorphous solids have a tendency to flow very slowly., Therefore, they are called pseudo solids or super cooled liquids.
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NOTE-1, Glass objects from ancient civilizations are milky in appearance, due to crystallization., Glass is an amorphous solid., On heating, glass become crystalline at some temperature.
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NOTE-2, Glass panes fixed to windows or doors of old buildings are found to, be slightly thicker at the bottom than at the top., Glass flows down very slowly and makes the bottom portion, slightly thicker.
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CLASSIFICATION OF CRYSTALLINE SOLIDS, Classification is based on the intermolecular forces., Crystalline solids are classified into four. They are, Molecular Solids, Ionic Solids, Metallic Solids, Covalent solids
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MOLECULAR SOLIDS, In molecular solids, molecules are the constituent particles., Molecular solids are classified into three types. They are, 1. Non Polar Molecular Solids, 2. Polar Molecular Solids, 3. Hydrogen Bonded Molecular Solids
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A. NON POLAR MOLECULAR SOLIDS, The atoms or molecules are held by weak dispersion forces or, London forces., They are Soft and non-conductors of electricity., They have low melting points., Exist in liquid or gaseous state at room temperature and pressure., Eg: H2, Cl2, I2 etc.
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B. POLAR MOLECULAR SOLIDS, The molecules are held together by dipole-dipole interactions., They are soft and non-conductors of electricity., Their melting points are higher than those of non polar molecular, solids., Eg: Solid SO2 and Solid NH3.
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C. HYDROGEN BONDED MOLECULAR SOLIDS, The molecules are held together by strong hydrogen bonding., They are non conductors of electricity., They are generally volatile liquids or soft solids under room, temperature and pressure., Eg: H2O.
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2. IONIC SOLIDS, Ions are the constituent particles., Formed by the 3 dimensional arrangements of, cations, , and, , anions, , bound, , by, , strong, , electrostatic forces., These solids are hard and brittle in nature., They have high melting and boiling points., They are insulators in solid state., In aqueous solutions, they conduct electricity.
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3. METALLIC SOLIDS, Consist of +ve ions in a sea of mobile, electrons., Held together by strong electrostatic force, of attraction., They are malleable and ductile., Good conductors of heat and electricity.
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4. COVALENT OR NETWORK SOLIDS, , The atoms are held together by strong, covalent bonds., They are very hard and brittle., They are insulators., Eg: Diamond, Silicon Carbide etc.
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NOTE - 3, Graphite is soft and conductor of electricity., The free electrons make graphite a good, conductor of electricity., Graphite is a soft solid and a good solid, lubricant., Because different layers can slide one over, the other.
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CRYSTAL LATTICE, , A regular 3 dimensional, arrangement of points in, space is called a crystal, lattice.
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CHARACTERISTICS OF CRYSTAL LATTICE, Each point in a lattice is called lattice point or lattice site., Lattice site represents one constituent particle., The constituent particle may be an atom, a molecule or an ion., Lattice points are joined by straight lines which give the geometry of the lattice.
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UNIT CELL, , Unit, , cell, , is, , the, , smallest, , repeating unit of a crystal, lattice which when repeated in, different direction generates, the entire crystal.
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CHARACTERISTICS OF UNIT CELL, The dimensions are along the three edges a, b and c., These edges may or may not be mutually perpendicular., Angles between the edges a, b and c are α, β and γ., A unit cell is characterized by six parameters a, b, c, α, β and γ.
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PRIMITIVE AND CENTERED UNIT CELLS
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PRIMITIVE UNIT CELL OR SIMPLE CUBIC, , The constituent particles are present at all the 8 corners of a cube.
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CENTERED UNIT CELLS
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BODY CENTERED CUBIC (BCC), The constituent particles are present at all the corners as well as at, the centre of the unit cell.
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FACE CENTERED CUBIC (FCC), The constituent particles are present at all the corners as well as at, the centre of each of the six faces.
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END CENTERED CUBIC, The constituent particles are present at all the corners as well as at, the centre of any two opposite faces.
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SEVEN PRIMITIVE UNIT CELLS
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BRAVAIS LATTICES, The 14 possible 3 dimensional lattices in which the atoms are arranged to form a crystal, are called Bravais lattices., These lattices are named after the French physicist Auguste Bravais.
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NUMBER OF ATOMS IN A UNIT CELL
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NUMBER OF ATOMS IN A UNIT CELL, , Simple cubic has one atom per unit cell., Body Centered Cubic has 2 atoms per unit cell., Face Centered Cubic unit cell has 4 atoms per unit cell.
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CO-ORDINATION NUMBER, , The number of nearest neighbour’s with which a given sphere is in, contact.
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CLOSE PACKED STRUCTURES, , A close packing is a way of arranging equidimensional object in, space., The available space is filled very effectively.
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CLOSE PACKING IN ONE DIMENSION, , Each sphere is in contact with two of its neighbour’s., The co-ordination number is 2.
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CLOSE PACKING, IN TWO DIMENSIONS
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A. SQUARE CLOSE PACKING, , The particles of second, third, fourth etc, are arranged vertically with the particles, of the first row., Each particle is in contact with four other, neighbouring particles., So it is called square close packing., Its co-ordination number (CN) is 4.
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B. HEXAGONAL CLOSE PACKING, The particles of the second row are, arranged in the depressions produced, by the particles of the first row., The particles in the third row will be, vertically aligned with those in the, first row., Each sphere is in contact with six other, spheres to form a hexagonal pattern., So it is called hexagonal close packing., Its coordination number is 6.
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CLOSE PACKING, IN THREE DIMENSIONS
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A. HEXAGONAL CLOSE PACKING, , The particles of every third layer are in vertical, alignment with those of the first layer., The third layer is a repetition of the first layer., This will form the sequence AB AB AB AB ……., This type of packing is called AB AB packing or, hexagonal close packing (hcp)., Eg: Mg, Zn, Cd etc.
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CO-ORDINATION NUMBER OF HCP STRUCTURE, , The Each atom is positioned in the empty space, formed by, Three adjacent atoms of the top layer, Three adjacent atoms in the bottom layer, It is surrounded by six neighbouring atoms., Thus, 12 atoms are in contact with each atom., Hence, Co-ordination Number is 12
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B. CUBIC CLOSE PACKING, The particles of every fourth layer are in, vertical alignment with those in the first layer., The fourth layer is the repetition of the first, layer., This will give rise to ABC ABC ABC ………….., sequence., This type of packing is called ABC ABC packing, or cubic close packing (ccp)., Eg: Cu, Ni, Au etc.
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CO-ORDINATION NUMBER OF CCP STRUCTURE, In CCP, there are three repeating layers of hexagonally, arranged atoms., Each atom is in contact with six atoms in its own layer., Three atoms are in contact with the layer above., And three atoms are in contact with the layer below., In this arrangement, each atom touches 12 of its nearest, neighbours., Hence, Co-ordination Number is 12
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INTERSTITIAL SITES OR VOIDS, The vacant space between the constituent particles in a closed, packed structure is called a void.
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TYPES OF VOIDS, Voids are classified into three types. They are, Triangular Voids, Tetrahedral Voids, Octahedral Voids
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TRIANGULAR VOID, The empty space produced in between three spheres in a close, packed structure is called a triangular void.
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TETRAHEDRAL VOID, The void formed by the close packing of four spheres which touch, each other at only one point is called a tetrahedral void.
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PACKING EFFICIENCY, It is the percentage of total space filled by the particles.
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PACKING EFFICIENCY IN A SIMPLE CUBIC CELL
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PACKING EFFICIENCY IN A BODY CENTERED CUBIC CELL
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RELATIONSHIP BETWEEN EDGE LENGTH (a), AND RADIUS (r) OF A SPHERE
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CALCULATION OF DENSITY OF A CRYSTAL
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FORMULA OF A COMPOUND
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IMPERFECTIONS, IN SOLIDS
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IDEAL CRYSTAL, In an ideal crystal the constituent particles are regularly arranged, throughout the crystal., , IMPERFECTION, Any deviation from the completely ordered arrangement in a crystal, is called a defect or imperfection.
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POINT DEFECTS, The irregularities or deviations from ideal arrangement around a, point or an atom in a crystalline substance., , LINE DEFECTS, The irregularities or deviations from ideal arrangement in entire, rows of lattice points.
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TYPES OF POINT DEFECTS, Point defects can be classified into three types., 1. Stoichiometric Defects, The number of +ve and ―ve ions are exactly in the ratio indicated by the, chemical formulae., 2. Non Stoichiometric Defects, The number of +ve and ―ve ions are not exactly in the ratio indicated by the, chemical formulae., 3. Impurity Defects, This type of defect arises due to the presence of some impurities in the crystal, lattice.
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SCHOTTKY DEFECT, Equal number of +ve and ―ve ions are missing from the lattice, site., Found in crystals of high co-ordination number., It decreases the density of the crystal., Shown by ionic substances in which the cation and anion are, almost similar sizes., The crystals are electrically neutral., Eg: NaCl, KCl, AgBr etc.
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FRENKEL DEFECT, A cation is dislocated from its normal site to an interstitial, site., Found in crystals having low coordination number. It does not, change the density of the solid., Shown by ionic substances in which there is a large difference, in the size of ions., Eg: ZnS, AgCl, AgBr and AgI
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AgBr shows both Schottky as well as Frenkel defect. Why?, The radius ratio of AgBr is intermediate., Schottky defect arise due to missing of ions from their lattice site, Frenkel defect arise when the missing ions occupy interstitial sites., In AgBr, Ag + ion is small in size., When removed from lattice site, they can occupy interstitial site., Therefore, it shows both frenkel and schottky defect.
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1. METAL EXCESS DEFECT
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A. DUE TO ANION VACANCY, A ―ve ion may be absent from its lattice site leaving a hole., This hole is occupied by an electron and the electrical neutrality is, maintained., The electrons entrapped in this anion vacancy are called ‘F’ centres., They impart colour to the crystals., The colour is due to the excitation of electrons which absorb energy, from the visible light falling on the crystals., Eg: Alkali halides like NaCl and KCl show this type of defect.
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When crystals of NaCl are heated in an atmosphere of Na vapour,, yellow colour is observed. Why?, , When heated, the Na atoms are deposited on the surface of the crystal., The Cl― ions diffuse to the surface of the crystal and combine with Na, atoms to give NaCl., This happens by the loss of electrons by Na atoms to form Na + ions., The released electrons diffuse into the crystal and occupy anionic sites., As a result, the crystal now has excess of Na., The anionic sites occupied by unpaired electrons are called F-centres., They impart yellow colour to the crystals of NaCl.
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B. DUE TO EXTRA CATION, An extra +ve ion occupies an, interstitial position., The, , electrical, , maintained, present, position., , in, , by, , neutrality, an, , another, , is, , electron, interstitial
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ZnO turns yellow on heating. Why?, , ZnO is white in colour at room temperature., On heating it loses oxygen and turns yellow., The excess Zn2+ ions move to interstitial sites., The electrons moves to neighbouring interstitial sites., The yellow colour is due to the Metal Excess Defect due to the presence of, Extra Cation on the interstitial position.
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2. METAL DEFICIENCY DEFECT
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A. DUE TO CATION VACANCY, , A cation is absent from its, lattice site., The electrical neutrality is, maintained, , by, , charge, , the, , metal., , on, , an, , extra, , adjacent
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B. DUE TO EXTRA ANION, , An extra anion occupies an, interstitial position., The, , electrical, , neutrality, , is, , maintained by an extra charge, on the adjacent cation.
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PROPERTIES OF SOLIDS
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ELECTRICAL PROPERTIES, OF SOLIDS
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CONDUCTORS, , Conductors are substances which allow electric current to flow through it., Metals are good conductors of electricity., In metals, the conduction is due to the movement of electrons under the, influence of an applied electric potential.
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SEMI CONDUCTORS, , Substances which allow electric current to flow through it partially., They have conductivity in between metals and Insulators., , INSULATORS, , These are substances which do not allow electric current to flow through, it.
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BAND THEORY OF SOLIDS, , Valence band is occupied by valence electrons., Conduction band is the empty band above the valence band.
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CONDUCTORS, , In a conductor, valence band overlaps the conduction band., Therefore, no energy is required to move an electron from the valence, band to the conduction band.
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SEMI CONDUCTORS, In In semiconductors, the gap between the valence band and, conduction band is small., Therefore, some electrons may jump from valence band to conduction, band, It shows some conductivity.
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INSULATORS, , In an insulator, the valence band and conduction bands are not, overlapped., There is a large gap in between the valence and conduction, bands., The, , electrons, , cannot, , conduction band., , jump, , from, , the, , valence, , band, , to
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CLASSIFICATION OF SEMICONDUCTORS, , INTRINSIC SEMICONDUCTOR, Semiconductors in their extremely pure state are very poor conductors of, electricity., They are called intrinsic semiconductors., Eg: Ge, Si., , EXTRINSIC SEMICONDUCTOR, With the addition of certain other elements in the crystal structure of, semiconductors, their conductivity can be improved., They are called extrinsic semiconductors.
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DOPING, The process of adding certain impurities in the crystal structure, of a semiconductor to improve its conductivity is called doping.
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N-TYPE SEMI CONDUCTOR, A semiconductor crystal having an excess of electrons by doping., The donor atoms are 15th group elements like P, As, Sb, Bi., Electrons are the charge carriers., When a pure semiconductor is doped by pentavalent impurity (P,, As, Sb, Bi) then, 4 electrons out of 5 valence electrons bonds with, the four electrons of Ge or Si., The fifth electron of the dopant is set free., The impurity atom donates a free electron for conduction in the, lattice and is called “Donor”., Eg. Silicon dopped with P, As, Sb and Bi., Eg: Germanium dopped with P, As, Sb and Bi.
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P-TYPE SEMI CONDUCTOR, A semiconductor material having an excess of holes by doping., Holes are the charge carriers., The donor atoms are 13th group elements like B, Al, Ga, In and, Tl (Trivalent Impurity)., When a pure semiconductor is doped with a trivalent impurity, then, the three valence electrons of the impurity bonds with, three of the four valence electrons of the semiconductor., This leaves an absence of electron (hole) in the impurity., These impurity atoms which are ready to accept bonded, electrons are called “Acceptors”., Eg: Silicon dopped with Boron, Silicon dopped Aluminium,, Germanium dopped with Boron etc., , 000
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MAGNETIC PROPERTIES, OF SOLIDS
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DIAMAGNETISM, Substances which are weakly repelled by the magnetic field, are called diamagnetic substances., Shown by those substances in which all the electrons are, paired and there are no unpaired electrons., Eg: H2O, Alcohol, C6H6, NaCl etc.
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FERROMAGNETISM, The substances which are strongly attracted by the magnetic, field are known as ferromagnetic substances., They show permanent magnetism even when the magnetic, field is removed., The magnetic moments are in the same direction., Eg: Fe, Co, Ni, Gd, CrO2 etc.
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CURIE TEMPERATURE, Curie point or Curie Temperature is the temperature at which certain magnetic, materials undergo a sharp change in their magnetic properties., Above this temperature, the magnetic materials lose their ferromagnetic, properties., At lower temperatures, the magnetic dipoles are aligned., Above the curie temperature, random thermal motions cause misalignment of, the dipoles.
