1. CHEMICAL BONDING
A chemical bond is the force which holds the atoms together in a molecule. It implies the force of attraction and repulsion balanced at a particular point.
2. CAUSE OF CHEMICAL COMBINATION
A. To acquire noble gas configuration
Noble gases have eight electrons in their valence shells.
The atoms of elements other than noble gases combine with another to have eight electrons. This tendency of atoms to acquire eight electrons in the valence shell is called octet rule. This rule was given by Lewis. However, this rule is not universally applicable. All stable molecules which contain less than eight electrons are electron-deficient while the molecules which contain more than eight electrons in valence shell are called hypervalent. For example: , etc. are electron –deficient while etc. are hypervalent.
? EXCEPTION
Octet rule is not obeyed in etc.
B. To acquire a state of minimum energy
According to modern theory of chemical bonding, atoms form chemical bond and they lead to decrease in energy. A molecule in lesser energy state is more stable than isolated atoms.
3. TYPES OF BONDS
i. Ionic bond ii. Covalent bond
iii. Co-ordinate bond iv. Metallic bond
v. Hydrogen bond vi. Van der Waal’s interactions
4. LEWIS SYMBOLS AND THEIR SIGNIFICANCE
i. The number of dots represents number of valence electrons.
ii. These number of valence electrons helps to calculate the common or group valence i.e., number of dots or 8 minus number of dots.
5. IONIC BOND
The concept of ionic bond was proposed by Kossel. Ionic bond is also known as electrovalent bond. It is formed by the complete transfer of one or more electrons from the valence shell of one atom to the valence shell of other atom. The atom which loses electron is called cation and acquires positive charge. The atom which accepts electron is called anion and acquires negative charge. The cation and anion are held together by electrostatic force of attraction known as ionic bond and the number of electrons gained or lost by the atom is called electrovalency. For example:
Red Alert
A. Favourable conditions for ionic bond
i. Low I.E. of the atom forming cation.
ii. High E.A. of the atom forming anion.
iii. High lattice energy of the crystal.
Elements of group 1 and 2 on combination with halogens, oxygen and sulphur generally form ionic bonds.
B. Lattice energy
The amount of energy released when one mole of ionic solid is formed from its gaseous ions is called lattice energy.
Or
It is the amount of energy required to break one mole of ionic solid into its gaseous ions.
C. Calculation of Lattice energy
Lattice energy is calculated by using Born-Haber cycle.
It is explained using the formation of NaCl.
Na(s) + NaCl (s)
+S + D
―U
Na(g) Cl (g)
+ I.E. – E.A. +
Na+(g)
Where S = Sublimation energy
I.E. = ionisation energy
E.A. = electron affinity
D = dissociation energy
U = lattice energy
= heat of formation
The heat of formation is given by
= S + I.E + D – E.A. –U
The higher is the lattice energy, The more will be the stability of ionic compound.
Note
D. Factors affecting lattice energy
i. Charge on ions : The higher the charge on the ions, The greater will be the force of attraction and higher will be the lattice energy.
ii. Size of ions: The smaller the size of ions, the smaller is the distance between them, and greater will be the force of attraction and lattice energy.
Lattice energy of solids containing bi-bivalent ions > bi-univalent ions > uni-univalent ions.
E. Characteristics of ionic compounds
i. Ionic compounds have high melting and boiling point.
ii. In solid state, ionic compounds are bad conductors of electricity. However, in molten state or in the form of aqueous solution, they conduct electricity.
ii. Ionic compounds are polar in nature. Hence, these are soluble in polar solvents like water.
iv. These compounds do not possess directional characteristics and hence do not show stereoisomerism.
v. Ionic compounds are hard and brittle.
Check Your Grasp
1. Which of the following is an example of hypervalent or super octet molecule?
A. B.
C. D. All of these
2. Among the following, which compound will show the highest lattice energy?
A. KF B. NaF
C. CsF D. RbF
3. A crystal is hard and has high melting point. It is
A. covalent crystal B. ionic
C. metallic D. molecular
6. COVALENT BOND
The concept of covalent bond was given by Lewis. Covalent bond is formed by the sharing of electrons between the atoms of combining elements.
or H–H
Red Alert
A. Covalency
The concept of covalency was given by Langmuir. The number of electrons contributed by each atom for sharing is called its covalency. For example:
Covalency of each N atom is 3.
B. Characteristics of covalent compounds
i. Low melting and boiling point.
ii. Insoluble in water but soluble in organic solvents such as etc.
iii. Bad conductor of electricity.
iv. Reactions are slow and molecular in nature and never proceed to completion.
v. Show stereoisomerism.
C. Electronegativity
It is the tendency of an atom to attract the shared pair of electrons towards itself. It has no unit. The following scales are used
i. Pauling scale : For the molecule A–B,
Where B.E.(A – B) – (B.EAA + B.EBB
= excess bond energy.
ii. Mullikan’s scale : =
Fluorine is the most electronegative element and casium is the least one in the periodic table.
If electronegativity difference between two elements is 1.7, the bond has 50% ionic and 50% ionic and 50% covalent character.
If electronegativity difference > 1.7, then the bond is predominantly ionic and if electronegativity difference < 1.7, then bond is predominantly covalent.
D. Types of covalent bond
i. Polar covalent bond: If a covalent bond is formed between two different atoms, then the shared pair of electrons is displaced towards more electronegative atom. As a result, one end of the bond acquires partial negative charge while other one acquires partial positive charge. Such a bond is called polar covalent bond. For example: etc.
ii. Non polar covalent bond: It the covalent bond is formed between similar atoms, then the shared pair of electron lies exactly in the middle and the electron cloud is uniformly distributed between them. Such a bond is know as non-polar covalent bond.
For example: etc.
7. FORMAL CHARGE
Formal charge on atom in molecule or ion = total no. of electrons in valence shell of free atom – total number of electrons as Ione pairs (total number of electrons involved in bonding). Formal charge helps to predict most stable Lewis structure.
Red Alert
Sum of formal charge of all atoms in a molecule should be zero. In case of ion should be equal to charge on ion.
For example:
In
Formal charge on O-atoms 1, 2, 3 is 0, +1, –1 respectively.
EXAMPLE
EXAMPLE:1 Zeise’s salt contains which type of bonds?
A. Ionic
B. Ionic and covalent
C. Hydrogen bonds
D. Ionic, covalent and coordinate bonds
SOLUTION: Zeisels salt is
It has ionic, covalent (in ethylene molecules) and coordinate bonds.
Red Alert
8. VALENCE BOND THEORY –ORBITAL CONCEPT
According to this theory, a covalent bond is formed between two atoms by partial overlapping of their half-filled atomic orbitals containing electrons with opposite spin.
A. Sigma bond
When two half-filled atomic orbitals overlap along their internuclear axis, then bond formed is known as -bond. It may be formed by the overlap of two s, two p and one s and one p-orbitals.
B. -bond
When two atomic orbitals overlap sidewise, then -bond is formed. Following points must be noted
i. All single bonds are -bonds
ii. Multiple bonds contain one - bond and rest of -bonds.
iii. A - bond is never formed alone, exception being molecule, contains only - bonds.
iv. -bond is stronger than - bonds.
Red Alert
9. SOME BONDING PARMETERS
A. Bond length
The average distance between the centres of the nuclei of the two bonded atoms is called its bond length.
Factors affecting bond length
i. Size of atoms : The bond length increases with increase in size of atoms. For example: bond length follows the order
HF <HCl <HBr <HI
ii. Multiplicity of bond: The bond length decreases with bond multiplicity. For example: bond length follows the order,
iii. Type of hybridisation: The greater the s-character, the shorter is the hybrid-orbital and hence shorter is the bond length. For example: -character 33% s-character 50% s-character
B. Bond Energy (B.E):
It is the amount of energy required to break one mole of bonds of a particular type. It is expressed in kJ .
B.E. bond strength: For example: B.E. in molecule is while in molecule, it is 495 kJ . Thus, bond strength in is greater than that of molecule.
Factors affecting bond energy
i. Size of atoms: The greater the size of atoms, the greater is the bond length, the lesser is the bond strength and hence the lesser is B.E.
ii. Multiplicity of bonds: B.E. multiplicity of bonds. For example: in case of CC bond, B.E. is 836 kJ while B.E. is 610 kJ/ in C=C bond.
iii. Number of Ione pairs of electrons
B.E.
For example:
Bond
Lone pair 0 1 2 3
B.E. 347 163 146 139
C. Bond angle:
The angle between the lines respresenting the directions of the bonds is known as bond angle.
D. Bond order:
Bond order is given by the number of bonds between the two atoms in a molecule.
Isoelectronic molecules and ions have identical bond order
For example: and have identical bond order. 1 CO and have bond order 3.
Stability of molecules increases with increase in bond order, bond enthalpy increases and bond length decreases.
Check Your Grasp
4. The distance between the two adjacent carbon atoms is the largest in
A. benzene B. ethene
C. butane D. ethyne
5. Which one of the following has the shortest carbon carbon bond length?
A. Benzene B. Ethene
C. Ethyne D. Ethane
6. The percentage of s- character in the hybrid orbitals and follows the pattern.
A. B.
C. D.
10. DIPOLE MOMENT
It is the product of magnitude of the positive or negative charge on each atom of molecule and distance between them.
Mathematically :
Its unit is debye, denoted by D
1D = esu-cm.
1 esu = It is vector quantity. Its direction if from +ve to –Ve end. Dipole moment is a measure of polar character of the molecule.
Dipole moment is a vector quantity. For polyatomic molecule. The resultant net dipole moment is calculated by vector summation of two vector moments.
In case of symmetrical molecule, the resultant dipole moment is zero. For example:
A. Application of dipole moments
i. It helps in determining whether molecule is polar or non-polar.
ii. It helps in the determination of geometry of the molecule.
iii. It helps to distinguish cis and trans forms of geometrical isomers.
EXAMPLE
EXAMPLE:2 The dipole mement of o, p and m-dichlorobenzene will be in the order
A. B.
C. D.
SOLUTION:
In p-dichlorobenzene, two C-Cl dipoles cancel each other
In o-dichlorobenzene, the two C-Cl dipoles (say x) are inclined at an angle of . Therefore, according to parallelogram law of forces, the resultant
=
In m- dichlorobenzene, the two dipoles are inclined to each other at an angle of , therefore, resultant
=
Thus, the decreasing order of dipole moments
Red Alert
B. Percentage ionic character in a covalent bond
It can be calculated as follows:
i. From electronegativity difference: The higher the electronegativity difference between the atoms, the higher is the ionic character percentage. For example:
Electronegativity difference 0.2 0.6 0.8 1.2 2.2 3.2
% Ionic character 1 10 15 30 70 92
ii. Pauling equation
% Ionic character =
iii. Henry – Smith equation % ionic character
iv. From dipole moment % ionic character
=
EXAMPLE
EXAMPLE:3
If the bond energy of ClF is 61 kcal/mol. Which of the following is the correct electronegativity of chlorine there in (Given
⦁ 3.1 B. 3.2
C. 3.3 D. 3.5
SOLUTION: Δ
11. CO-ORDINATE BOND
This bond is formed in which shared pair of electrons comes only from one atom. The atom which donates electron pair is known as donor atom while other which accepts is the acceptor atom. It is represented by
For example: +
The co-ordinate bond has some polar character. Due to this reason, it is also known as semi-polar bond. It is also known as dative bond. In terms of V.B. theory a co-ordinate bond is formed by the overlap of a fully filled orbital containing a lone pair of electron with an empty orbital of another atom.
Characteristics of co-ordinate compounds
i. Insoluble in water but soluble in organic solvents.
ii. Generally do not conduct electricity.
iii. M.pts. and B.pts. are higher than covalent compounds but lower than that of ionic compounds.
iv. Show steroisomerism.
Check Your Grasp
7. In the co-ordinate valency
A. electrons are equally shared by the atoms
B. electron of one atom are shared by two atoms
C. hydrogen bond is formed
D. none of the above.
8. Which of the following has a net dipole moment?
A. B.
C. D.
12. HYBRIDISATION
The concept of hybridisation was given by Pauling and Slater. It involves the intermixing of two or more atomic orbitals of slightly different energies but of the same atom, so that redistribution of energy takes place between them resulting in the formation of an equal number of new orbitals having the same energy, size and shape.
Following points must be noted
A. Empty, half filled as well as completely filled orbitals can take part in hybridisation.
B. Hybridisation never takes place in an isolated atom. It takes place only at the time of bond formation.
C. Hybrid orbitals form stronger bond than pure atomic orbitals.
Note
Hybrid orbitals form stronger bonds because they provide more extent of overlapping than the pure atomic orbitals. Thus overlapping (i.e., bond) results in a stronger bond than the p-p overlapping (i,e., bond).
Red Alert
D. Types of hybridisation
The types of hybridisation with examples are given in the following table
Sr.No. Types of hybridisation Orbitals mixed Shape of molecule or ion Examples
1. sp one s + one p Linear
2. one s + two p Triangular planar , ,
3. one s + three p Tetrahedral
4. one done s+ two p Square planar 2-2-
5. one s + three p+ one d
Trigonal bipyramidal ,
6. one s + three p+ two d ) Octahedral planar 3-
7. One s + three p + three d
Pentagonal bipyramidal
TRICK
E. Method for finding hybridisation
i. Method I: H
Where H= Hybridisation
V= No. of valence electrons in central atom
M = No. of monovalent atoms attached to central atom
C = Charge on a cation
A = Charge on anion
For example: - ion.
Central atom = I
The no. of valence electron in I, (V) =7
The no. of monovalent atom, (M) =2
Charge on anion, (A) =1
Hybridisation
Note
Value of H 2 3 4 5 6 7
Hybridisation sp
Ii. Method II : Various rules are
1. Add no. of valence electron of all atoms present in molecule or ion.
2. If ion cation then subtract electrons equal to charge on cation. However, if it is anion, then add electrons equal to charge on it.
3. If these electrons are less than eight then eight then divide it by 2 and find the sum of quotient and remainder.
4. If these electrons are 9-56, then divide by eight, find the quotient say . Divide the remainder by 2, find quotient say . Find the sum of last remainder.
5. Report final result as X. The type of hybridisation (H) can be decided as
X 2 3 4 5 6 7
Type of H sp
For examples:
i. molecule: Total number of valence electrons = of O-atom;+ of two H-atoms =
X =
ii. ion .: Total number of valence electrons =5of N-atom; +18of three O-atoms +(due to negative charge)=24
Thus, H=
iii. ion: Total number of valences electrons= of Cl-atom + of 4 O- atoms +(due to negative charge) = 32
Thus, H=
Check Your Grasp
9. Both and hybrid carbons are present in which one of the following compounds?
A. B.
C. D.
10. In which of the following, the central atom does not use hybrid orbitals in its bonding?
A. B.
C. D.
11. What type of hybridisation is possible in square planar molecules?
A. B.
C. D.
12. In which one of the following molecules can the central atom be said to adopt hybridisation?
A. B.
C. D.
13. VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) THEORY
It was given by Sidgwick and Powell. According to this theory, the electron pairs present around the central atom repel each other and move far apart as far as possible so that there is no further repulsion between them. As a result, the molecule has minimum energy and maximum stability. The directions of pairs give definite geometry to the molecule. The magnitude of repulsive forces follow the order.
Red Alert
l.p.-l.p.-b.p.>b.p.-b.p., Where l.p.= Ione pair b.p. = bond pair.
Depending upon the number of electron pairs, the geometries of various molecules can be determined. The following table gives the geometry, type of hybridisation etc.
Check Your Grasp
13. The shape of sulphate ion is
A. square planar B. tetrahedral
C. trigonal bipyramidal D. hexagonal
14. Which of the following has a linear structure?
A. B.
C. D.
15. The bond angles of and are in the order
A.
B.
C.
D.
Sr.
No. Type of molecules Total No. of electron pairs No. of
Bond
pairs No. of
Ione pairs involved Type of
hybridisation Geometry of molecule Examples
1. 2 2 0 Linear
2. 3 3 0 Trigonal planar
3. 3 2 1 V-shaped
4. 4 4 0 Tetrahedral
5. 4 3 1 Trigonal pyramidal
6. 4 2 2 V-shaped
7. 5 5 0 Trigonal bipyramidal
8. 5 4 1 See saw
9. 5 3 2 T-shaped
10. 5 2 3 Linear
11. 6 6 0 octahedral
12. 6 5 1 Square pyramidal
13. 6 4 2 Square planar
14. 7 7 0 Pentagonal bipyramidal
14. MOLECULAR ORBITAL THEORY (MOT)
A. The main points of this theory are
i. When two A.O’s combine, then they lose their identity and form two new orbitals called M.O’s. One M.O. has lower energy than that of A.O. It is called bonding M.O. other M.O. other M.O. has higher energy than that of A.O. It is called antibonding.
ii. Only A.O. of comparable energy and proper orientation combine to form M.O’s.
iii. According to LCAO principle, bonding M.O’s. are formed by addition of wave function of electrons while antibonding M.O’s. are formed by subtraction of wave functions of electrons.
iv. Bonding M.O’s. contribute towards the stability molecules, while antibonding M.O’s. decrease the stability of molecule.
v. Bonding M.O’s. are represented by etc. while antibonding M.O’s. are represented by etc.
vi. The shape of M.O’s depends upon the shapes of A.O’s.
vii. The filling of M.O’s. takes place according to three principles namely pauli’s exclusion principle, Aufbau principle, and Hund’s rule of multiplicity.
B. Linear combination of atomic orbitals
The formation of molecular orbitals described by the linear combination of atomic orbitals that can take place by addition and by subtraction of wave function of individual atomic orbitals are shown below
Therefore two molecular orbitals and are formed as
is called bonding molecular orbital
is called anti bonding molecular orbital
B. Conditions for the combination of atomic orbitals
i. The combining atomic orbitals must have the same or nearly the same energy.
ii. The combining atomic orbitals must have the same symmetry about the molecular axis.
iii. The combining atomic orbitals must overlap to the maximum extent.
d Comparison of bonding and antibonding molecular orbitals
Sr.No. Bonding molecular orbital Antibonding molecular orbital
1. A bonding molecular orbital is formed by the linear combination of two atomic orbitals when their wavefunction are added. An antibonding molecular orbital is formed by the linear combination of the two atomic orbitals when their wave functions are subtracted.
2. It is formed when the same sign
(+and+or – and–) lobes atomic orbitals combine.
It is formed when opposite sign (+ and -) lobes atomic orbitals combine.
3. Its energy is less than combining atomic orbitals. Its energy is more than that of the combining atomic orbitals.
4. It favours the formation of bond and stabilises the molecule. It does not favour the bond formation and destabilise the molecule.
5. It increases the electron density in the space between the nuclei of the atomic orbitals. It decreases electron density in the space between the nuclei of the atomic orbitals.
6. It can be represented as and It is represented by the sign , and
15. ENERGY LEVEL DIAGRAM
It is the arrangement of filled M.O’s. of a molecule. There are two types of energy level diagrams.
Red Alert
For molecules upto the order of filling of M.O’ is
For molecules after the order of filling of M.O’s is
A. Information obtained from Energy Level Diagram
i. Bond order (B.O.): It is the number of covalent bonds present in a molecule. Mathematically
B.O.=
Where number of present in bonding M.O’s.
number of present in antibonding M.O’s.
Bond order indicates about
1. Formation of molecule
If B.O.>0, molecule is formed
If B.O. 0, molecule is not formed.
2. B.E. –B.E B.O.: The greater is the bond order, more is the bond energy.
3. Stability – B.O. stability: The greater is the bond order, more is the stability of molecule.
4. Bond length (B.L.) – B.O.
Higher is the bond order, the shorter is the bond length.
5. Nature of bond – Intergral bond order values of 1, 2 or 3 corresponds to single, double or triple bonds respectively.
ii. Magnetic Properties: Energy level diagram predicts whether molecule is paramagnetic or diamagnetic.
1. If there are one or more unpaired electrons in M.Os, then molecule is paramagnetic.
2. If there is no unpaired electron in M.Os, then molecule is diamagnetic.
Note
A. Order of stability and bond dissociation energies are
i.
ii.
B. Order of bond lengths
i.
ii.
EXAMPLE
EXAMPLE:4 The correct order of Cl –O bond lengths in , and is
A.
B.
C.
D.
Total no. of bonds between Cl and O
SOLUTION: Bond order =
SSpecies Bond order
Cl– 1
O=Cl – 1.5
B. Molecular orbital electronic configuration of some common heteronuclear molecules
The configuration of heteronuclear molecules can be written in similar manner as in the case of homonuclear molecules.
i) NO: The number of electrons = 7+8=15
NO : KK
Bond order =
: KK
Bond order
ii) CN : Total number of electrons =6+7=13
CN:
Bond order
: KK
Bond order
iii) CO : Total number of electrons =6+8=14
CO : KK
Bond order =
Check Your Grasp
16. The M.O. configuration of will be
A.
B. ,
C.
D. ,
17. The bond order for a species with the configuration will be
A. 1 B.
C. zero D. 1
18. Which of the following is paramagnetic?
A. B.
C. D. CO
19. Which of the following diamagnetic?
A. B.
C. D.
20. The bond length in the species and are in the order
A. B.
C. D.
Molecules or Ions Electric
Configuration Bond order Magnetic Character
1 Diamagnetic
½ Paramagnetic
0 Does not exist
½ Paramagnetic
KK where KK stands for 1 Diamagnetic
KK 3 Diamagnetic
KK 21/2 Paramagnetic
KK 2 Paramagnetic
KK 21/2 Paramagnetic
KK 11/2 Paramagnetic
KK 1 Diamagnetic
16. RESONANNCE
When a molecule has more than one structure and none of these structures define its all properties, then the actual structure lies between these structures. This structure is known as resonance hybrid structure while the rest of structures are known as resonation or contributing or canonical structures
For example:
For ozone ( molecules,
For carbon monoxide molecule,
⦁ Resonance Energy
The difference between the energy of resonance hybrid and that of the most stable resonating structure is called resonance energy.
⦁ Rules for writing resonance structures
i) Resonance structures differ in the position of electrons and not in the position of atoms.
ii) All these structures should have the same number of unpaired electrons.
iii) These structure should not differ much in energy.
iv) Structure in which negative charge is on electronegative atom and positive charge is on more electropositive atom, contribute more towards hybrid than the other ones.
v) Resonance structure should be written in such a way that unlike charge reside on adjacent atoms.
vi) Structure with greater number of covalent bonds contribute more towards hybrid than the other.
Misconcepts
1. The canonical forms have no real existence.
2. There is no actual equilibrium between canonical forms.
EXAMPLE:5 Which of the following is not considered as the canonical structure of ion?
SOLUTION: Position of nuclei is changed in (D), that’s why it is not considered as canonical structure.
17. HYDROGEN BOND
It was given by More and Winmill.When in a molecule, H-atom is linked to a highly electronegative atom such as F,O,N, then this end becomes negatively charged, while H- end acquires positive charge. The negative end of one molecule attracts the positive end of other molecule. The bond thus formed is known as H-bond. It is represented by dotted line.
The energy of H-bonding is -42 kJ/mol. The actual strength of H- bond depends on the electronegativity difference of the bonded atoms.
A. Conditions for hydrogen bonding
i. The molecule should contain an atom of high electronegativity such as F,O or N bonded to hydrogen atom.
ii. The size of electronegative atom should be small for strong hydrogen bonding.
B. H- bonds are of two types
i. Intermolecular H-bond: It is formed between two same or different molecule
ii. Intramolecular H-bond: it is formed within the same molecule.
Red Alert.
⦁ Effect of H- bonding
i) Intermolecular H-bonding increases the melting end boiling point., solubility, viscosity, surface tension etc.
ii) Intramolecular H- bonding has opposite effects. This is because intramolecular H-bonding prevents the association of other molecules.
18. METALLIC BOND
According to electron gas model, there is a sea of valence electrons in which kernel are immersed. A metallic bond is formed between positive kernel and mobile electrons.
Characteristics of Metallic bonds
i) Metals are good conductors of heat an electricity due to presence of mobile electrons. The conductivity decreases with rise in temperature due to vibrations of positive kernels which, in turn, interfere the movement of free electrons.
ii) Metal possesses bright luster due to and from oscillations of electrons.
iii) Metallic bonds have no directional characteristics.
19. VAN DER WAAL’S FORCES
These are very weak attractive forces. These are of three types
A. Dipole – Dipole interactions
In case of polar molecules, the van der Waal’s forces are mainly due to electrical interactions between oppositely charged ends of molecules. These are called dipole-dipole interactions.
For example: Gases such as etc. have permanent dipole moments. The greater is the dipole moment, the greater is the magnitude of dipole – dipole interactions.
B. Dipole – induced dipole interactions
These interactions exist between a polar molecule and a non-polar molecule. The polar molecule has definite dipole moment. This induces a temporary dipole moment in non polar molecule. The magnitude of interactions depends upon the dipole moment of polar molecule and polarisability of non polar molecule.
For example: Solubility of inert gases in increases due to increase in dipole induced dipole interactions.
C. Momentary dipole – induced dipole interactions
These interactions exist between non-polar molecules. The electrons of non-polar molecules keep on oscillating w.r.t. the nuclei of the atoms. As a result, at a given instant, one side of the molecule may have a slight excess of electrons relative to opposite side. Thus, a non-polar molecule may become momentarily self polarised. This polarised molecule will induce dipole moment in another non-polar molecule. These forces are also known as London or dispersive forces.
The magnitude of these forces depends upon
i.Size of the molecule
ii. Shape of the molecule
Note Order of strength of bonds
Ionic bond > Covalent bond > Metallic bond > H–bond > van der Waal’s forces.
20. FANJAN’S RULE
Factors Condition Electrovalency Covalency
Charge Low High Low
High Low High
Size of anion Small High Low
Large Low High
Size of cation Small Low High
Large High Low
According to Fajan rule, in ionic bond, some covalent character is introduced because of the tendency of the cation to polarise the anion. In fact cation attracts the electron cloud of the anion and pulls electron density between the two nuclei.
According to Fajan’s rule, the magnitude of covalent character in the ionic bond depends upon the extent of polarization caused by cation. In general,
i. The smaller the size of cation, the larger is its polarizing power.
ii. Among two cations of similar size, the polarizing power of cation with pseudonoble gas configuration is larger than cation with noble gas configuration e.g., Polarising power of is more than .
iii. The larger the anion, the more will be its polarizability.
iv. The higher the charge, the more is polarizing power and polarizability.
Note is more covalent than because polarising power of is more than that of Similarly is more covalent than .
Certain increasing order of covalent nature are
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
Check Your Grasp
21. Which one shows maximum hydrogen bonding?
A. B.
C. D. HF
22. Which of the following does not apply to metallic bond?
A. Overloading valence orbitals
B. Mobile valency electrons
C. Delocalized electrons
D. Highly directed bonds
23. The weakest among the following types of bonds is
A. ionic B. covalent
C. metallic D. H-bond
24. According to Fajan’s rules covalent bonding is favourable by
A. small cation and large anion
B. small cation and small anion
C. large cation and large anion
D. large cation and small anion
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