ARBIND SINGH ACADEMY Solid State Introduction Intermolecular forces and thermal energy are the two factors on which physical states of matter depend. While the intermolecular forces of attraction tend to keep the particles closer; the thermal energy tends to keep the particles apart from each other by making them move faster. When the net resultant of these two opposing forces, i.e. intermolecular forces and thermal energy, makes the particles cling together and forces them to occupy fixed positions, matters exist in solid state. Characteristic properties of solid state: a. Solids have definite mass, volume and shape b. Solids are incompressible and rigid c. In solids, intermolecular distances are very short d. In solids, intermolecular forces are very strong e. The constituent particles of solids have fixed positions. f. The constituent particles of solids can only oscillate about their mean positions. Classification of solids – Solids can be classified into two types on the basis of the arrangements of their constituent particles (atoms, molecules or ions). These two types are Crystalline Solid and Amorphous Solid. Crystalline Solid Solids having large number of crystals; each with definite characteristic geometrical shape; are called crystalline solids. The constituent particles of crystalline solid are arranged in regular pattern which is repeated periodically over the entire crystal. Such type of arrangement is called long range order. Crystalline solids are anisotropic in nature, i.e. many physical properties, such as electrical resistance, refractive index, etc. www.arbindsingh.com B-1 /44 , Sector-18 , Rohini ,New Delhi-110089 Ph-9999906400 Page 1
Transcript
ARBIND SINGH ACADEMY
Solid State Introduction
Intermolecular forces and thermal energy are the two factors on which physical states of matter depend. While the intermolecular forces of attraction tend to keep the particles closer; the thermal energy tends to keep the particles apart from each other by making them move faster.
When the net resultant of these two opposing forces, i.e. intermolecular forces and thermal energy, makes the particles cling together and forces them to occupy fixed positions, matters exist in solid state.
Characteristic properties of solid state:
a. Solids have definite mass, volume and shapeb. Solids are incompressible and rigidc. In solids, intermolecular distances are very shortd. In solids, intermolecular forces are very stronge. The constituent particles of solids have fixed positions.f. The constituent particles of solids can only oscillate about their mean positions.
Classification of solids – Solids can be classified into two types on the basis of the arrangements of their constituent particles (atoms, molecules or ions). These two types are Crystalline Solid and Amorphous Solid.
Crystalline Solid
Solids having large number of crystals; each with definite characteristic geometrical shape; are called crystalline solids.
The constituent particles of crystalline solid are arranged in regular pattern which is repeated periodically over the entire crystal. Such type of arrangement is called long range order. Crystalline solids are anisotropic in nature, i.e. many physical properties, such as electrical resistance, refractive index, etc. are different along different axes. Crystal of NaCl, Quartz, Ice, HCl, Iron, etc. are some examples of crystalline solid.
a. Crystalline solids have definite characteristic geometrical shape.b. Crystalline solids have sharp characteristic melting point.c. Crystalline solids have definite and characteristic heat of fusion.d. Crystalline solids produce pieces with plain and smooth surface when cut with a
tool of sharp edge.e. Crystalline solids are anisotropic in nature.f. Crystalline solids are true solid.g. Constituent particles of crystalline solids are arranged in long range order.
Amorphous Solid
Solids having irregular shapes of particles are known as Amorphous Solids. The word ‘Amorphous’ came from Greek ‘Amorphos’ which means no shape.
The constituent particles of amorphous solids have only short range order of arrangement, i.e. regular and periodical arrangement of particles is seen to a short distance only. The structures of amorphous solids are similar to that of liquids. Glass, rubber, plastics, etc. are some of the examples of amorphous solids. Amorphous solids are isotropic in nature, i.e. physical properties of amorphous solids are same in all directions.
In old buildings, it is often seen that glasses of windows get slightly thickened at bottom, this happens because glass which is an amorphous solid; flows down very slowly. Some very old glasses get milky appearance because of some crystallization. This happens because on heating, glasses get crystallized at some temperature. This is the cause; amorphous solids are also known as Pseudo Solids or Super Cooled Liquids.
Characteristic of amorphous solid –
a. Particles of amorphous solids are irregular in shape.b. Amorphous solids soften gradually over a range of temperature.c. Amorphous solids produce pieces of irregular shapes when they are cut into two
pieces.d. Amorphous solids do not have definite heat of fusion.e. Amorphous solids are isotropic in nature, i.e. they have same physical properties
in all directions.f. Amorphous solids are not true solids and hence these are also known as Pseudo
Solid or Super Cooled Liquid.g. The arrangement of constituent particles is in short range order.
Amorphous solids are very useful but most of the solids are crystalline in nature. Crystalline solids are classified into four types; based on the intermolecular forces operating in them.
1 - Molecular Solids – Solids having molecules as their constituent particles are called Molecular solids. For, example, Hydrogen, Chlorine, Water, HCl, solid carbon dioxide, sucrose, etc.
Molecular solids are classified into three types on the basis of their bond:
(a) Non Polar Molecular Solids – Solids which are comprised of atoms only, such as helium and argon or molecules; formed because of the non polar covalent bonds are known as Non-Polar Molecular Solids. For example – H2, Cl2, I2, etc.
Characteristic of Non-Polar Molecular Solids –
The molecules of non-polar molecular solids are held together by weak dispersion forces or London forces.
Non-Polar Molecular Solids are soft. Non-polar molecular solids are non-conductor of electricity. Non-polar molecular solids have low melting points. Non-polar molecular solids are usually in liquid or gaseous state at the room
temperature and pressure.
(b) Polar Molecular Solids – The solids which are formed by polar covalent bonds are known as Polar Molecular solids. For example – HCl, SO2, NH3, etc.
Characteristic of Polar Molecular Solids –
The molecules in polar molecular solids are held together with dipole-dipole interactions.
Polar molecular solids are generally soft in nature. Polar molecular solids are non-conductor of electricity. Polar molecular solids have higher melting points in comparison to non-polar
Most of the polar molecular solids are gases or liquids at room temperature and pressure.
Solid SO2 and solid NH3 are some examples of polar molecular solids.
(c) Hydrogen bonded Molecular Solids – The molecules of hydrogen bonded molecular solids contain polar covalent bond between H and O, F or N. In solids such as H2O (ice) molecules are bound together strongly with hydrogen bond. HF, H2O (ice), etc are the examples of hydrogen bound molecular solids.
Characteristics –
Hydrogen bound molecular solids are generally volatile liquid or soft solids at room temperature and pressure.
Hydrogen bound molecular solids are non-conductor of electricity.
2 - Ionic Solids – Solids, in which ions are the constituent particles, are called ionic solids. These solids are formed because of three dimensional arrangements of cations and anions bound together with strong electrostatic forces (coulombic forces). For example NaCl.
Characteristics of Ionic Solids –
High melting and boiling points. Non-conductor of electricity in solid state. Conductor of electricity in molten state. Conducted electricity when dissolved in water.
3 - Metallic Solids – All metals are referred as Metallic solids. Their constituent particles are positive ions. These positive ions are surrounded by free moving electrons. For example – iron, aluminium, etc.
Characteristics –
High melting points. Good conductors of electricity and heat. Lustrous, and are of specific colors. Hard but malleable and ductile in nature
4 - Covalent Solids – Crystalline solids are formed by non metals because of formation of covalent bonds between the adjacent molecules throughout the crystal. These are also known as Network Solids. These are also called giant molecules. For example – diamond, graphite, silicon carbide, etc.
Characteristic of Covalent Solids –
They are very hard and brittle except graphite which is soft.
Very high melting points. Do not conduct electricity except graphite. Also called giant molecules.
Crystal Lattices and Unit Cells
Crystal lattice is the depiction of three dimensional arrangements of constituent particles (atoms, molecules, ions) of crystalline solids as points. Or the geometric arrangement of constituent particles of crystalline solids as point in space is called crystal lattice.
Characteristics of crystal lattice:
Each constituent particle is represented by one point in a crystal lattice. These points are known as lattice point or lattice site. Lattice points in a crystal lattice are joined together by straight lines. By joining the lattice points with straight lines the geometry of the crystal lattice is
formed.
Unit Cell – The smallest portion of a crystal lattice is called Unit Cell. By repeating in different directions unit cell generates the entire lattice.
A unit cell is characterized by six parameters. These parameters are three edges (a, b and c) and angles between them (α, β and γ).
Dimensions along the edges of a unit cell is represented by a, b and c. Edges of unit cell may or may not be mutually perpendicular. The angle between b and c is represented by α, between a and c by β and
between a and b by γ.
Types of Unit Cell: - There are two types of unit cells – Primitive and Centred Unit Cells.
Primitive Unit Cells: – When particles in unit cell are present only at the corners, it is called the primitive unit cell.
Centred Unit Cells: – When particles are present at other positions in addition to those at corners in a unit cell, it is called a Centred Unit Cell.
There are three types of Centred Unit Cell.
(a) Body Centred Unit Cells: – If one constituent particle lies at the centre of the body of a unit cell in addition to the particles lying at the corners, it is called Body-Centred Unit Cell.
(b) Face-Centred Unit Cells: – If one constituent particle lies at the centre of each face besides the particles lying at the corner, it is known as Face-Centred Unit Cells.
(c) End-Centred Unit Cell: – If one constituent particle lies at the centre of any two opposite faces besides the particles lying at the corners, it is known as End-Centred Unit Cell. It is also known as base-centred unit cell.
There are seven types of unit cell formed. These are Cubic, Tetragonal, Orthorhombic, Monoclinic, Hexagonal, Rhombohedral or Trigonal and Triclinic.
Bravais Lattices
There are only 14 possible crystal lattices, which are called Bravais Lattices.
Cubic Lattice – There are three types of lattice possible for cubic lattice.
Primitive or Simple, Body centred, Face centred lattices. In these types of lattices all sides are of equal length. The angles between their faces are 900 in a cubic lattice.
Tetragonal Lattice – There are two possible types of tetragonal lattices. Primitive and Body centred unit cells. In these lattices one side is different in length and angles between faces are equal to 900.
Orthorhombic Lattice – Four types of orthorhombic lattice are possible. They are Primitive, End-centred, Body centred and Face centred. They have unequal sides. The
Monoclinic Lattice – There are two possible types of monoclinic lattice. They are Primitive and End centred. They have unequal sides and two faces have angles other than 900.
Hexagonal lattice – Hexagonal lattice is of one type only. It has one side is different in length to the other two and the angles on two faces are 600.
Rhombohedral Lattice – Only one type of lattice is possible for Rhombohedral lattice. It has all sides equal and angles on two faces are less than 900.
Triclinic Lattice – Triclinic lattice has only one type of lattice. It has unequal sides and none of the angles between faces are equal to 900.
A crystal lattice is made of very large number of unit cells and lattice points are the representation of constituent particles. Therefore, the number of atoms in a unit cell of a crystal lattice can be calculated.
Number of atoms in Primitive Cubic Unit Cell –
In primitive unit cell, atoms are present at corners only. In a crystal lattice every corner is shared by eight adjacent unit cells. Therefore, only 1/8 of an atom, or other constituent particles, belong to a particular unit cell.
Therefore,
Since, there are 8 atoms present in a unit cell on every corner,
Thus, 1 atom is present in a Primitive Cubic Unit Cell.
Body Centred Cubic (bcc) Unit Cell – There are eight atoms at each corner and one atom present at the centre of body in a body centred cubic (bcc) unit cell.
Therefore, the number of atoms present in a Body Centred Cubic (bcc) Unit Cell
Face – Centred Cubic (fcc) Unit Cell –
In a face centred cubic unit cell, there are eight atoms present at each corner. A cube has six faces, therefore total six atoms are present at the centre of each of the face.
Each atom present at corners is shared by adjacent eight atoms and each atom present at the centre of face is shared between adjacent two atoms.
Matters exist in solid state because of close packing of their constituent particles. There are two types of close packing found in solids. These are Cubic Close Packed (ccp) and Hexagonal Close Packed (hcp) lattice.
Cubic Close packed (ccp):
In this type of packing, the spheres of molecules are adjacent to each other that each row of spheres in a particular dimension is a repetition of the pervious row. The spheres of a particular row don’t fit in the depressions between two adjacent spheres of the previous row. This types of arrangement is called AAAA type arrangement. This is also known as face centered cubic (fcc). This type of close packing of constituent particles is found in metals like copper, silver, etc.
Lattice of this cubic close packed is simple cubic and its unit cell is primitive cubic unit cell.
Hexagonal Close packed (hcp):
In this type of packing, the spheres of molecules of a particular row in a particular dimension are in a position that they fit into depressions between adjacent spheres of the previous row. This type of arrangement is called ABAB type arrangement. This type of packed lattice is found in many metals such as magnesium, zinc, etc.
Coordination number: The number of adjacent particles of atoms is called coordination number.
In both ccp and hcp, each sphere is surrounded by 12 adjacent atoms, thus coordination number is equal to 12 in each case.
Formation of voids in close packing:
Empty space left after the packing is called void. Two types of voids are formed in ccp and hcp structures. These are tetrahedral voids and octahedral voids.
Tetrahedral voids are formed because of formation of tetrahedron between the layers of atoms. Thus, voids in the shape of tetrahedron are called tetrahedral voids.
Octahedral voids are formed because of formation of octahedron between the layers of atoms. Thus, voids in the shape of octahedron are called octahedral voids.
Number of voids:
The number of formation of voids depends upon the number of close packed spheres. The number of tetrahedral voids is formed twice as the number of octahedral voids while close packing of atoms in ccp and hcp structures.
Thus, if number of close packed spheres is equal to ‘N’.
Therefore, number of octahedral voids formed = N
And, the number of tetrahedral voids formed = 2N
Formula of a compound and number of voids filled:
Bigger ions, usually anions, form close packed structure and smaller ions, usually cations occupy the voids in ionic solids. If cations are bigger in size, they occupy octahedral voids and if are smaller enough then they occupy tetrahedral voids.
The occupation of number of voids depends upon the chemical formula of compound. It may be possible to occupy all the voids or fraction of voids.
Example –
(a) If cation of an ionic solid occupies all the octahedral voids, then the formula of the compound can be obtained as follows:
Let ‘A’ are cations and ‘B’ are anions in the compound.
Since the number of close packed sphere is equal to the number of octahedral voids formed, thus the cations and anions must be in the ratio of 1:1.
Therefore, A and B will be combined in the ratio of A:B.
(b) If there are two ions A and B in an ionic compound and cations occupy all the tetrahedral voids formed because of close packing, then the formula of the compound can be obtained as follows:
Let A is the cation and B is the anion in given compound.
Since, number of tetrahedral voids formed = 2 X number of close packed spheres.
This means A and B will combined in the ratio of 1:2
Therefore, formula of the compound will be AB2
Packing Efficiency of Close Packed Structure - 1
Both ccp and hcp are highly efficient lattice; in terms of packing. The packing efficiency of both types of close packed structure is 74%, i.e. 74% of the space in hcp and ccp is filled. The hcp and ccp structure are equally efficient; in terms of packing.
The packing efficiency of simple cubic lattice is 52.4%. And the packing efficiency of body centered cubic lattice (bcc) is 68%.
Calculation of pacing efficiency in hcp and ccp structure:
The packing efficiency can be calculated by the percent of space occupied by spheres present in a unit cell.
Let the side of an unit cell = a And diagonal AC = b
In body centered cubic unit cell, one atom is present in body center apart from 4 atoms at its corners. Therefore, total number of atoms present in bcc unit cell is equal to 2.
Let a unit cell of bcc structure with side a.
Let FD (diagonal) = b and diagonal AF = c
Let the radius of atom present in unit cell = r
Now, in ∆EFD
After subtituting the value of a from equation (vi) we get
Thus,packing efficiency of bcc structure=68%
Packing Efficiency of Close Packed Structure - 2
Packing efficiency in Simple Cubic Lattice:
A unit cell of simple cubic lattice contains one atom.
Where, d is density, z is number of atoms present in unit cell, a is length of edge, and NA is Avogadro constant.
Above expression has five parameters, d, z, a, m and NA . By knowing any four of them fifth can be calculated.
Imperfections in Solids or Crystal defects
Irregularity in the arrangement of constituent particles in solids is called crystal defect or imperfection in solids. There are two types of crystal defects - Point Defects and Line Defects.
Point Defects: Irregularities or deviation from ideal arrangement of constituent particles around the point or atom in a crystalline solid is known as point defects.
Line Defects: Irregularities or deviation from ideal arrangement of constituent particles in entire row of lattice is known as line defects.
Point Defects: Point Defects are divided into three types:
(i) Stoichiometric Defects
(ii) Impurities Defects
(iii) Non-stoichiometric Defects
(i) Stoichiometric Defects: – It is a type of point defects which does not disturb the stoichiometry of solid. This is also known as Intrinsic or Thermodynamic Defects.
Vacancy defects and Interstitial defects are found in non-ionic compounds while similar defects found in ionic compounds are known as Frenkel Defects and Schottky Defects.
(a) Vacancy Defects: When some lattice sites left vacant while the formation of crystal, the defect is called Vacancy Defects.
In vacancy defects, an atom is missing from its regular atomic site. Because of missing of atom the density of substance decreases, i.e. because of vacancy defects.
The vacancy defect develops on heating of substance.
(b) Interstitial Defects: - Sometime in the formation of lattice structure some of the atoms occupy interstitial site, the defect arising because of this is called Interstitial Defects.
In interstitial defect, some atoms occupy sites at which; generally there is no atom in the crystal structure. Because of the interstitial defects, the number of atoms becomes larger than the number of lattice sites.
Increase in number of atoms increases the density of substance, i.e. interstitial defects increase the density of substance.
The vacancy defects and interstitial defects are found only in non-ionic compounds. Such defects found in ionic compounds are known as Frenkel Defects and Schottky Defects.
(c) Frenkel Defects: It is a type of vacancy defect. In ionic compounds, some of the ions (usually smaller in size) get dislocated from their original site and create defect. This defect is known as Frenkel Defects. Since this defect arises because of dislocation of ions, thus it is also known as Dislocation Defects. As there are a number of cations and anions (which remain equal even because of defect); the density of the substance does not increase or decrease.
Ionic compounds; having large difference in the size between their cations and anions; show Frenkel Defects, such as ZnS, AgCl, AgBr, AgI, etc. These compounds have smaller size of cations compared to anions.
(d) Schottky Defects: Schottky Defect is type of simple vacancy defect and shown by ionic solids having cations and anions; almost similar in size, such as NaCl, KCl, CsCl, etc. AgBr shows both types of defects, i.e. Schottky and Frenkel Defects.
When cations and anions both are missing from regular sites, the defect is called Schottky Defect. In Schottky Defects, the number of missing cations is equal to the
number of missing anions in order to maintain the electrical neutrality of the ionic compound.
Since, Schottky Defects arises because of mission of constituent particles, thus it decreases the density of ionic compound.
(ii) Impurities Defects: Defects in ionic compounds because of replacement of ions by the ions of other compound is called impurities defects.
In NaCl; during crystallization; a little amount of SrCl2 is also crystallized. In this process, Sr++ ions get the place of Na+ ions and create impurities defects in the crystal of NaCl. In this defect, each of the Sr++ ion replaces two Na+ ions. Sr++ ion occupies one site of Na+ ion; leaving other site vacant. Hence it creates cationic vacancies equal number of Sr++ ions. CaCl2, AgCl, etc. also shows impurities defects.
(iii) Non-stoichiometric Defects: There are large numbers of inorganic solids found which contain the constituent particles in non-stoichiometric ratio because of defects in their crystal structure. Thus, defects because of presence of constituent particles in non-stoichiometric ratio in the crystal structure are called Non-stoichiometric Defects.
Non-stoichiometric Defects is mainly of two types – Metal Excess Defects and Metal Deficiency Defects.
Metal Excess Defects: Metal excess defects are of two types:
(a) Metal excess defects due to anionic vacancies:
These type of defects seen because of missing of anions from regular site leaving a hole which is occupied by electron to maintain the neutrality of the compound. Hole occupied by electron is called F-centre and responsible for showing colour by the compound.
This defect is common in NaCl, KCl, LiCl, etc. Sodium atoms get deposited on the surface of crystal when sodium chloride is heated in an atmosphere of sodium vapour. In this process, the chloride ions get diffused with sodium ion to form sodium chloride. In this process, sodium atom releases electron to form sodium ion. This released electron gets diffused and occupies the anionic sites in the crystal of sodium chloride; creating anionic vacancies and resulting in the excess of sodium metal.
The anionic site occupied by unpaired electron is called F-centre. When visible light falls over the crystal of NaCl, the unpaired electron present gets excited because of absorption of energy and impart yellow colour.
Because of similar defect if present, crystal of LiCl imparts pink colour and KCl imparts violet.
(b) Metal excess defect due to presence of extra cations at interstitial sites:
Zinc oxide loses oxygen on heating resulting the number of cations (zinc ion) become more than anions present in zinc oxide.
The excess cations (Zn++ions) move to interstitial site and electrons move to neighbouring interstitial sites. Because of this zinc oxide imparts yellow colour when heated. Such defects are called metal excess defects.
Metal Deficiency Defects:
Many solids show metal deficiency defects as they have less metals compare to ideal stoichiometric proportion. The less proportion of metal is compensated by same metals having higher valency. Such defects are shown generally by transition elements. Thus, when metal present less than ideal stoichiometric proportion in a solid, it is called metal deficiency defect.
Example – FeO is generally found in composion of Fe0.95O. In the crystal of FeO, missing Fe++ ions are compensated with Fe+++ ions in order to maintain neutrality.
Electrical Properties
Solids show amazing range of electrical conductivities. Electrical conductivity is the reciprocal of resistivity. Whereas resistivity is the property of solids to resist flow of electricity, conductivity is the property to conduct electricity.
The SI unit of resistivity is ohm meter. Since, conductivity is the reciprocal of resistivity, thus its unit is reciprocal of ohm meter, i.e. ohm -1 m -1. Conductivity is generally
represented by Greek letter σ (sigma). The SI unit of conductivity is Siemens per meter, i.e. S/m.
On the basis of magnitude of range of conductivities, i.e. from 10 -20 to 107 ohm-1 m-1, solids can be classified into three types:
(a) Conductor: - Solids having magnitude of range of conductivities from 104 to 107 ohm-
1 m-1 are classified as conductors. Metals are good conductor of electricity. Silver has conductivity in the order of 107 ohm-1 m-1 is considered as very good conductor.
(b) Insulator: - Solids having range of conductivity from 10-20 to 10-10 ohm-1 m-1 are considered as insulators.
(c) Semiconductor: - Solids having intermediate range of conductivity, i.e. from 10-6 to 104 ohm-1 m-1 are called semiconductors.
Conduction of Electricity in Metals:
Metals show electrical conductivity because of movement of electrons. Electrolytes show electrical conductivity because of movement of ions. Metals show electrical conductivity in solid and molten states both while electrolytes show electrical conductivity in molten state and aqueous solution.
Conductivity in metals depends upon presence of unpaired electrons in their valence shell per atom. Electrons present in valence shell of metals are free to move and allow conducting electricity in metals.
Energy level (atomic orbital) with electrons and vacant energy levels present in metals have if minute difference in energy they together are called energy band or simply band. The empty energy levels or unoccupied energy levels are known as conduction band also since they helps in conduction of electricity.
When partially filled energy levels (atomic orbital) are too close or overlapped with unoccupied energy level or conduction band; electrons can easily flow between them under the electrical field. Because of flows of unpaired electrons from occupied energy level to conduction band metals conduct electricity.
Conduction of electricity in Insulators:
In insulators the difference in energy between occupied energy level and unoccupied energy level (conduction band) is higher because of which electrons do not flow from occupied energy band to the next higher unoccupied energy band resulting insulators do not conduct electricity as electrons do not flow.
In semiconductors like silicon and germanium, the energy gap between valence shell and conduction band is so smaller that electrons may jump from filled orbital to conduction band when put under electrical field. Because of this behavior, i.e. lower gap between valence band and conduction band semiconductor show the conduction of electricity.
The conduction of electricity in semiconductors increases with increase in temperature. Elements such as silicon and germanium show such behavior and are called intrinsic semiconductors.
Doping:
Intrinsic semiconductors show very low conductivity and thus cannot be used practically. Thus, the conductivity of intrinsic semiconductors is increased by adding suitable impurities. Addition of appropriate amount of suitable impurities to elements, such as intrinsic semiconductors is called doping.
Doping is done with electron rich or electron deficient element (impurities) to the intrinsic semiconductors. Doping with electron rich or electron deficient elements creates electronic defects in semiconductors.
(a) Doping with electron rich impurities: n-type of semiconductor:
Silicon and/or germanium are doped with electron rich impurities to increase their electrical conductivity. Semiconductors so formed after are called n-type semiconductors.
Silicon and germanium, each has four valence electrons as they belong to 14th group of periodic table. Arsenic and phosphorous belong to 15th group of periodic table and they
have valence electrons equal to 5. When silicon or germanium is doped with phosphorous or arsenic, four electrons of phosphorous or arsenic out of five; make covalent bonds with four electrons of silicon or germanium leaving one electron free; which increases the electrical conductivity of silicon or germanium.
Since the electrical conductivity of silicon or phosphorous is increased because of negatively charged particle (electron), thus this is known as n-type of semiconductor.
(b) Doping with electron deficient impurities – p-type semiconductor:
Electrical conductivity of silicon or germanium is doped with elements, such as Boron, Aluminium or Gallium which belong to group 13th in periodic table also. Elements belong to group 13th have valence electrons equal to 3. Three valence electrons present in these elements make covalent bonds with three electrons present in valence shell out of four of silicon or germanium leaving one electron delocalized. The place from where one electron is missing is called electron hole or electron vacancy.
When the silicon or germanium is placed under electrical field, electron from neighbouring atom fill the electron hole, but in doing so another electron hole is created at the place of movement of electron. In the influence of electrical filed electron moves toward positively charge plate through electron hole as appearing the electron hole as positively charged and are moving towards negatively charged plate.
Semiconductor formed by the doping with electron deficient impurities; are called p-type semiconductors.
Applications of n-type and p-type semiconductors
Both n-type and p-type semiconductors are used in making electronic components.
As diode which is the combination of n-type and p-type semiconductors. As integrated circuit (ICs). In photoelectric cell As transistors, to amplify radio and audio signal
Magnetic Properties:
Substance shows magnetic properties because of presence of electrons in them. Each electron in an atom behaves like a magnet because of its two types of motions - one is around their axis and other around the nucleus. Electrons in an atom because of charge over then and in motion continuously; possess small loop of current which shows the magnetic moment.
Substances are classified into five types on the basis of magnetic properties:
b. Diamagneticc. Ferromagneticd. Antiferromagnetice. Ferrimagnetic
(a) Paramagnetism: Substances which are attracted slightly by magnetic field and do not retain the magnetic property after removal of magnetic field are called paramagnetic substances. For example O2, Cu2+, Fe3+, Cr3+, Magnesium, molybdenum, lithium, etc.
Substances show paramagnetism because of presence of unpaired electrons. These unpaired electrons are attracted by magnetic field.
(b) Diamagnetism: Diamagnetic substances are just opposite to that of paramagnetic. Substances which are repelled slightly by magnetic field are called diamagnetic substances. For example; H2O, NaCl, C6H6, etc. Diamangetic substances are magnetized slightly when put under magnetic field but in opposite direction.
Substances show diamagnetic property because of presence of paired electrons and no unpaired electron. Thus, pairing of electrons cancel the magnetic property.
(c) Ferromagnetism: Substances that are attracted strongly with magnetic field are called ferromagnetic substances, such as cobalt, nickel, iron, gadolinium, chromium oxide, etc. Ferromagnetic substances can be permanently magnetized also.
Metal ions of ferromagnetic substances are randomly oriented in normal condition and substances do not act as a magnet. But when metal ions are grouped together in small regions, called domains, each domains act like a tiny magnet and produce strong magnetic field, in such condition ferromagnetic substance act like a magnet. When the ordering of domains in group persists even after removal of magnetic field a ferromagnetic substance becomes a permanent magnet.
(d) Antiferromagnetism: Substances in which domain structure are similar to ferromagnetic substances but are oriented oppositely, which cancel the magnetic property are called antiferromagnetic substances and this property is called antiferromagnetism. For example; MnO.
(e) Ferrimagnetism: Substances which are slightly attracted in magnetic field and in which domains are grouped in parallel and anti-parallel direction but in unequal number, are called ferromagnetic substances and this property is called ferrimagnetism. For example, magnetite (Fe3O4), ferrite (MgFe2O4), ZnFe2O4, etc.
Ferrimagnetic substances lose ferrimagnetism on heating and become paramagnetic.
NCERT Solution
In Text Questions and Answer - 1
Question: 1.1 - Why are solids rigid?
Answer: - The particles of solids are close packed and can only oscillate about their fixed positions. These properties make solids rigid.
Question: 1.2 - Why do solids have a definite volume?
Answer: The intermolecular force of attraction make the particles of solid closely packed and force them to only oscillate at their fixed positions. These give solids a definite volume.
Question: 1.3 - Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.
Question: 1.4 - Why is glass considered a super cooled liquid?
Answer: - Glass is an amorphous solids, it has tendency to flow but very slowly. This is the cause that glass is considered as super cooled liquid.
Question: 1.5 - Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show cleavage property?
Answer: Amorphous solids are isotropic in nature, i.e. they have short range order of arrangement of particles. Because of this amorphous solids have same value of refractive index along all directions.
Amorphous solids do not show cleavage property, i.e. when cut into two pieces with a sharp knife, they give pieces with irregular surface.
Question: 1.6 - Classify the following solids in different categories based on the nature of intermolecular forces operating in them:
Graphite, silicon carbide – Covalent solids or network solids
Question: 1.7 - Solid A is a very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. What type of solid is it?
Answer: Given solid ‘A’ is a covalent solids, such as diamond.
Question: 1.8 - Ionic solids conduct electricity in molten state but not in solid state. Explain.
Answer: Ionic solids conduct electricity because of movement of their ions. In solid state ions present in ionic solids do not move hence do not conduct electricity while in molten state ions can move and thus conduct electricity.
Question: 1.9 - What type of solids are electrical conductors, malleable and ductile?
Answer:
Metallic solids are conductor of electricity, malleable and ductile.
Question: 1.10 - Give the significance of a ‘lattice point’.
Lattice point denotea the position of constituent particles (molecule, atom or ion) in space. When lattice points are joined together by straight line they give the geometry of lattice.
Question: 1.11 - Name the parameters that characterise a unit cell.
Answer:
Unit cells are characterize on six parameters – dimensions along three edges and three angles between their edges, i.e. a, b, c which are edges and α, β and γ which are angles between the edges.
Question: 1.12 - Distinguish between
(i) Hexagonal and monoclinic unit cells
Answer:
(ii) Face-centred and end-centred unit cells.
Answer:
There are four atoms present in face centered unit cell while there are only 2 atoms present in end centered unit cell.
In face centered unit cell one constituent particles are present at the center of each of the faces besides one at each corner.
In end centered unit cell two constituent particles are present at the center of any of the two faces besides one at each corner of the unit cell.
Question: 1.13 - Explain how much portion of an atom located at (i) corner and (ii) bodycentre of a cubic unit cell is part of its neighbouring unit cell.
Answer: -
(i) Atom located at the corner is shared among eight adjacent unit cell, thus only 1/8 th portion of the atom is located at corner.
(ii) Atom located at body center does not share any part of its neighbouring unit cell, thus whole portion of atom is located at body center of cubic unit cell.
Question: 1.14 - What is the two dimensional coordination number of a molecule in square close-packed layer?
Answer: -The coordination number of a molecule in two dimensions in square close packed layer is 4.
Question: 1.15 - A compound forms hexagonal close-packed structure. What is the total number of voids in 0.5 mol of it? How many of these are tetrahedral voids?
Answer:
Question: 1.16 - A compound is formed by two elements M and N. The element N forms ccp and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the compound?
Let the number of octahedral voids occupied by element N = a
Therefore total number of tetrahedral voids = 2a
Question: 1.17 - Which of the following lattices has the highest packing efficiency (i) simple cubic (ii) body-centred cubic and (iii) hexagonal close-packed lattice?
Answer:
The packing efficiency of simple cubic lattice is 52.4%, body centered is 68% and that of hexagonal close packed lattice is 74%.
Therefore, (iii) hexagonal close packed lattice has highest packing efficiency, i.e. 74%.
Question: 1.18 - An element with molar mass 2.7×10-2 kg mol-1 forms a cubic unit cell with edge length 405 pm. If its density is 2.7×103 kg m-3, what is the nature of the cubic unit cell?
Answer:
By knowing the number of atom in the cubic unit cell of given lattice, its nature can be determined.
Question: 1.19 - What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?
Answer:
When a solid is heated a vacancy defect may arise. Because of vacancy defect the density of the solid decreases because of leaving of some of the constituent particles.
Question: 1.20 - What type of stoichiometric defect is shown by:
(i) ZnS (ii) AgBr
Answer:
(i) ZnS – Since there is large difference in the size of ions, thus it shows Frenkel defect.
(ii) AgBr – AgBr shows Frenkel defects and Schottky defects both.
Question: 1.21 - Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.
Answer: -
In ionic solids, such as NaCl; during crystallization; a little amount of SrCl2 is also crystallized. In this process, Sr++ ions get the place of Na+ ions and create defects in the crystal of NaCl. In this defect, each of the Sr++ ion replaces two Na+ ions. Sr++ ion occupies one site of Na+ ion; leaving other site vacant. Hence it creates cationic vacancies equal number of Sr++ ions. CaCl2, AgCl, etc and is called vacancy defects. This is also known as impurities defects.
Question: 1.22 - Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.
Answer: -
Metal excess defects are seen because of missing of anions from regular site leaving a hole which is occupied by electron to maintain the neutrality of the compound. Hole occupied by electron is called F-centre and responsible for showing colour by the compound.
This defect is common in NaCl, KCl, LiCl, etc. Sodium atoms get deposited on the surface of crystal when sodium chloride is heated in an atmosphere of sodium vapour.
In this process, the chloride ions get diffused with sodium ion to form sodium chloride and sodium atom releases electron to form sodium ion. This released electron gets diffused and occupies the anionic sites in the crystal of sodium chloride; creating anionic vacancies and resulting in the excess of sodium metal.
The anionic site occupied by unpaired electron is called F-centre. When visible light falls over the crystal of NaCl, the unpaired electron present gets excited because of absorption of energy and impart yellow colour. Because of similar defect if present, crystal of LiCl imparts pink colour and KCl imparts violet.
Question: 1.23 - A group 14 element is to be converted into n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong?
Answer:
Silicon and germanium, each has four valence electrons and they belong to 14th group of periodic table. Arsenic and phosphorous belong to 15th group of periodic table and they have valence electrons equal to 5. When silicon or germanium is doped with phosphorous or arsenic, four electrons of phosphorous or arsenic out of five; make covalent bonds with four electrons of silicon or germanium leaving one electron free; which increases the electrical conductivity of silicon or germanium. Since the electrical conductivity of silicon or phosphorous is increased because of negatively charged particle (electron), thus this is known as n-type of semiconductor.
The impurities belong to group 15 which is introduced with elements of group 14 to produce n-type of semiconductor.
Question: 1.24 - What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic. Justify your answer.
Answer: Metal ions of ferromagnetic substances are randomly oriented in normal condition and substances do not act as a magnet. But when metal ions are grouped together in small regions, called domains, each domains act like a tiny magnet and produce strong magnetic field, in such condition ferromagnetic substance act like a magnet. When the ordering of domains in group persists even after removal of magnetic field a ferromagnetic substance becomes a permanent magnet. While domains are grouped in parallel and anti-parallel direction but in unequal number in ferromagnetic compounds.
Thus, Ferromagnetic substances, such as Ni, Co, Fe would make better permanent magnets rather than ferromagnetic substances.
