Crystalline & Solid State
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CHM361 CHAPTER 4:
CRYSTALLINE & SOLID STATE
CONTENTS
Crystal Structures
Metallic crystal structure
Ionic lattice (NaCl and CsCl)
Giant molecule crystal structures
Crystal Defects
Stoichiometric defect
Non-stoichiometric defect
Semiconductor
Metallic Bonding
Electron Sea Model
Band Theory of Metals
LEARNING OUTCOMES 1)
Explain the crystal structures.
2)
Explain the type of defect in crystal structure.
3)
Explain semiconductor.
4)
Explain Electron Sea Model & Band Theory.
SOLID
Atoms, molecules or ions that are arranged closely together & regularly in a complete order.
Can only vibrate & rotate about fixed position.
Rigid arrangement.
Cannot move freely without disrupting the whole structure.
The shape is independent of its container.
SOLID
CRYSTALLINE -exist in regular arrangement. -Occupy specific position. -Rigid & long range order. -have repeating 3-D geometric patterns. -salt, sugar, diamond
AMORPHOUS - Do not have particular & regular geometric arrangement. -Without shape or form. -plastic, glass, gels
TERM
Unit Cell
Smallest part of a crystal which if repeated in 3D space would produce the entire cell.
Basic building block of the crystal.
Lattice Points
Points or corner where atoms, ions or molecules are placed.
TERM
TERM
Coordination number
The number of atoms or ions surrounding an atom or ions in a crystal lattice.
The value is a measurement of how tightly the spheres are packed together.
The larger the value, the closers the spheres are to each other.
CUBIC LATICE SYSTEM (SIMPLE CUBIC)
Also known as the primitive cubic unit cell.
Consist of 8 lattice which are placed at each corner of the cube.
Eg: Oxygen molecule
CUBIC LATICE SYSTEM (BODY-CENTERED CUBIC)
One lattice point at the centre
Consist of 8 lattice point at the corners of the cube & 1 lattice point at the centre of the unit cell.
Eg: Na, Fe
CUBIC LATICE SYSTEM (FACE-CENTERED CUBIC)
Consist of 8 lattice point at the corners of the cube & 1 lattice point at the centre of each face.
Eg: Cu, Au
COUNTING THE NUMBER OF PARTICLES IN A UNIT CELL
Particles in a unit cell can be seen at the lattice point but may not contribute the whole entities to the unit.
Particles located on edges,faces & corner areactually shared by neighboring unit cell.
COUNTING THE NUMBER OF PARTICLES IN A UNIT CELL Particles at the corner Shared by 8 unit cell. Contributes only 1/8 towards its own unit cell.
Particles at the edge Shared by 4 unit cell. Contributes only ¼ towards its own unit
cell.
COUNTING THE NUMBER OF PARTICLES IN A UNIT CELL Particles at the face Shared by 2 unit cell. Contributes only ½ towards its own unit
cell.
Particles at the centre Belongs only to one cell.
METALLIC CRYSTAL STRUCTURE
Simplest crystal structure.
Tend to be densely packed.
Every lattice point in a crystal is occupied by an metal.
Bonding- bonding electron are delocalized over the entire crystal.
The great cohesive force resulting from delocalization is responsible for metal strength.
The mobility makes the metal good conductor of heat and electricity.
atom of the same
METALLIC CRYSTAL STRUCTURE
STRUCTURE
Body Centered Cubic (BCC)
Face Centered Cubic (FCC)
Hexagonal Closed Packed (HCP)
Cr, Fe
Cu, Al, Ag, Au
Cd, Mg, Zn, Ti
METALLIC CRYSTAL STRUCTURE (BCC)
Coordination number of
8.
The bcc unit cell consists of a net total of 2 atoms.
1
1 at the centre
: 8 1 8 : 1 1 1
Total of atom
:
8 at the corner
2 atoms
The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements.
METALLIC CRYSTAL STRUCTURE (FCC)
coordination number of 12.
The fcc unit cell consists of a net total of four atoms; eight from corners atoms and six of the face atoms. 1
8 at the corner
: 8 8 1
6 at the faces
: 6 2 3
Total of atom
: 4 atoms
1
METALLIC CRYSTAL STRUCTURE (FCC)
t h e a t o m s can pack closer together than can in the bcc structure.
The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer.
they
METALLIC CRYSTAL STRUCTURE (HCP)
coordination number is 12 . Six nearest neighbors in the same close
METALLIC CRYSTAL STRUCTURE (HCP)
there are 6 atoms in the hcp unit cell. 12 atoms in the corners of the top and bottom layers, the two atoms in the center of the hexagon of both the top and bottom layers and each of the three atom in the middle layer. 12 at the corner (top & bottom)
: 12 6 2
2 at the center (top & bottom)
: 2 2 1 :313
1
1
3 at the middle Total of atom
: 6 atoms
METALLIC CRYSTAL STRUCTURE (HCP)
The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer just like in the fcc structure.
has three layers of atoms. In each the top and botom layer, there are six atoms that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of the hexagon.
IONIC CRSYTAL
The binding between the ions is mostly electrostatic and rather strong.
it has no directionality.
There are no free electrons, ionic crystals are insulators.
IONIC CRSYTAL (NaCl) The unit cell can be drawn with either the Na+ ions at the corners, or with the Cl- ions at the corners. Na+ & Cl- are arranged alternately. Face-centered Cubic (FCC) Na+ Cl-
6:6-co-ordinated.
Only those ions joined by lines are actually touching each other. The Cl- ion in theis being touched by 6 chloride ions. centre
IONIC CRSYTAL (NaCl) 1
8 Na ions at the corner
: 8 8 1 Na ions
6 Na ions at the face
: 6 2 3 Na ions
Total of Na ions
: 4 Na ions
1
1
12 Cl ions at the edge
: 12 4 3 Cl ions
1Cl ions at the centre
: 1 1 1 Cl ions
Total of Cl ions
: 4 Cl ions
**4 sodium ions & 4 chlorides ion contained within the unit cell.
IONIC CRSYTAL (NaCl) PHYSICAL PROPERTIES 1) High melting point.
Strong electrostatic forces between ions of opposite charge.
Requires more energy to overcome the attraction.
2) Hard but brittle.
Strong electrostatic force make it hard but, if a strong force is applied, the ion layers shifts slightly.
Ions of the same charge are brought side-by-side and so the crystal repels itself.
IONIC CRSYTAL (NaCl) PHYSICAL PROPERTIES 3)
Does not conduct electricity in solid but does so in molten or aqueous solution.
I o n s arenot free to move because areheld by electrostatic force in fixed position.
When the solid melt, ions are free to move thus conducting electricity.
strongly
IONIC CRSYTAL (CsCl)
Simple Cubic. 8:8-co-ordinated. Each caesium ion is touched by eight chloride ions. Cs+ Cl-
IONIC CRSYTAL (CsCl)
Cs
+
Cl-
1Cs ions at the centre
: 1 1 1 Cs ions
Total of Cs ions
:1 Cs ions
8 Cl ions at the corner
: 8 8 1 Cl ions
Total of Cl ions
:1Cl ions
1
**1 caesium ions & 1 chlorides ion contained within the unit cell.
GIANT MOLECULE CRYSTAL STRUCTURE
Giant molecule consist of particles held together by covalent bonds.
Non-metals.
Allotropes of carbon.
-
pure forms of the same element that differ in structure.
GIANT MOLECULE CRYSTAL STRUCTURE (DIAMOND)
of Composed single network.
element (C) arranged
in 3D
GIANT MOLECULE CRYSTAL STRUCTURE (DIAMOND) Each C atom covalently bonded to 4 other C in tetrahedral arrangement by strong covalent bonding.
so no free moving electron to conduct electricity.
Diamond does not conduct electricity.
Very strong covalent bond.
So have high melting point 3550°C.
Very hard.
Used in cutting tools & grinding.
GIANT MOLECULE CRYSTAL STRUCTURE (GRAPHITE)
Each C is connected with 3 other C by covalent bond forming 6-member rings that form flat layer which are held loosely by weak VDW forces.
C use only 3 of its 4 valence electron, so it has one unused electron .
The electron are able to move freely thus conduct electricity.
Graphite is an electric conductor.
GIANT MOLECULE CRYSTAL STRUCTURE (GRAPHITE)
has a high melting point, similar to that of diamond.
In order from another.
have to break the covalent bonding throughout the whole structure.
6-member rings form flat layer which are held loosely by weak VDW forces.
The layers are able to slide past each other, resulting in softness of graphite.
Used as lead in pencil & lubricant.
to melt graphite, it isn't enough to loosen one sheet
CRSYTAL DEFECT -Irregularity in the arrangement of constituent particles in solids. - Ideal crystal does not exist. -all contain large no. of defects or -imperfections .
STOICHIOMETRIC DEFECT Irregularities or deviation from ideal arrangement of constituent particles around the point or atom in a crystalline solid
VACANC Y
SCHOTTKY
INTERSTITIAL
NON-STOICHIOMETRIC DEFECT Presence of constituent particles In non-stoichiometric ratio in the crystal structure
FRENKEL
META EXCESS L
METAL DEFICIENCY
CRYSTAL DEFECT (VACANCY )
An atom is missing from its regular atomic site.
When some lattice sites left vacant while the formation of crystal.
Because of missing the atom, the density of substance decreases.
The vacancy defect develops on heating of substance.
When the temperature is sufficiently high, the atoms vibrate around their regular positions, some acquire enough energy to leave the site completely.
CRYSTAL DEFECT (SCHOTTKY) A pair of one cation and one anion missing from regular sites in crystal. ionic
Almost similar in size, such as NaCl, KCl, CsCl the number of missing cations is equal to the number of missing anions in order to maintain the electrical neutrality of the ionic compound.
CRYSTAL DEFECT (INTERSTITIAL)
when an atom occupies a definite position in the lattice that is not normally occupied in the perfect crystal.
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
CRYSTAL DEFECT (FRENKEL)
an ion displaced from a regular site to an interstitial site in ionic crystal.
As cations are generally the smaller ones, it is possible for them to
get displaced into the void space.
Anions do not get displaced as the void space is too small compared to the size of the anions.
As there are a number of cations and anions (which remain equal even because of defect); the density of the substance & the overall electrical neutrality of the crystal does not change.
such as ZnS, AgCl, AgBr, AgI
CRYSTAL DEFECT (METAL EXCESS DEFECT) (a)
Metal excess defects due to anionic vacancies: missing of anions from regular site leaving a hole which is occupied by electron to maintain the neutrality of the compound.
The trapped electrons are called F-centers or color centers they are responsible for imparting color to the because crystal.
This defect is common in NaCl, KCl, LiCl,.
CRYSTAL DEFECT (METAL EXCESS DEFECT) (b) Metal excess defect due to presence of extra cations at interstitial sites
Extra cations occupying interstitial sites while electrons occupy another interstitial site to maintain electrical neutrality
Similar to Frenkel defect.
CRYSTAL DEFECT (METAL DEFICIENCY DEFECT) Solids which have less metals compare to ideal stoichiometric proportion.
CRYSTAL DEFECT (METAL DEFICIENCY DEFECT) Arise due to cationic vacancies. It occurs when positive ions are missing from its crystal lattice.
To maintain electrical neutrality, one of the nearest metal ion acquires two positive charge. This type of defect occurs in compounds where the same metal
canexhibit variable valency.
Transition metal compounds like NiO, FeO, FeS.
SEMICONDUCTOR
Has intermediate conductivity between a conductor & insulator.
Can resist & allow flow of electron depending on the energy applied.
Commonly used in electronics as transistor or diodes.
Silicon is the most commonly used semiconductor material
SEMICONDUCTOR
Element that normally are not conductor but will conduct electricity at elevated temperatures or when combined with a small amount of certain other element.
The electrons are held in place so they cannot move or change their energy. Electrons in a bond are not considered "free" and cannot participate in current flow, absorption or other physical processes of interest.
SEMICONDUCTOR
At elevated temperatures, the electron can gain enough energy to escape from its bond, the electron is free to move about the crystal lattice and participate in conduction.
At room temperature, a semiconductor has enough free electrons to allow it to conduct current, while at, or close to absolute temperatures, a semiconductor behaves like an insulator.
Doping is a process wherein impurities are added to the semiconductor to improve conductivity.
SEMICONDUCTOR (N-TYPE)
The addition of pentavalent impurities (atoms with 5 Ve-) such as antimony, arsenic or phosphorous contributes free electrons.
This electron can move through the structure & function as a charge carrier.
Greatly increasing semiconductor.
the
conductivity
of
the
intrinsic
SEMICONDUCTOR (N-TYPE)
Known as Donor Impurities.
N-negative (charge of the extra electron)
SEMICONDUCTOR (P-TYPE)
The addition of trivalent impurities (atoms with 3 Ve-) such as boron, aluminum or gallium creates deficiencies of valence electrons, called "holes".
so the charge carrier is effectively a positively charged hole.
Known as acceptor impurities. P-positive (holes)
METALLIC BONDING
Attraction between a positive ions and the sea of delocalized electrons.
strong bonds.
The strength of metallic bond is proportional to the number of ve- & inversely proportional to the size of atom.
Eg: the metallic bond in Mg is stronger than in Na because Mg atom has 2 ve- and smaller while Na has only one ve-
PROPERTIES OF METALLIC BONDS High melting and boiling points Metallic bonds are strong and a lot of energy is needed to break them. Conducting electricity Since the valence & reemit Delocalized valence electron can electron are absorb electromagnetic radiation. free to move. Malleable & ductile Metallic The layers of cations can easily slide past each luster other without breaking the lattice structure.
ELECTRON SEA MODEL
Atoms in metallic element are packed closely as possible.
As a result Ve- of each atom become attracted to the positively charged nuclei of the neighbor and become free from the attraction of its own nucleus.
These free electron are said to be delocalized over the entire solid embedded in a sea of electron.
The electron-sea is acting as glue holding the positive
ELECTRON SEA MODEL
BAND THEORY OF METALS
Theory : delocalized electrons move freely through bands formed by overlapping MO.
Used the concept of orbital & based on the modern MO.
It explain the metalsaregood conductor, capable of conducting electric current.
as they are
BAND THEORY OF METALS
VALENCE BAND: Band of orbitals that are filled or partially filled by valance electron.
- lower MOs are occupied by valence e
CONDUCTION BAND : Higher energy unoccupied bands in which electron are free to migrate.
- empty MOs that are higher in energy
BAND GAP : The energy valence and conduction band.
difference between the
INSULATOR S
SEMICONDUCTORS
CONDUCTORS
Conduction Band
ENERGY
Conduction Band
Band Gap
Valence Band Large band gap between the valence & conduction band
Conduction Band
Band Gap
Valence Band
Small band gap between the valence & conduction band
Valence Band
Overlap between the valence & conduction band
Current flows when electrons move from the valence band to the conduction band. For a filled band to conduct, e- must be promoted from the highest occupied MO to the lowest unoccupied MO. The amount of energy required to promote these will determine how well the substance conducts.
BAND THEORY OF METALS
In metal, the valence band & conducting band are adjacent to each other.
Means, electron can travel freely through the metal (from the valence band to the conduction band).
Current flows when electrons to the conduction band.
move
from
So, the amount of energy needed to promote electronis negligible.
the valence band
a
valence
REVISION TYPE OF CRYSTAL
IONIC
GIANT
METALLIC
FORCES / BONDING
GENERAL PROPERTIES
CRYSTALLINE & SOLID STATE
CONTENTS
Crystal Structures
Metallic crystal structure
Ionic lattice (NaCl and CsCl)
Giant molecule crystal structures
Crystal Defects
Stoichiometric defect
Non-stoichiometric defect
Semiconductor
Metallic Bonding
Electron Sea Model
Band Theory of Metals
LEARNING OUTCOMES 1)
Explain the crystal structures.
2)
Explain the type of defect in crystal structure.
3)
Explain semiconductor.
4)
Explain Electron Sea Model & Band Theory.
SOLID
Atoms, molecules or ions that are arranged closely together & regularly in a complete order.
Can only vibrate & rotate about fixed position.
Rigid arrangement.
Cannot move freely without disrupting the whole structure.
The shape is independent of its container.
SOLID
CRYSTALLINE -exist in regular arrangement. -Occupy specific position. -Rigid & long range order. -have repeating 3-D geometric patterns. -salt, sugar, diamond
AMORPHOUS - Do not have particular & regular geometric arrangement. -Without shape or form. -plastic, glass, gels
TERM
Unit Cell
Smallest part of a crystal which if repeated in 3D space would produce the entire cell.
Basic building block of the crystal.
Lattice Points
Points or corner where atoms, ions or molecules are placed.
TERM
TERM
Coordination number
The number of atoms or ions surrounding an atom or ions in a crystal lattice.
The value is a measurement of how tightly the spheres are packed together.
The larger the value, the closers the spheres are to each other.
CUBIC LATICE SYSTEM (SIMPLE CUBIC)
Also known as the primitive cubic unit cell.
Consist of 8 lattice which are placed at each corner of the cube.
Eg: Oxygen molecule
CUBIC LATICE SYSTEM (BODY-CENTERED CUBIC)
One lattice point at the centre
Consist of 8 lattice point at the corners of the cube & 1 lattice point at the centre of the unit cell.
Eg: Na, Fe
CUBIC LATICE SYSTEM (FACE-CENTERED CUBIC)
Consist of 8 lattice point at the corners of the cube & 1 lattice point at the centre of each face.
Eg: Cu, Au
COUNTING THE NUMBER OF PARTICLES IN A UNIT CELL
Particles in a unit cell can be seen at the lattice point but may not contribute the whole entities to the unit.
Particles located on edges,faces & corner areactually shared by neighboring unit cell.
COUNTING THE NUMBER OF PARTICLES IN A UNIT CELL Particles at the corner Shared by 8 unit cell. Contributes only 1/8 towards its own unit cell.
Particles at the edge Shared by 4 unit cell. Contributes only ¼ towards its own unit
cell.
COUNTING THE NUMBER OF PARTICLES IN A UNIT CELL Particles at the face Shared by 2 unit cell. Contributes only ½ towards its own unit
cell.
Particles at the centre Belongs only to one cell.
METALLIC CRYSTAL STRUCTURE
Simplest crystal structure.
Tend to be densely packed.
Every lattice point in a crystal is occupied by an metal.
Bonding- bonding electron are delocalized over the entire crystal.
The great cohesive force resulting from delocalization is responsible for metal strength.
The mobility makes the metal good conductor of heat and electricity.
atom of the same
METALLIC CRYSTAL STRUCTURE
STRUCTURE
Body Centered Cubic (BCC)
Face Centered Cubic (FCC)
Hexagonal Closed Packed (HCP)
Cr, Fe
Cu, Al, Ag, Au
Cd, Mg, Zn, Ti
METALLIC CRYSTAL STRUCTURE (BCC)
Coordination number of
8.
The bcc unit cell consists of a net total of 2 atoms.
1
1 at the centre
: 8 1 8 : 1 1 1
Total of atom
:
8 at the corner
2 atoms
The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements.
METALLIC CRYSTAL STRUCTURE (FCC)
coordination number of 12.
The fcc unit cell consists of a net total of four atoms; eight from corners atoms and six of the face atoms. 1
8 at the corner
: 8 8 1
6 at the faces
: 6 2 3
Total of atom
: 4 atoms
1
METALLIC CRYSTAL STRUCTURE (FCC)
t h e a t o m s can pack closer together than can in the bcc structure.
The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer.
they
METALLIC CRYSTAL STRUCTURE (HCP)
coordination number is 12 . Six nearest neighbors in the same close
METALLIC CRYSTAL STRUCTURE (HCP)
there are 6 atoms in the hcp unit cell. 12 atoms in the corners of the top and bottom layers, the two atoms in the center of the hexagon of both the top and bottom layers and each of the three atom in the middle layer. 12 at the corner (top & bottom)
: 12 6 2
2 at the center (top & bottom)
: 2 2 1 :313
1
1
3 at the middle Total of atom
: 6 atoms
METALLIC CRYSTAL STRUCTURE (HCP)
The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer just like in the fcc structure.
has three layers of atoms. In each the top and botom layer, there are six atoms that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of the hexagon.
IONIC CRSYTAL
The binding between the ions is mostly electrostatic and rather strong.
it has no directionality.
There are no free electrons, ionic crystals are insulators.
IONIC CRSYTAL (NaCl) The unit cell can be drawn with either the Na+ ions at the corners, or with the Cl- ions at the corners. Na+ & Cl- are arranged alternately. Face-centered Cubic (FCC) Na+ Cl-
6:6-co-ordinated.
Only those ions joined by lines are actually touching each other. The Cl- ion in theis being touched by 6 chloride ions. centre
IONIC CRSYTAL (NaCl) 1
8 Na ions at the corner
: 8 8 1 Na ions
6 Na ions at the face
: 6 2 3 Na ions
Total of Na ions
: 4 Na ions
1
1
12 Cl ions at the edge
: 12 4 3 Cl ions
1Cl ions at the centre
: 1 1 1 Cl ions
Total of Cl ions
: 4 Cl ions
**4 sodium ions & 4 chlorides ion contained within the unit cell.
IONIC CRSYTAL (NaCl) PHYSICAL PROPERTIES 1) High melting point.
Strong electrostatic forces between ions of opposite charge.
Requires more energy to overcome the attraction.
2) Hard but brittle.
Strong electrostatic force make it hard but, if a strong force is applied, the ion layers shifts slightly.
Ions of the same charge are brought side-by-side and so the crystal repels itself.
IONIC CRSYTAL (NaCl) PHYSICAL PROPERTIES 3)
Does not conduct electricity in solid but does so in molten or aqueous solution.
I o n s arenot free to move because areheld by electrostatic force in fixed position.
When the solid melt, ions are free to move thus conducting electricity.
strongly
IONIC CRSYTAL (CsCl)
Simple Cubic. 8:8-co-ordinated. Each caesium ion is touched by eight chloride ions. Cs+ Cl-
IONIC CRSYTAL (CsCl)
Cs
+
Cl-
1Cs ions at the centre
: 1 1 1 Cs ions
Total of Cs ions
:1 Cs ions
8 Cl ions at the corner
: 8 8 1 Cl ions
Total of Cl ions
:1Cl ions
1
**1 caesium ions & 1 chlorides ion contained within the unit cell.
GIANT MOLECULE CRYSTAL STRUCTURE
Giant molecule consist of particles held together by covalent bonds.
Non-metals.
Allotropes of carbon.
-
pure forms of the same element that differ in structure.
GIANT MOLECULE CRYSTAL STRUCTURE (DIAMOND)
of Composed single network.
element (C) arranged
in 3D
GIANT MOLECULE CRYSTAL STRUCTURE (DIAMOND) Each C atom covalently bonded to 4 other C in tetrahedral arrangement by strong covalent bonding.
so no free moving electron to conduct electricity.
Diamond does not conduct electricity.
Very strong covalent bond.
So have high melting point 3550°C.
Very hard.
Used in cutting tools & grinding.
GIANT MOLECULE CRYSTAL STRUCTURE (GRAPHITE)
Each C is connected with 3 other C by covalent bond forming 6-member rings that form flat layer which are held loosely by weak VDW forces.
C use only 3 of its 4 valence electron, so it has one unused electron .
The electron are able to move freely thus conduct electricity.
Graphite is an electric conductor.
GIANT MOLECULE CRYSTAL STRUCTURE (GRAPHITE)
has a high melting point, similar to that of diamond.
In order from another.
have to break the covalent bonding throughout the whole structure.
6-member rings form flat layer which are held loosely by weak VDW forces.
The layers are able to slide past each other, resulting in softness of graphite.
Used as lead in pencil & lubricant.
to melt graphite, it isn't enough to loosen one sheet
CRSYTAL DEFECT -Irregularity in the arrangement of constituent particles in solids. - Ideal crystal does not exist. -all contain large no. of defects or -imperfections .
STOICHIOMETRIC DEFECT Irregularities or deviation from ideal arrangement of constituent particles around the point or atom in a crystalline solid
VACANC Y
SCHOTTKY
INTERSTITIAL
NON-STOICHIOMETRIC DEFECT Presence of constituent particles In non-stoichiometric ratio in the crystal structure
FRENKEL
META EXCESS L
METAL DEFICIENCY
CRYSTAL DEFECT (VACANCY )
An atom is missing from its regular atomic site.
When some lattice sites left vacant while the formation of crystal.
Because of missing the atom, the density of substance decreases.
The vacancy defect develops on heating of substance.
When the temperature is sufficiently high, the atoms vibrate around their regular positions, some acquire enough energy to leave the site completely.
CRYSTAL DEFECT (SCHOTTKY) A pair of one cation and one anion missing from regular sites in crystal. ionic
Almost similar in size, such as NaCl, KCl, CsCl the number of missing cations is equal to the number of missing anions in order to maintain the electrical neutrality of the ionic compound.
CRYSTAL DEFECT (INTERSTITIAL)
when an atom occupies a definite position in the lattice that is not normally occupied in the perfect crystal.
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
CRYSTAL DEFECT (FRENKEL)
an ion displaced from a regular site to an interstitial site in ionic crystal.
As cations are generally the smaller ones, it is possible for them to
get displaced into the void space.
Anions do not get displaced as the void space is too small compared to the size of the anions.
As there are a number of cations and anions (which remain equal even because of defect); the density of the substance & the overall electrical neutrality of the crystal does not change.
such as ZnS, AgCl, AgBr, AgI
CRYSTAL DEFECT (METAL EXCESS DEFECT) (a)
Metal excess defects due to anionic vacancies: missing of anions from regular site leaving a hole which is occupied by electron to maintain the neutrality of the compound.
The trapped electrons are called F-centers or color centers they are responsible for imparting color to the because crystal.
This defect is common in NaCl, KCl, LiCl,.
CRYSTAL DEFECT (METAL EXCESS DEFECT) (b) Metal excess defect due to presence of extra cations at interstitial sites
Extra cations occupying interstitial sites while electrons occupy another interstitial site to maintain electrical neutrality
Similar to Frenkel defect.
CRYSTAL DEFECT (METAL DEFICIENCY DEFECT) Solids which have less metals compare to ideal stoichiometric proportion.
CRYSTAL DEFECT (METAL DEFICIENCY DEFECT) Arise due to cationic vacancies. It occurs when positive ions are missing from its crystal lattice.
To maintain electrical neutrality, one of the nearest metal ion acquires two positive charge. This type of defect occurs in compounds where the same metal
canexhibit variable valency.
Transition metal compounds like NiO, FeO, FeS.
SEMICONDUCTOR
Has intermediate conductivity between a conductor & insulator.
Can resist & allow flow of electron depending on the energy applied.
Commonly used in electronics as transistor or diodes.
Silicon is the most commonly used semiconductor material
SEMICONDUCTOR
Element that normally are not conductor but will conduct electricity at elevated temperatures or when combined with a small amount of certain other element.
The electrons are held in place so they cannot move or change their energy. Electrons in a bond are not considered "free" and cannot participate in current flow, absorption or other physical processes of interest.
SEMICONDUCTOR
At elevated temperatures, the electron can gain enough energy to escape from its bond, the electron is free to move about the crystal lattice and participate in conduction.
At room temperature, a semiconductor has enough free electrons to allow it to conduct current, while at, or close to absolute temperatures, a semiconductor behaves like an insulator.
Doping is a process wherein impurities are added to the semiconductor to improve conductivity.
SEMICONDUCTOR (N-TYPE)
The addition of pentavalent impurities (atoms with 5 Ve-) such as antimony, arsenic or phosphorous contributes free electrons.
This electron can move through the structure & function as a charge carrier.
Greatly increasing semiconductor.
the
conductivity
of
the
intrinsic
SEMICONDUCTOR (N-TYPE)
Known as Donor Impurities.
N-negative (charge of the extra electron)
SEMICONDUCTOR (P-TYPE)
The addition of trivalent impurities (atoms with 3 Ve-) such as boron, aluminum or gallium creates deficiencies of valence electrons, called "holes".
so the charge carrier is effectively a positively charged hole.
Known as acceptor impurities. P-positive (holes)
METALLIC BONDING
Attraction between a positive ions and the sea of delocalized electrons.
strong bonds.
The strength of metallic bond is proportional to the number of ve- & inversely proportional to the size of atom.
Eg: the metallic bond in Mg is stronger than in Na because Mg atom has 2 ve- and smaller while Na has only one ve-
PROPERTIES OF METALLIC BONDS High melting and boiling points Metallic bonds are strong and a lot of energy is needed to break them. Conducting electricity Since the valence & reemit Delocalized valence electron can electron are absorb electromagnetic radiation. free to move. Malleable & ductile Metallic The layers of cations can easily slide past each luster other without breaking the lattice structure.
ELECTRON SEA MODEL
Atoms in metallic element are packed closely as possible.
As a result Ve- of each atom become attracted to the positively charged nuclei of the neighbor and become free from the attraction of its own nucleus.
These free electron are said to be delocalized over the entire solid embedded in a sea of electron.
The electron-sea is acting as glue holding the positive
ELECTRON SEA MODEL
BAND THEORY OF METALS
Theory : delocalized electrons move freely through bands formed by overlapping MO.
Used the concept of orbital & based on the modern MO.
It explain the metalsaregood conductor, capable of conducting electric current.
as they are
BAND THEORY OF METALS
VALENCE BAND: Band of orbitals that are filled or partially filled by valance electron.
- lower MOs are occupied by valence e
CONDUCTION BAND : Higher energy unoccupied bands in which electron are free to migrate.
- empty MOs that are higher in energy
BAND GAP : The energy valence and conduction band.
difference between the
INSULATOR S
SEMICONDUCTORS
CONDUCTORS
Conduction Band
ENERGY
Conduction Band
Band Gap
Valence Band Large band gap between the valence & conduction band
Conduction Band
Band Gap
Valence Band
Small band gap between the valence & conduction band
Valence Band
Overlap between the valence & conduction band
Current flows when electrons move from the valence band to the conduction band. For a filled band to conduct, e- must be promoted from the highest occupied MO to the lowest unoccupied MO. The amount of energy required to promote these will determine how well the substance conducts.
BAND THEORY OF METALS
In metal, the valence band & conducting band are adjacent to each other.
Means, electron can travel freely through the metal (from the valence band to the conduction band).
Current flows when electrons to the conduction band.
move
from
So, the amount of energy needed to promote electronis negligible.
the valence band
a
valence
REVISION TYPE OF CRYSTAL
IONIC
GIANT
METALLIC
FORCES / BONDING
GENERAL PROPERTIES
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