A
+X
–B
+Y
–Highly ionic compound Highly ionic compound
Large cationic size Large cationic size
Highly covalent compound Highly covalent compound
small cationic size small cationic size
Large cationic sizeLarge cationic size
Small anionic sizeSmall anionic size
small cationic sizesmall cationic size
large anionic sizelarge anionic size
• The covalency properties of a molecule is dependent on the cation and anion where they can be explained qualitatively via • Polarisation power of cation
3.6.1.1
3.6.1.1 PolarisationPolarisation Power of Power of CationCation
PolarisationPolarisation Power of Power of CationCation –– measure the ability of a measure the ability of a cationcation
to
to polarisepolarise the electron cloud of the anion.the electron cloud of the anion.
2 factors determining the 2 factors determining the polarisationpolarisation power of power of cationcation Charge of cation Size of cation ⇒ Greater the charge of ion, higher the
effective nuclear charge of cation, hence it will be able to attract the
neighboring electron density of anion.
⇒ Smaller the size of cation, closer the neighboring anion to the nucleus of cation, hence easier for the cation to polarise the anion and result an
neighboring electron density of anion. This will caused the polarization power of cation increase, hence increase the covalent characteristic of cation.
polarise the anion and result an
increment in the polarization power of cation, and increase the covalent
characteristic of cation.
♦ Both factors can be explained in another term called as charge density where
Charge Density = Charge / Ionic Radius
♦ From the equation above, Charge Density will have a greater value, provided that cation has a high charge and small cationic radius.
♦ Greater the charge density, higher the polarization power, greater the covalent characteristic of the cation.
3.6.1.2
3.6.1.2 PolarisabilityPolarisability of Anionof Anion
PolarisabilityPolarisability of an anion ~ ability of the anion to allow the electron of an anion ~ ability of the anion to allow the electron
density to be
density to be polarisedpolarised by by cationcation..
2 factors determining the 2 factors determining the polarisabilitypolarisability of an anionof an anion
Charge of anion Size of anion
⇒ Greater the charge of anion, lower the
effective nuclear charge of anion. This will weakened the electrostatic attraction forces between nucleus and the outermost
⇒ Larger the size of anion, further the outermost electron from the nucleus of the anion, easier for the cation to polarise the anion, and cause the
Unlike Unlike cationcation, anion does not have a term that combined both , anion does not have a term that combined both
factors of charge and ionic radius. However, information of factors of charge and ionic radius. However, information of polarisability
polarisability of anion enable the prediction of the covalent of anion enable the prediction of the covalent
characteristic of a molecule, since in order to form a covalent bond, characteristic of a molecule, since in order to form a covalent bond, it depend on both
it depend on both polarisationpolarisation power of power of cationcation and and polarisabilitypolarisability of of the anion
the anion
between nucleus and the outermost electron in anion, and increase the
polarisability of the anion, hence increase the covalent characteristic of anion
polarise the anion, and cause the polarisability to increase, hence increase the covalent characteristic of anion.
3.6.2
3.6.2 Prediction of Chemical Bond :Prediction of Chemical Bond :FajansFajans’ Rule’ Rule
In 1923In 1923, , KazimierzKazimierz FajansFajans formulated an easy guidance to predict formulated an easy guidance to predict
whether a chemical bond will be covalent or ionic, and depend on whether a chemical bond will be covalent or ionic, and depend on the charge on the
the charge on the cationcation and the relative sizes of the and the relative sizes of the cationcation and and anion. They can be summarized in the following table
anion. They can be summarized in the following table
Ionic compound Low positive charge Large cation Small anion
Covalent compound High positive charge Small cation Large anion
Based on these guidance, the bonding of a few compounds Based on these guidance, the bonding of a few compounds
shall be discussed to understand the application of
shall be discussed to understand the application of FajansFajans’ ’ Rule in the chemical bonding
Lithium halide (
Lithium halide (LiXLiX))
Lithium ion, LiLithium ion, Li++ (1s(1s22) has a small size due to only 1 shell ) has a small size due to only 1 shell
present in its ion. But since it has a low charge, so its charge present in its ion. But since it has a low charge, so its charge density is not too high. That is why, all lithium halide are ionic density is not too high. That is why, all lithium halide are ionic compound. The
compound. The covalencycovalency of lithium halide varies from a of lithium halide varies from a highly
highly ioniccharacteristicioniccharacteristic to highly to highly covalencycovalency, depending on , depending on the
the polarisabilitypolarisability of the anion next to Liof the anion next to Li++
When a group of halide, FWhen a group of halide, F–– ; ; ClCl––; Br; Br––; I; I–– is put close to Liis put close to Li++, the , the
covalency
covalency of lithium halide of lithium halide increase when going down to increase when going down to covalency
covalency of lithium halide of lithium halide increase when going down to increase when going down to Group 17 halide.
Group 17 halide. LiFLiF is highly ionic, since the fluoride ion has is highly ionic, since the fluoride ion has small ionic size and low charge, hence has low
small ionic size and low charge, hence has low polarisabilitypolarisability. . Ionic size increase with the increasing shell when going down Ionic size increase with the increasing shell when going down to Group 17 halide, hence increase the
to Group 17 halide, hence increase the polarisabilitypolarisability, which , which allowed lithium ion to
allowed lithium ion to polarisepolarise the anion’s electron density, the anion’s electron density, hence increase the
Li
+F
–Br
–Cl
–Aluminium
Aluminium halidehalide (AlX(AlX33)) andand aluminiumaluminium oxideoxide (Al(Al22OO33))
AluminiumAluminium ionion (Al(Al33++)) hashas highhigh chargecharge density,density, duedue toto itsits highhigh
charge
charge unitunit andand itsits smallsmall ionicionic radiusradius.. So,So, dependingdepending onon thethe anion,anion, aluminium
aluminium hashas aa highhigh tendencytendency toto formform covalentcovalent compoundcompound.. ForFor example,
example, whenwhen goinggoing downdown toto GroupGroup 1717 halide,halide, aluminiumaluminium fluoridefluoride (AlF
(AlF33)) formsforms ionicionic compoundcompound (since(since FF-- hashas aa lowlow polarisabilitypolarisability),), while
while aluminiumaluminium trichloridetrichloride (AlCl(AlCl33),), aluminiumaluminium tribromidetribromide (AlBr(AlBr33)) and
and aluminiumaluminium iodideiodide (AlI(AlI33)) formform covalentcovalent compoundcompound (since(since chloride,
chloride, bromidebromide andand iodideiodide havehave highhigh polarisabilitypolarisability)).. ThisThis explained
explained whywhy aluminiumaluminium fluoridefluoride hashas aa highhigh meltingmelting pointpoint explained
explained whywhy aluminiumaluminium fluoridefluoride hashas aa highhigh meltingmelting pointpoint ((1040104000C),C), whilewhile aluminiumaluminium trichloridetrichloride andand tribromidetribromide areare 19219200CC andand
78
7800CC respectivelyrespectively..
AsAs forfor aluminiumaluminium oxideoxide (Al(Al22OO33),), itit isis anan ionicionic compoundcompound withwith highhigh
covalent
covalent characteristic,characteristic, asas aluminiumaluminium ionion hashas highhigh covalentcovalent characteristic
characteristic duedue toto itsits highhigh chargecharge densitydensity.. ThisThis explainedexplained thethe high
high meltingmelting pointpoint ofof AlAl22OO33 ((2050205000C)C) yetyet itit isis insolubleinsoluble inin waterwater.. ItIt
also
also explainedexplained thethe amphotericamphoteric propertiesproperties ofof aluminiumaluminium oxideoxide wherewhere aluminium
aluminium oxideoxide cancan actact asas anan acidacid (covalent(covalent characteristic),characteristic), asas well
Metallic Bonding Metallic Bonding
The properties of metals cannot be explained in terms of the The properties of metals cannot be explained in terms of the
ionic / covalent bond. In ionic / covalent compound, electron ionic / covalent bond. In ionic / covalent compound, electron are not free to move under the influence of applied potential are not free to move under the influence of applied potential (charge) difference. Therefore, ionic solid and covalent
(charge) difference. Therefore, ionic solid and covalent compound are
compound are insulatorinsulator..
In metal, electron are In metal, electron are delocaliseddelocalised and metal atoms are and metal atoms are
effectively
effectively ionisedionised. .
Metallic bond ~ electrostatic attraction between the positively Metallic bond ~ electrostatic attraction between the positively
Metallic bond ~ electrostatic attraction between the positively Metallic bond ~ electrostatic attraction between the positively
charged metal ion and the electron
charged metal ion and the electron delocaliseddelocalised..
Because of this, electron now can freely move from cathode Because of this, electron now can freely move from cathode
to anode when a metal is subjected to an electrical potential. to anode when a metal is subjected to an electrical potential. The mobile electron can also conduct heat by carrying the The mobile electron can also conduct heat by carrying the kinetic energy from a hot part of the metal to a cold part. This kinetic energy from a hot part of the metal to a cold part. This electron
electron delocaliseddelocalised can also use to explain the electrical can also use to explain the electrical and thermal conductivities of metal
The Band Theory : Overlapping of Orbital The Band Theory : Overlapping of Orbital
The number of molecular orbitals produced is equal to the The number of molecular orbitals produced is equal to the
number of atomic orbitals that overlap. number of atomic orbitals that overlap.
In a metal, the number of atomic orbitals that overlap is very In a metal, the number of atomic orbitals that overlap is very
large. Thus the number of molecular orbital produced is also large. Thus the number of molecular orbital produced is also very large.
very large.
The energy separations between these metal orbitals are The energy separations between these metal orbitals are
extremely small. So, we may regard the orbital as merging extremely small. So, we may regard the orbital as merging together to form a continuous band of allowed energy state. together to form a continuous band of allowed energy state. together to form a continuous band of allowed energy state. together to form a continuous band of allowed energy state. This collection of very closed molecular orbital energy levels This collection of very closed molecular orbital energy levels is called an energy band. This theory for metal is called band is called an energy band. This theory for metal is called band theory
Electrical Conductors Electrical Conductors
Molecular orbital model == 2 group of energy level.Molecular orbital model == 2 group of energy level.
Lower energy level Lower energy level –– valence band valence band → form from overlap of outer → form from overlap of outer most orbital containing valence electron of each atom.
most orbital containing valence electron of each atom.
Higher energy level Higher energy level –– conduction band conduction band → energy level filled with → energy level filled with mobile electron
mobile electron
But there are some case where valence band can also serve But there are some case where valence band can also serve
as conduction band (caused by the movement of delocalised as conduction band (caused by the movement of delocalised molecular orbital)
molecular orbital) molecular orbital) molecular orbital)
Electrical conductivities decrease when temperature Electrical conductivities decrease when temperature
increase
increase –– vibration of the lattice of ion impedes the free vibration of the lattice of ion impedes the free movement of electron in conduction band.
movement of electron in conduction band. conduction band
conduction band valence band valence band
Insulator Insulator
Difference between conductors, semiDifference between conductors, semi--conductors, and conductors, and
insulator depend on the energy gap between the 2 bands. insulator depend on the energy gap between the 2 bands.
Conductor Conductor –– 2 bands overlaps so conduction band always 2 bands overlaps so conduction band always
partly filled. partly filled.
Insulator Insulator –– gap between the band is large and no electron gap between the band is large and no electron
exist in the conduction band. E.g. insulator
exist in the conduction band. E.g. insulator –– diamonddiamond
When 2s and 2p orbital of C is combine to form 2 energy When 2s and 2p orbital of C is combine to form 2 energy
bands, valence band is filled with electron. bands, valence band is filled with electron.
In insulator, the energy gap between the band is large. Under In insulator, the energy gap between the band is large. Under
normal condition, few electrons in valence band can jump normal condition, few electrons in valence band can jump across to conduction band. If electron cannot reach
across to conduction band. If electron cannot reach
conduction band across the gaps, the electrical conduction conduction band across the gaps, the electrical conduction cannot take place.
Semiconductor Semiconductor
There’s still energy gaps between 2 bands in semiconductor, There’s still energy gaps between 2 bands in semiconductor,
but it is smaller than insulator. but it is smaller than insulator.
In semiconductor, some electrons have sufficient energy to In semiconductor, some electrons have sufficient energy to
jump across the energy gaps and electron can move freely jump across the energy gaps and electron can move freely in conduction band thus enable electrical conduction.
in conduction band thus enable electrical conduction.
Still, the electrical activity is not as good as metal (conductor) Still, the electrical activity is not as good as metal (conductor)
Increasing temperature can help to improve the conductivity Increasing temperature can help to improve the conductivity because electron gain thermal energy and are able to reach because electron gain thermal energy and are able to reach because electron gain thermal energy and are able to reach because electron gain thermal energy and are able to reach conduction band.
conduction band.
It can also improve its effectiveness by adding small amount It can also improve its effectiveness by adding small amount
of substance. This adding is what we called doping. It can of substance. This adding is what we called doping. It can help to increase electrons to fill in valence band.
help to increase electrons to fill in valence band.
Example of doping is Si dope P (nExample of doping is Si dope P (n--type). Si dope Ge (ptype). Si dope Ge (p--type) type)
Depend on the needs, this process can help to create the Depend on the needs, this process can help to create the various type of semiconductor in electronic characteristic. various type of semiconductor in electronic characteristic.
7.1
7.1 Van Van derder Waals forcesWaals forces
Van Van DerDer Waals forces are the intermolecular forces formed Waals forces are the intermolecular forces formed
between covalently bond molecules which exist as simple between covalently bond molecules which exist as simple molecules.
molecules.
There are 2 types of Van There are 2 types of Van DerDer Waals forces namelyWaals forces namely
♥
♥ Permanent Dipole Permanent Dipole –– Permanent dipole forcesPermanent dipole forces ♥
7.1.1
7.1.1 DipoleDipole--dipole attraction forcesdipole attraction forces
1. Polar molecule possessed dipole moment. Each of the polar 1. Polar molecule possessed dipole moment. Each of the polar
molecules have an overall magnitude. For example in hydrogen molecules have an overall magnitude. For example in hydrogen chloride chloride H H –––––––– ClCl δ δ+ + δδ––
2. The dipole inside polar molecules is permanent and the forces 2. The dipole inside polar molecules is permanent and the forces
between the molecule form as the positive end of dipole will between the molecule form as the positive end of dipole will attract to the
attract to the negative end of another molecule’s dipole.negative end of another molecule’s dipole. attract to the
attract to the negative end of another molecule’s dipole.negative end of another molecule’s dipole.
3. This kind of forced are called permanent dipole
3. This kind of forced are called permanent dipole--dipole forces.dipole forces. 4. The strength of the attraction depends on two factors : dipole 4. The strength of the attraction depends on two factors : dipole
moment and relative molecular mass moment and relative molecular mass
5. Higher the dipole moment
5. Higher the dipole moment –– the more polar the molecule the more polar the molecule –– stronger the Van
stronger the Van DerDer Waals forcesWaals forces
6. Comparisons were made between 4 molecules that have 6. Comparisons were made between 4 molecules that have
nearly equaled of molecular mass, but with different dipole nearly equaled of molecular mass, but with different dipole moment
moment
Compounds RMM DM Boiling point (°C) Propane , CH3CH2CH3 44 0.1 - 18.0
Methyl methoxide, CH3–O–CH3 44 1.3 4.0 Chloromethane 50.5 1.9 6.0
7. Methyl cyanide exhibit the highest boiling point among the 3 7. Methyl cyanide exhibit the highest boiling point among the 3 molecules as it has the highest dipole moment among these molecules as it has the highest dipole moment among these molecules, which makes the attraction between the dipole molecules, which makes the attraction between the dipole-- dipole attraction become stronger, and
dipole attraction become stronger, and required a higher required a higher temperature to break the attraction forces among CH
temperature to break the attraction forces among CH33CNCN--- CH
CH33CNCN..
Chloromethane 50.5 1.9 6.0 Methyl cyanide, CH3CN 41 3.9 56.0
8. Another factor which influence the strength of permanent 8. Another factor which influence the strength of permanent
dipole
dipole--dipole forces, are the factor of relative molecular dipole forces, are the factor of relative molecular mass.
mass.
9. Higher the mass, stronger the forces of attraction ( Van
9. Higher the mass, stronger the forces of attraction ( Van DerDer Waals forces ), higher the boiling point or melting point of the Waals forces ), higher the boiling point or melting point of the substance substance RMM Melting point (°C) Boiling point (°C) RMM point (°C) point (°C) Hydrogen chloride, H – Cl 36.5 - 114 - 85 Hydrogen bromide, H – Br 81.0 - 87 - 66 Hydrogen iodide, H – I 128 - 51 - 35
7.1.2 Temporary dipole
7.1.2 Temporary dipole –– induce dipole forcesinduce dipole forces