1 intermediate type of bonding 9.1incomplete electron transfer in ionic compounds...

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Intermediate Type Intermediate Type of Bondingof Bonding

9.19.1 Incomplete Electron Transfer in Ionic Incomplete Electron Transfer in Ionic CompoundsCompounds

9.29.2 Electronegativity of ElementsElectronegativity of Elements

9.39.3 Polarity of Covalent BondsPolarity of Covalent Bonds

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Pure ionic and covalent bonds are only extremes of a continuum.

Most chemical bonds are intermediate between the two extremes.

Pure covalent Intermediate Pure ionic

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Equal sharing of electrons

Symmetrical distribution of electron cloud

Non-polar molecule

Complete transfer of electrons

Spherical electron clouds

Electron cloud of D is not

polarized by C+

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Incomplete transfer of electronsOr

Unequal sharing of electrons

Polar molecule with partial –ve charge on B and partial +ve charge on A

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Polarization of a covalent bond means the displacement of shared electron cloud towards the more electronegative atom (Cl).

Polarization of a covalent bond results in a covalent bond with ionic character.

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Polarization of an ionic bond means the distortion of the electron cloud of an anion towards a cation by the influence of the electric field of the cation.

Polarization of an ionic bond results in an ionic bond with covalent character.

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Pure ionic bond does not exist

Li+ F

LiF(g)Electron clouds are not perfectly spherical

Slight distortion or sharing of electron cloud

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Polarization of ionic bond Polarization of ionic bond - Incomplete Transfer of - Incomplete Transfer of

ElectronElectron

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Determination of Lattice Determination of Lattice EnthalpyEnthalpy

1. Experimental method : - from Born-Haber cycle

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-349

-791.4

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Determination of Lattice Determination of Lattice EnthalpyEnthalpy

1. Experimental method : - from Born-Haber cycle

2.Theoretical calculation : -based on an ionic model

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1. Ions are spherical and have no distortion of electron cloud, I.e. 100% ionic.

Ionic model : Assumptions

2. Oppositely charged ions are in direct contact with each other.

r+ + r

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3. The crystal has certain assumed lattice structure.

5. Repulsive forces between oppositely charged ions at short distances are ignored.

4. The interaction between oppositely charged ions are electrostatic in nature.

)r(r4πQMLQ

ΔH0

lattice

o

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Comparison of theoretical and Comparison of theoretical and experimental values of lattice experimental values of lattice enthalpyenthalpy

Discrepancy : -

Reveals the nature of the bond in the compound

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0.14-631.8-630.9KI

0.87-672.3-666.5KBr0.84-697.8-692.0KCl0.38-688.3-685.7NaI0.74-733.0-730.5NaBr0.04-766.4-766.1NaCl

% deviationExperimentalTheoretical

Lattice enthalpy (kJ mol-1)Compound

Good agreement between the two values for alkali halides The simple ionic model used for calculating the theoretical value holds true All alkali halides are typical ionic compounds

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5.5-3615.0-3427.0Zns12-867.0-774.0AgI8.5-877.0-808.0AgBr6.8-890.0-833.0AgCl



Silver halides and zinc sulphide show large discrepancies between the two values. Silver halides and zinc sulphide are NOT purely ionic compounds

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5.5-3615.0-3427.0Zns12-867.0-774.0AgI8.5-877.0-808.0AgBr6.8-890.0-833.0AgCl



The experimental values are always more negative than the theoretical values Polarization of a chemical bond always results in a stronger bond.

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The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms.

E.g. Ag+ Cl Ag–Cl

Large % deviation of lattice enthalpy greater b and more covalent

character

Purely ionic

Purely covalen

tClAgClAgAgCl ba

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The real picture of the polarized bond can be considered as a resonance hybrid of the two canonical forms.

E.g. Ag+ Cl Ag–Cl

Small % deviation of lattice enthalpy smaller b and less covalent

character

Purely ionic

Purely covalen

tClAgClAgAgCl ba

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Factors that Favour Polarization of Ionic Bond – Fajans’ Rules

For cations

Polarizing power : - The ability of a cation to polarize the electron cloud of an anion.

Polarizing power as the of the cation

sizecharge

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Q.50(a)

Charge : Al3+ > Mg2+ > Na+

Size : Al3+ < Mg2+ < Na+

: Size

ChargeAl3+ > Mg2+ > Na+

Polarizing power :Al3+ > Mg2+ > Na+

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Q.50(b)

Charge : Li+ = Na+ = K+

Size : Li+ < Na+ < K+

: Size

ChargeLi+ > Na+ > K+

Polarizing power :Li+ > Na+ > K+

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For anionsPolarizability : - A measure of how easily the electron cloud of an anion can be distorted or polarized by a cation.

Polarizability as the size of the anion

Polarizability as the charge of the anion

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Larger size of anion

outer electrons are further away from the nucleus

electrons are less firmly held by the nucleus and are more easily polarized by cations

I > Br > Cl > F

S2 > O2

Polarizability as the size of the anion

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5.5-3615.0-3427.0ZnS12-867.0-774.0AgI8.5-877.0-808.0AgBr6.8-890.0-833.0AgCl



Polarizability : I > Br > Cl

% deviation : AgI > AgBr > AgCl

Covalent character : AgI > AgBr > AgCl

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5.5-3615.0-3427.0ZnS12-867.0-774.0AgI8.5-877.0-808.0AgBr6.8-890.0-833.0AgCl



Great % deviation of ZnS due to high polarizability of the large S2 ion

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Polarizability as the charge of the anion

Higher charge in the anion results in greater repulsion between electrons

electrons are less firmly held by the nucleus and are more easily polarized by cations

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12-867.0-774.0AgI8.5-867.0-808.0AgBr6.8-890.0-833.0AgCl

0.38-688.3-685.7NaI0.74-733.0-730.5NaBr0.04-766.4-766.1NaCl



Ionic radius : Ag+ > Na+

Why are AgX more covalent than NaX ?

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Polarizing power : Ag+ > Na+

Ag+ = [Kr] 5s1 4d9

Na+ = Ne

The valence 4d electrons are less penetrating They shield less effectively the electron cloud of the anion from the nuclear attraction of the cation The electron cloud of the anion experiences a stronger nuclear attraction

Ag+ has a higher ENC than Na+

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Polarizing power : Ag+ > Na+

Ag+ = [Kr] 5s1 4d9

Na+ = Ne

Noble gas configuration of the cation produces better shielding effect and less polarizing power

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Q.51(a)

Solubility in water : NaX >> AgX

AgX has more covalent character due to higher extent of bond polarization.

Thus, it is less soluble in water

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Q.51(b)

Solubility in water : AgF > AgCl > AgBr > AgI

Polarizability : F < Cl < Br < I

Extent of polarization : F < Cl < Br < I

Ionic character : AgF > AgCl > AgBr > AgI

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Q.51(c)

Solubility in water : -

Gp I carbonates >> other carbonates

However, ions of group I metals have very small charge/size ratio and thus are much less polarizing than other metal ions.

Gp I carbonates have less covalent character

Carbonate ions are large and carry two negative charges. Thus, they can be easily polarized by cations to exhibit more covalent character.

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Q.51(d)

Solubility in water : LiX << other Gp I halide

Li+ is very small and thus is highly polarizing.

LiX has more covalent character

Example 9-1Example 9-1

Check Point 9-1Check Point 9-1

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Fajans’ rules – A summary

Ionic Covalent

Low charge on ions High charge on ions

Large cation Small cation

Small anion Large anion

Noble gas configuration

Valence shell electron configuration with incom

plete d/f subshell

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Apart from those compounds mentioned on p.63, list THREE ionic compounds with high covalent character.

AlCl3 , MgI2 , CuCO3

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Polarization of Covalent Bond : – Unequal Sharing of electrons

Evidence : -

1. Deflection of a jet of a polar liquid(e.g. H2O) in a non-uniform electrostatic field

2. Breakdown of additivity rule of covalent radii

3. Breakdown of additivity rule of bond enthalpies

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Liquid shows deflection

Contains polar

molecules

Liquid shows no deflection

Contains non-polarmolecules

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a charged rod

deflectionof water

Deflection of a polar liquid (water) under the influence of a charged rod.

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a polar moleculea positively charged rod

Orientation of polar molecules towards a positively charged rod.

DemonstrationDemonstration

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TetrachloromethaneCyclohexaneBenzeneCarbon disulphide

Trichloromethane, CHCl3

Ethanol,CH3CH2OHPropanoneWater, H2O

Solvents showing no deflection

Solvents showing a marked deflection

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A stream of water is attracted (deflected) to a charged rod, regardless of the sign of the charges on the rod. Explain.

O

H

H+

+

O

H

H

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Additivity rule of covalent radii

Assumption : Electrons are equally shared between A and B

Pure covalent bond

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9.91%5.59%12.12%-1.54%% deviation

0.12750.15100.14800.1910Estimated bond

length/nm

0.11600.14300.13200.1940Experimental

value/nm

CO in CO2

CO in CH3OH

CF in CF4

CBr in CBr4

Bond

Failure of additivity rule indicates formation of

covalent bond with ionic character due to polarization of shared electron cloud to the more electronegative atom.

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9.91%5.59%12.12%-1.54%% deviation

0.12750.15100.14800.1910Estimated bond

length/nm

0.11600.14300.13200.1940Experimental

value/nm

CO in CO2

CO in CH3OH

CF in CF4

CBr in CBr4

Bond

Polarization of a covalent bond always results in the formation a stronger bond with shorter bond length.

C F+

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Breakdown of additivity rule of bond enthalpy

E(H – H) = 436 kJ mol1

E(F – F) = 158 kJ mol1

E(H – F) = 565 kJ mol1 >> A.M. or G.M.

1mol kJ 2972

F)E(FH)E(H

A.M.

1mol kJ 262F)E(FH)E(H G.M.

Equal sharing of electrons

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E(H – F) = 565 kJ mol1 >> A.M. or G.M.Greater difference

Higher extent of bond polarization Greater difference in electronegativity values of bonding atoms

Pauling Scale of Electronegativity (1932)

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X)E(AX)E(XA)E(Ann96 2XA

For the molecule A–X

nA and nX are the electronegativity values of A and X respectively

nF = 4.0

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Q.52

565158436)n96(4.0 2H

nH = 2.2

4312424362.2)96(n 2Cl

nCl = 3.3

More electronegative

Given : E(H–H) 436 kJ mol1 , E(F–F) 158 kJ mol1 , E(H–F) 565 kJ mol1 , E(Cl–Cl) 242 kJ mol1 , E

(H–Cl) 431 kJ mol1 Calculate the electronegativity values of H and Cl.

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Estimation of Ionic Character of Chemical BondsTwo methods : -1. The difference in electronegativity between

the bonding atoms nA – nX (Qualitative)

2. The electric dipole moment of diatomic molecule (Quantitative)

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nA – nX 2.0 ionic or nearly ionic bond

nA – nX 0.4 covalent or nearly covalent bond

0.4 nA – nX 2.0

covalent bond with ionic character orionic bond with covalent character

e.g. C – H bond (2.5 – 2.1) = 0.4

e.g. Li – F bond (4.0 – 1.0) = 3.0

1. The difference in electronegativity between the bonding atoms nA – nX (Qualitative)

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2. The electric dipole moment of diatomic molecule (Quantitative)

= q d = q d

SI units : - Coulomb meter

1 Debye (D) = 3.3361030 Coulomb meter

1 Debye (D) = 3.3361030 Coulomb meter

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Electric dipole moment is a vector pointing from the positive pole to the negative pole

Centre of postive charge

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Estimating the % ionic character of H–Cl bond by dipole moment

Molecule

Dipole moment (Coulomb meter)

Bond length meter

H–Cl 3.6891030 1.2841010

Electronic charge, e 1.6021019 Coulomb

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If H–Cl is 100% ionic,

dipole moment 1.6021019 Coulomb1.2841010

meter 2.0571029 CmThe measured dipole moment of H–Cl 3.6891030 Cm

17.9%100%Cm 102.057Cm 103.689

character ionic % 29

30

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Q.53

70.041.111.314.82.87% ionic

character

7.8841.8270.8880.4480.159Dipole moment(D)

2.3470.9261.6321.6201.154Bond len

gth(Å)

CsFHFClFHINOMolecule


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Q.53

83.980.182.279.3% ionic

character

6.32710.26

98.5939.001

Dipole moment(D)

1.5702.6712.1762.365Bond len

gth(Å)

LiFKClKFNaClMolecule


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Calculated from dipole moment

nA – nX

Good correlation between two

methods

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How do you expect the bond type to change for the chlorides of the third period elements, NaCl, MgCl2, AlCl3, SiCl4, PCl5, SCl2 and Cl2, going from left to right?

Explain the change in the bond type.

NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2

Purely Ionic

Purely covale

nt

Ionic with covalent

character

Polar covalent

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difference in electronegativity val

ues

difference in electronegativity val

ues


Purely Ionic

Purely covale

nt

Ionic with covalent

character

Polar covalent

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extent of polarization of ionic bon

d

extent of polarization of covalent

bond


Purely Ionic

Purely covale

nt

Ionic with covalent

character

Polar covalent

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Polarity of Moleculesdepends on : -

1. Polarity of bonds

nA – nX or dipole moment

2. Geometry of molecules

Symmetrical molecules are usually non-polar

due to symmetrical arrangements of dipole moments

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Non-polarSymmetricalNon-polar

Non-polarAsymmetrica

lNon-polar

Non-polarSymmetricalPolar

PolarAsymmetrica

lPolar

Polarity of molecule

Geometry of molecule

Bond polarity

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The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs.

C OO

Net dipole moment (the vector sum) is zero

Non-polar

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B

F

F F


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Non-polar

B

F

F F

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Cl

CCl Cl

Cl



Non-polar

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The overall dipole moment of a molecule is the vector sum of dipole moments of individual bondsand lone pairs.

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NF F

FN

H HH

+

+

+

or

Q.54

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SO O

S

O

O O

Non-polar

Polar

Q.55

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SF

F F

F

F

F

Q.55

Symmetrical Non-polar

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Xe

F

F

F

F

Dipole moments of the two lone pairs point in opposite directions

XeF

F F

F

Non-polar

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Q.55

C C

H

H

H

H

Non-polar

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Non-zero vector sum

Polar molecule

Q.55

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Cl

CH H

H

I

CH H

H

Br

CH H

H

Q.56(a)

> >

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Cl

Cl

Cl

Cl

Cl

Cl

> >

Q.56(b)

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Explain the following phenomena:(a)PCl3 is polar but BCl3 is non-polar.

BCl3 has three polar B−Cl bonds and is trigonal planar in shape. As the dipole moments of the three polar bonds cancel out each other, the molecule is non-polar.

B

Cl

Cl Cl

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Explain the following phenomena:(a)PCl3 is polar but BCl3 is non-polar.

PCl3 has three polar P−Cl bonds and is trigonal pyramidal in shape. As there is a resultant dipole moment arising from the three polar bonds, the molecule is polar.

P

ClCl

Cl

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Explain the following phenomena:(b) Both NBr3 and NF3 are polar but their molecules

align differently in a non-uniform electrostatic field.

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(b) As the order of electronegativity is F > N > Br, the resultant dipole moments of NBr3 and NF3 are pointing to different directions. The situations are shown below:

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In a non-uniform electrostatic field, the nitrogen end of NBr3 will point to the positive pole while the nitrogen end of NF3 will point to the negative pole.

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Non-polar Non-polar moleculesmolecules

Tetrahedral

Trigonal planar

Linear

Cancelling out of dipole moments

MoleculeShape

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Non-polar Non-polar moleculesmolecules

Octahedral

Trigonal bipyramidal

Cancelling out of dipole moments

MoleculeShape

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Polar Polar moleculesmolecules

Tetrahedral

Trigonal pyramidal

V-shaped

( or bent)

Net resultant dipole

moment

Dipole moment of individual

polar bonds

MoleculeShape

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Use of dipole momentsUse of dipole moments

• Provide important structural information about molecules

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9.1 Incomplete electron transfer in ionic compounds (SB p.250)

The following gives the theoretical and experimental values of the lattice enthalpies of two metal bromides. X+Br- and Y+Br-.

(a)There is a high degree of agreement between the theoretical and experimental values in the case of X+Br-(s) but a large discrepancy in the case of Y+Br-(s). What can you tell about the bond type of the two compounds? Answer

Compound Theoretical lattice

enthalpy (kJ mol-1)

Experimental lattice

enthalpy (kJ mol-1)

X+Br-(s) -665 -670

Y+Br-(s) -758 -890

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(a) Since the theoretical value of the lattice enthalpy is calculated based on a simple ionic model, the good agreement for X+Br-(s) suggests that the compound is nearly purely ionic. The ions are nearly spherical with nearly uniform distribution of charges. The bond type in the compound is thus nearly purely ionic.

For Y+Br-(s), the large discrepancy suggests that the simple ionic model does not hold due to the distortion of the electron cloud of the anion. Thus the bond type in this compound has a certain degree of covalent character.

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(b)To which group in the Periodic Table does metal X belong? Explain your answer.

Answer(b) As X+ ion must have a low polarizing power, its

charge to size ratio should be small. X is a Group I metal.

Back

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9.3 Polarity of covalent bonds (SB p.252)

Pure ionic bond and pure covalent bond are two extreme bond types. Why?

In pure ionic bonding, the bonded atoms are so different that one or more electrons are transferred to form oppositely charged ions. Two identical atoms share electrons equally in pure covalent bonding. This type of bonding results from the mutual attraction of the two nuclei for the shared electrons. Between these extremes are intermediate cases in which the atoms are not so different that electrons are incompletely transferred and unequal sharing results, forming polar covalent bond.

Answer

Back

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How do you expect the bond type to change for the chlorides of the third period elements, NaCl, MgCl2, AlCl3, SiCl4, PCl5, SCl2 and Cl2, going from left to right?

Explain the change in the bond type.

Back9.3 Polarity of covalent bonds (SB p.252)


Purely Ionic

Ionic with covalent

character

Polar covalent Purely covale

nt

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Explain the variation in dipole moment of the following molecules.

Answer


Molecule Dipole moment (D)

CH4 0

NH3 0.35

H2O 0.65

HF 1.07

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The dipole moment of a molecule is based on two factors:

1. Bond polarity

This depends on the electronegativity of the atoms involved in a bond. A bond is said to be polar if there is a difference in electronegativity between two bonded atoms. The larger the difference, the more polar is the bond.

H C N OF

2.1 2.5 3.0 3.54.0

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2. The geometry

If the molecule have symmetrical arrangements of polar bonds, the dipole moments of the bonds will cancel out each other.

CH4 NH3

No net dipole moment Net dipole moment resulted

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Back


H2O HF

Net dipole moment resulted Net dipole moment resulted

(Note: Lone pair(s) is/are not shown in the above diagrams)

Hence, zero dipole moment is only observed in CH4. HF has the largest dipole moment since the difference in electronegativity between the hydrogen atom and the fluorine atom is the largest. H2O comes the second, followed by NH3.

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Give the shapes and structural formulae of the following molecules. State whether each molecule is polar or non-polar.

(a) BCl3

(b) NH3

(c) CHCl3

Answer

96


Back

PolarTetrahedral(c) CHCl3

PolarTrigonal pyramidal

(b) NH3

Non-polarTrigonal planar(a) BCl3

Polar or non-polar

Structural formula

ShapeMolecule

1 intermediate type of bonding 9.1incomplete electron transfer in ionic compounds...

Documents

ionic model slide

electron cloud slide

pure ionic bond

compound slide

ionic bond results

ionic character

c slide

covalent bond results