Solid State
Three states of matter are solid, liquid and gas. Amongst them liquid and gases are called fluids because of their ability to flow. But solid cannot flow. The reason for that is particles in solid are not free to move in available space. There is strong intermolecular force of attraction in between the particles in solid. The constituents particles in solid have fixed position and can only oscillate about their mean position. This gives the rigidity to the solid and hence a fixed shape also. In brief we can say that:
“Solid state of matter possesses fixed mass, volume, shape and rigidity “.
Solids are classified on the basis of arrangement of constituent particles. Due to their specific arrangements, it shows wide range of properties and hence varied applications like as superconductors, magnetic materials, polymers etc.
GENERAL CHARACTERISTICS OF SOLID STATE:
In nature the particular state of matter is governed by two opposing forces at given set of temperature and pressure. These forces are intermolecular force of attraction and thermal energy. If intermolecular force of attraction is high as compared to thermal energy, particles remains in closest position and hence very less movement in particles is observed. In this case solid state is the preferred state of matter.
Let us revise the general characteristics of solid:
i) Fixed mass, volume and shape
ii) Strong intermolecular force of attraction
iii) Least intermolecular space
iv) Fixed position of constituent particles
v) Incompressible and rigid
CLASSIFICATION OF SOLIDS:
Solids are classified on the basis of arrangement of their constituent particles. If the arrangement of constituent particles is same throughout the solid (long range order) it is called crystalline. If the arrangement of particles does not follow any regular pattern throughout the solid (short range order) it is called amorphous solid.
Characteristics of crystalline solid:
- It consists of large number of small crystals having a definite geometrical shape.
- The arrangement of constituent particles is regular throughout the solid (long range order). That is a fixed pattern of constituent particles repeat itself periodically over the entire range of solid.
- They have sharp melting points.
- They are anisotropic in nature.
Anisotropy is defined as” Difference in properties when measured along different axes or different directions”.
Crystalline solid show different values of some of the physical properties like electrical resistance, refractive index etc.when measured along the different directions. The anisotropy arises due to the different arrangement of particles in different directions. Look the different arrangement of particles along the axis AB and CD in diagram given below.
Characteristics of amorphous solid:
- The arrangement of constituent particles is irregular throughout the solid. Regular pattern of constituent particle is visible in small areas only. That is it shows short-range order.
- Melting point is not sharp. Amorphous solid melts over a range of temperature.
- They have tendency to flow at slower rate.
- They are isotropic in nature.
“Isotropic means no difference in properties taken from any direction.”
CLASSIFICATION OF CRYSTALLINE SOLIDS:
Crystalline solid can be classified on the basis constituent particles and intermolecular force of attraction in between them. Constituent particles are molecules, ions, metal kernel in sea of electrons and atoms. Force of attraction operate in between the particles are dispersion force, dipole-dipole interaction, hydrogen bonding, electrostatic attraction, metallic bonding and covalent bonding.
Classification with properties of solid in tabular form:
CRYSTAL LATTICE AND UNIT CELL
The characteristic feature of crystalline solid is regular and repeating arrangement of constituent particles in space. The regular three dimensional arrangement of constituent particle in a crystal is known as crystal lattice.
The smallest part of the crystal lattice is known as unit cell. Look at the role of unit cell in crystal lattice.
A unit cell is characterised by 6 parameters:
- 3- edges a, b and c (refer the diagram given above). The edges may or may not be perpendicular to each other.
- 3 angles α (between b and c) ,β (between a and c) and γ (between a and b).
Classification of unit cell:
i) Primitive unit cells: The constituent particles are only present at corners of unit cell.
ii) Centred unit cells: The constituent particles occupy other positions also beside the corners.
Centred unit cell can be further classified into 3-types:
a) Body- centred unit cells: Other than corner it contains one constituent particle at the centre of the body of unit cell.
b) Face-centred unit cells: Other than corner it contains one constituent particle at the centre of each face of unit cell.
c) End- centred unit cells: Other than corner it contains one constituent particle at the centre of any two opposite faces of unit cell.
On the basis of 6 parameters (3 edges and 3 angles) of unit cell mentioned above, total seven types of primitive unit cells are possible. The primitive unit cells show variations in form of centred unit cells. The total number of possible unit cells (primitive and centred) in 3-dimensional lattice is 14. This is called Bravais Lattice. The detail of the lattice structure in tabular form is given below.
Point to be noted is primitive unit cell structure is 7 but total (primitive + centred) is 14.
The structure of above mentioned 14 Bravias lattice is as follows:
NUMBER OF ATOMS IN UNIT CELL
Primitive unit cell: In Primitive unit cell, atoms are present at corner only. An atom is shared by 8 unit cell. Hence only 1/8 th of the atom belong to particular unit cell. There are total 8 corners in each unit cell therefore, 1/8 part of 8 atoms would give the value of number of atoms per unit cell. In brief
8 corners , 1 atom at each corner
1/8 of each atom in unit cell
Number of atoms present at different position is as follows:
- 8 corners, 1 atom at each corner (1/8 of each atom in unit cell)
- 1 atom at centre of body ( 1 atom completely present in the unit cell)
Total number of atom present in each unit cell:
= Contribution of atoms at corner + contribution of atom at the centre of the body
a) Face-centred unit cell -
Number of atoms present at different position is as follows:
- 8 corners, 1 atom at each corner (1/8 of each atom in unit cell)
- 6 faces, 1 atom at each face of unit cell ( ½ of each atom in the unit cell)
Total number of atom present in each unit cell:
= Contribution of atoms at corner + contribution of atom at the face of unit cell
CLOSED PACKED STRUCTURE
As we the constituent particles in solid are in form of sphere. The spheres in solid are arranged in different way to leave minimum vacant space. These arrangements of spheres in different layers form the closed packed structure of solid. The crystals are formed in closed packed structures. Similarly spheres in solid are also arranged in 3-dimension to form close-packed structure.
a) Close-packing in one-dimension :
- Spheres are arranged in a row with touching each other as shown below :
In this type of arrangement spheres of both the layers are perfectly aligned horizontally and vertically. This type of arrangement is known as AAA type closed packing of sphere in 3-dimension. The possible smallest geometrical 3-dimensional shape would be cube. Thus this type of arrangement generate simple cubic lattice with primitive type of unit cell.
(i) Hexagonal closed packing in three dimension:
- Placing second layer over the first layer-
- Spheres in the second layer are fit in the depression of first layer.
Let the number of close packed spheres be N, then:
The number of octahedral voids generated = N
The number of tetrahedral voids generated = 2N
- Placing the third layer above second layer –
- Covering tetrahedral voids : If all the tetrahedral voids of second layer is covered by third layer, then third layer would be exactly aligned with the first layer as shown below :
This forms the ABCABC... type of arrangement of sphere and is known as Cubic closed packing (ccp) or face centred cubic (fcc) structure.
Coordination number: The number of nearest neighbour touching a particle in closed packed structure is known as the coordination number of constituent particles.
In both type of crystal lattice (hcp, ccp or fcc) the coordination number for the constituent particle is 12.
Formula of compound:
Formula of the compound is deduced by calculating number of atoms present at lattice point and number of atoms present in voids. Generally anions are bigger and they occupy the lattice point while cations are occupied in voids.
In a given compound, it is not necessary that all the voids are occupied by constituent particles. Some time only fraction of voids are occupied depend on the formula of compound. Therefore it is necessary to know the position of voids in crystal lattice.
Total number of octahedral voids = 1(body-centre) + 3 (edge-centre) = 4
As we know number of atom per unit cell for ccp is 4. Therefore:
The number of octahedral voids generated = N(number of atoms per unit cell)
PACKING EFFICIENCY:
Packing efficiency is defined as percentage of total space filled by the constituent particles in crystal.
Packing efficiency in hcp and ccp structures:
Let us consider the unit cell of ccp in which sphere ABCDEFGH are occupied at corner. And sphere a,b,c,d,e,f are placed at centre of face.
IMPERFECTION IN SOLIDS
Although in crystalline solid there is regular arrangement of constituent particles but yet the crystals are not perfect. There is always some king of irregularity in arrangement of constituent particles in small crystals. These
Irregularities are known as defects in crystal. There are 2 types of defects known in crystal lattice.
- Point defect
- Line defect
In our syllabus only point defects are included so we will focus our study on point defects only.
POINT DEFECTS:
- Impurity defect
- Stoichiometric defects
- Non- stoichiometric defects
Types of stoichiometric defects and Non- stoichiometric defects are mentioned in tabular form.
Stoichiometric defects | Non- stoichiometric defects |
| Vacancy defect | Metal excess defect |
| Interstitial defect | Metal deficiency defect |
| Frenkel defect | |
| Schottky defect |
Impurity defect:
Impurity defect is arises due to addition of small amount of impurity in ionic solid. It actually creates some kind of cationic vacancy in ionic solid. For example, if some SrCl2 is added in molten salt of NaCl it takes the position of Na+ in the crystal. But charge on Sr is 2+. After removing 2 ions of Na+, Sr2+ occupy one point only. In this way it causes vacancy of 1 point.
Vacancy defect:
It arises when some of the lattice point remains unoccupied during the crystal formation.- It occurs in non-ionic compounds
- It decreases the density of solid
- It can be created by heating
Interstitial defect:
It arises when some of the constituent particles occupy the interstitial sites other than the lattice points.It occurs in non-ionic compounds
It increases the density of solid
Impurity defect:
Impurity defect is arises due to addition of small amount of impurity in ionic solid. It actually creates some kind of cationic vacancy in ionic solid. For example, if some SrCl2 is added in molten salt of NaCl it takes the position of Na+ in the crystal. But charge on Sr is 2+. After removing 2 ions of Na+, Sr2+ occupy one point only. In this way it causes vacancy of 1 point.Vacancy defect:
It arises when some of the lattice point remains unoccupied during the crystal formation.- It occurs in non-ionic compounds
- It decreases the density of solid
- It can be created by heating
Interstitial defect:
It arises when some of the constituent particles occupy the interstitial sites other than the lattice points.- It occurs in non-ionic compounds
- It increases the density of solid
Frenkel defect:
- It occurs in ionic compounds
- It does not change the density of solid
- Shown by ionic compound having large difference in their constituent ions.
For example: ZnS, AgBr, AgI etc.
Schottky defect:
It is actually a vacancy defect. But it causes vacancy of cation and anion both.It occurs in ionic compounds
It decrease the density of solid
Shown by ionic compound having similar size of cation and anion.
It is actually the combination of vacancy defect and interstitial defect in ionic compound. It arises when smaller ion is dislocated from its normal site and occupies an interstitial site. It is also known as dislocation defect.
It occurs in ionic compounds
It does not change the density of solidShown by ionic compound having large difference in their constituent ions.
For example: ZnS, AgBr, AgI etc.
Schottky defect:
It is actually a vacancy defect. But it causes vacancy of cation and anion both.- It occurs in ionic compounds
- It decrease the density of solid
- Shown by ionic compound having similar size of cation and anion
For example: NaCl, KCl, AgBr.
Important point: AgBr shows both Frenkel and schottky defect.
Metal excess defect:
Due to anionic vacancies:
anionic vacancies are created due to reaction of anion of the crystal with excess of metal present in the atmosphere. For example when crystal of NaCl is heated in presence of sodium vapour, the sodium atoms are deposited on the surface of crystal and later on forms the bond with Cl- ion by losing an electron.
NaCl – yellow colour
LiCl – pink
KCl-violet.
- It occurs in ionic compounds
- It is responsible for imparting colour to the crystal through F-centres.
For example: NaCl, KCl, LiCl.
Due to presence of extra cations:
Excess cationic sites are generated due to the loss of anion of crystal after any reaction. The excess of cation occupies the interstitial site. ZnO on heating loss oxide ion from the crystal. The excess of Zn2+ ions occupy the interstitial site and electron lost by oxide ion occpy the neighbouring site, in this way maintain the electrical neutrality of crystal.
- It occurs in ionic compounds
- It is responsible for imparting colour to the crystal through free electrons.
For example: ZnO (white at room temperature, yellow on heating)
Metal deficiency defect:
Number of metal cations is less as required by metallic crystal. For example in FeO the actual composition ranges from Fe0.93O to Fe0.95O.
ELECRTICAL PROPERTIES:
- Conductor: electrical conductivity range 104 to 107 ohm-1m-1
- Semi-Conductor: electrical conductivity range 10-6 to 107 ohm-1m-1.
- Insulator: electrical conductivity range 10-20 to 10-10 ohm-1m-1
CONDUCTION OF ELECTRICITY
- In case of conductor there is an overlapping of valance band and conduction band.
- In semi-conductor there is small gap of energy between valance band and conduction band. Some of the electron may jump and show some activity. The metal showing this type of activity is known as intrinsic semi-conductors. For example : Silicon and Germanium
- In case of insulator there is a large energy difference between valance band and conduction band
Classification of semi-conductors:
The electrical conductivity of semi-conductors can be increased by adding some electron rich impurity or electron deficient impurity. This is called doping.n-type semiconductor : When the conductivity of semi-conductor is increased by adding electron rich impurity. It generates n-type semi-conductor. For Example: when Silicon is doped with phosphorus, phosphorus also occupies some lattice site. The covalency of Si is 4 but that of P is 5. So, one electron per atom left unused and it delocalise from its location. This delocalise electron help in increasing conductivity of semi-conductor.
p-type semiconductor : When the conductivity of semi-conductor is increased by adding electron deficient impurity. It generates p-type semi-conductor. For Example: when Silicon is doped with boron, boron also occupies some lattice site. The covalency of Si is 4 but that of B is 3. So, one of the position of electron is left unused. This is called electron hole. The electron from neighbouring atom moves to fill this hole but creates a new hole. This looks like movement of electron hole throughout the system. Electrons moves through theses hole under the influence of electric fields. In this way electrical conductivity increases.
Application of n-type and p-type semiconductors:
By the combination of n-type and p-type semi-conductors many electronic devices are prepared.
- Diode (is a device that allows the current in one direction only) is used as amplifier
- pnp and npn sandwiching is done in making transistors
- As photo diode in solar cells
- In lasers
MAGNETIC PROPERTIES:
Electrons are charged particles and it generates a magnetic field around itself. The magnetic field arises due to the spinning of electron at its own axis and movement of atomic orbital around nucleus.Classification of magnetic properties:
I. Paramagnetism :
- Weakly attracted by magnetic fields
- Alignment of magnetic dipole in the same direction of magnetic field
- Loose magnetism in absence of magnetic field
- They have unpaired electrons
Ex. Cu 2+, Fe 3+
II. Diamagnetism :
- Weakly repelled by magnetic fields
- Alignment of magnetic dipole in the opposite direction of magnetic field
- They have all the electrons paired
Ex. H2O, NaCl
III. Ferromagnetism :
- strongly attracted by magnetic fields
- Permanently magnetised.
Ex. Fe, Co, Ni etc.
IV. Anti-Ferromagnetism :
- strongly repelled by magnetic fields
- Alignment of magnetic domains (group of metal ions)in the opposite direction of magnetic field..
Ex. MnO
V. Ferrimagnetisms :
- Weakly attracted by magnetic fields
- Alignment of magnetic domains (group of metal ions)in the parallel and anti parallel direction with direction of magnetic field and unequal in number.
- Loss magnetic moment on heating.
Ex. Fe3O4(magnetite)
Summary:
- Solids have definite mass, volume and shape. This is due to the fixed position of their constituent particles and strong intermolecular force of attraction.
- Solids are classified as amorphous and crystalline.
- In amorphous solids, the arrangement of constituent particles has only short
- Range order. The melting point is not sharp. They are isotropic in nature. In crystalline solids there is long range order in the arrangement of their constituent particles. The melting point is sharp. They are anisotropic in nature.
- Properties of crystalline solids depend upon the nature of interactions between their constituent particles. On this basis, they can be divided into four categories, namely: molecular, ionic, metallic and covalent solids.
- The constituent particles in crystalline solids are arranged in a regular pattern which extends throughout the crystal. This arrangement is often depicted in the form of a three dimensional array of points which is called crystal lattice. Each lattice point gives the location of one particle in space. In all, fourteen different types of lattices are possible which are called Bravais lattices. Each lattice can be generated by repeating its small characteristic portion called unit cell
- A unit cell is the smallest portion of space lattice which when repeated in different directions , generate the entire lattice. A unit cell is characterised by its edge lengths and three angles between these edges. Unit cells can be either primitive which have particles only at their corner positions or centred. The centred unit cells have additional particles at their body centre (body-centred),at the centre of each face (face-centred) or at the centre of two opposite faces (end-centred)
- There are seven types of primitive unit cells. Taking centred unit cells also into account, there are fourteen types of unit cells in all, which result in fourteen Bravais lattices
- Close-packing of particles is obtained in two highly efficient lattices, hexagonal close-packed (hcp) and cubic close-packed (ccp). The latter is also called facecentredcubic (fcc) lattice. In both of these packings 74% space is filled. Other types of packing are not close-packings and have less efficient packing of particles. While in body-centred cubic lattice (bcc) 68% space is filled, in simple cubic lattice only 52.4 % space is filled
- The remaining space is present in the form of two types of voids-octahedral voids and tetrahedral voids
- Solids are not perfect in structure. There are different types of imperfections or defects in them. Point defects and line defects are common types of defects
- Point defects are of three types - stoichiometric defects, impurity defects and non-stoichiometric defects
- Vacancy defects and interstitial defects are the two basic types of stoichiometric point defects. In ionic solids, these defects are present as Frenkel and Schottky defects
- Impurity defects are caused by the presence of an impurity in the crystal. In ionic solids, when the ionic impurity has a different valence than the main compound, some vacancies are created
- Nonstoichiometric defects are of metal excess defect and metal deficient defect
- On the basis of electrical conductivity solids are classified as conductor, semiconductor and insulator
- Sometimes calculated amounts of impurities are introduced by doping in semiconductors that change their electrical properties. Such materials are widely used in electronics industry. They are of two types namely, n-type semiconductor and p-type semiconductor
- Solids show many types of magnetic properties like paramagnetism, diamagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism
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