Intermediate Type of Bonding 9 Intermediate Type of Bonding 9.1 Incomplete Electron Transfer in Ionic Compounds 9.2 Electronegativity of Elements 9.3 Polarity of Covalent Bonds

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

Pure covalent Intermediate Pure ionic 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+

Pure covalent Intermediate Pure ionic Incomplete transfer of electrons Or Unequal sharing of electrons Polar molecule with partial –ve charge on B and partial +ve charge on A

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.

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.

Pure ionic bond does not exist Li+ F LiF(g) Electron clouds are not perfectly spherical Slight distortion or sharing of electron cloud

Polarization of ionic bond - Incomplete Transfer of Electron

Determination of Lattice Enthalpy 1. Experimental method : - from Born-Haber cycle

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Determination of Lattice Enthalpy 1. Experimental method : - from Born-Haber cycle 2. Theoretical calculation : - based on an ionic model

r+ + r Ionic model : Assumptions 1. Ions are spherical and have no distortion of electron cloud, I.e. 100% ionic. 2. Oppositely charged ions are in direct contact with each other. r+ + r

3. The crystal has certain assumed lattice structure. 4. The interaction between oppositely charged ions are electrostatic in nature. 5. Repulsive forces between oppositely charged ions at short distances are ignored.

Comparison of theoretical and experimental values of lattice enthalpy Discrepancy : - Reveals the nature of the bond in the compound

Lattice enthalpy (kJ mol-1) 0.14 -631.8 -630.9 KI 0.87 -672.3 -666.5 KBr 0.84 -697.8 -692.0 KCl 0.38 -688.3 -685.7 NaI 0.74 -733.0 -730.5 NaBr 0.04 -766.4 -766.1 NaCl % deviation Experimental Theoretical 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

Lattice enthalpy (kJ mol-1) 5.5 -3615.0 -3427.0 Zns 12 -867.0 -774.0 AgI 8.5 -877.0 -808.0 AgBr 6.8 -890.0 -833.0 AgCl % deviation Experimental Theoretical Lattice enthalpy (kJ mol-1) Compound Silver halides and zinc sulphide show large discrepancies between the two values.  Silver halides and zinc sulphide are NOT purely ionic compounds

Lattice enthalpy (kJ mol-1) 5.5 -3615.0 -3427.0 Zns 12 -867.0 -774.0 AgI 8.5 -877.0 -808.0 AgBr 6.8 -890.0 -833.0 AgCl % deviation Experimental Theoretical Lattice enthalpy (kJ mol-1) Compound The experimental values are always more negative than the theoretical values  Polarization of a chemical bond always results in a stronger bond.

Large % deviation of lattice enthalpy 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 Purely ionic Purely covalent Large % deviation of lattice enthalpy  greater b and more covalent character

Small % deviation of lattice enthalpy 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 Purely ionic Purely covalent Small % deviation of lattice enthalpy  smaller b and less covalent character

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 

Q.50(a) Charge : Al3+ > Mg2+ > Na+ Size : Al3+ < Mg2+ < Na+ Al3+ > Mg2+ > Na+ Polarizing power : Al3+ > Mg2+ > Na+

Q.50(b) Charge : Li+ = Na+ = K+ Size : Li+ < Na+ < K+ Li+ > Na+ > K+ Polarizing power : Li+ > Na+ > K+

For anions Polarizability : - 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 

I > Br > Cl > F S2 > O2 Polarizability  as the size of the anion  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

Lattice enthalpy (kJ mol-1) 5.5 -3615.0 -3427.0 ZnS 12 -867.0 -774.0 AgI 8.5 -877.0 -808.0 AgBr 6.8 -890.0 -833.0 AgCl % deviation Experimental Theoretical Lattice enthalpy (kJ mol-1) Compound Polarizability : I > Br > Cl % deviation : AgI > AgBr > AgCl Covalent character : AgI > AgBr > AgCl

Lattice enthalpy (kJ mol-1) 5.5 -3615.0 -3427.0 ZnS 12 -867.0 -774.0 AgI 8.5 -877.0 -808.0 AgBr 6.8 -890.0 -833.0 AgCl % deviation Experimental Theoretical Lattice enthalpy (kJ mol-1) Compound Great % deviation of ZnS due to high polarizability of the large S2 ion

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

Lattice enthalpy (kJ mol-1) 12 -867.0 -774.0 AgI 8.5 -808.0 AgBr 6.8 -890.0 -833.0 AgCl 0.38 -688.3 -685.7 NaI 0.74 -733.0 -730.5 NaBr 0.04 -766.4 -766.1 NaCl % deviation Experimental Theoretical Lattice enthalpy (kJ mol-1) Compound Ionic radius : Ag+ > Na+ Why are AgX more covalent than NaX ?

Ag+ = [Kr] 5s1 4d9 Na+ = Ne Polarizing power : Ag+ > Na+ 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+ Polarizing power : Ag+ > Na+

Ag+ = [Kr] 5s1 4d9 Na+ = Ne Polarizing power : Ag+ > Na+ Noble gas configuration of the cation produces better shielding effect and less polarizing power Polarizing power : Ag+ > Na+

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

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

Q.51(c) Solubility in water : - Gp I carbonates >> other carbonates Carbonate ions are large and carry two negative charges. Thus, they can be easily polarized by cations to exhibit more covalent character. 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

Example 9-1 Check Point 9-1 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 9-1 Check Point 9-1

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 incomplete d/f subshell

Apart from those compounds mentioned on p Apart from those compounds mentioned on p.63, list THREE ionic compounds with high covalent character. AlCl3 , MgI2 , CuCO3

Polarization of Covalent Bond : – Unequal Sharing of electrons Evidence : - Deflection of a jet of a polar liquid(e.g. H2O) in a non-uniform electrostatic field Breakdown of additivity rule of covalent radii Breakdown of additivity rule of bond enthalpies

Contains polar molecules Liquid shows deflection Contains non-polar molecules Liquid shows no deflection

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

a positively charged rod a polar molecule a positively charged rod Orientation of polar molecules towards a positively charged rod. Demonstration

Solvents showing no deflection Solvents showing a marked deflection Trichloromethane, CHCl3 Ethanol,CH3CH2OH Propanone Water, H2O Tetrachloromethane Cyclohexane Benzene Carbon disulphide

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

Additivity rule of covalent radii Assumption : Electrons are equally shared between A and B Pure covalent bond

Experimental value/nm 0.1940 0.1320 0.1430 0.1160 Bond CBr in CBr4 CF in CF4 CO in CH3OH CO in CO2 Experimental value/nm 0.1940 0.1320 0.1430 0.1160 Estimated bond length/nm 0.1275 0.1510 0.1480 0.1910 % deviation 9.91% 5.59% 12.12% -1.54% Failure of additivity rule indicates formation of covalent bond with ionic character due to polarization of shared electron cloud to the more electronegative atom.

Experimental value/nm 0.1940 0.1320 0.1430 0.1160 Bond CBr in CBr4 CF in CF4 CO in CH3OH CO in CO2 Experimental value/nm 0.1940 0.1320 0.1430 0.1160 Estimated bond length/nm 0.1275 0.1510 0.1480 0.1910 % deviation 9.91% 5.59% 12.12% -1.54% Polarization of a covalent bond always results in the formation a stronger bond with shorter bond length. + 

Equal sharing of electrons Breakdown of additivity rule of bond enthalpy E(H – H) = 436 kJ mol1 E(F – F) = 158 kJ mol1 Equal sharing of electrons A.M. G.M. E(H – F) = 565 kJ mol1 >> A.M. or G.M.

Pauling Scale of Electronegativity (1932) E(H – F) = 565 kJ mol1 >> 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)

For the molecule A–X nA and nX are the electronegativity values of A and X respectively nF = 4.0

Q.52 Given : E(H–H)  436 kJ mol1 , E(F–F)  158 kJ mol1 , E(H–F)  565 kJ mol1 , E(Cl–Cl)  242 kJ mol1 , E(H–Cl)  431 kJ mol1 Calculate the electronegativity values of H and Cl. More electronegative nH = 2.2 nCl = 3.3

Estimation of Ionic Character of Chemical Bonds Two methods : - 1. The difference in electronegativity between the bonding atoms nA – nX  (Qualitative) 2. The electric dipole moment of diatomic molecule (Quantitative)

1. The difference in electronegativity between 1. The difference in electronegativity between the bonding atoms nA – nX  (Qualitative) nA – nX  2.0 ionic or nearly ionic bond e.g. Li – F bond (4.0 – 1.0) = 3.0 nA – nX  0.4 covalent or nearly covalent bond e.g. C – H bond (2.5 – 2.1) = 0.4 0.4  nA – nX  2.0 covalent bond with ionic character or ionic bond with covalent character

1 Debye (D) = 3.3361030 Coulomb meter 2. The electric dipole moment of diatomic molecule (Quantitative)  = q  d SI units : - Coulomb meter 1 Debye (D) = 3.3361030 Coulomb meter

Centre of postive charge Electric dipole moment is a vector pointing from the positive pole to the negative pole

Dipole moment (Coulomb meter) Estimating the % ionic character of H–Cl bond by dipole moment Molecule Dipole moment (Coulomb meter) Bond length meter H–Cl 3.6891030 1.2841010 Electronic charge, e  1.6021019 Coulomb

If H–Cl is 100% ionic, dipole moment 1.6021019 Coulomb1.2841010 meter  2.0571029 Cm The measured dipole moment of H–Cl  3.6891030 Cm

Q.53 Electronic charge, e  1.6021019 Coulomb Molecule NO HI ClF HF CsF Bond length(Å) 1.154 1.620 1.632 0.926 2.347 Dipole moment(D) 0.159 0.448 0.888 1.827 7.884 % ionic character 70.0 41.1 11.3 14.8 2.87

Q.53 Electronic charge, e  1.6021019 Coulomb Molecule NaCl KF KCl LiF Bond length(Å) 2.365 2.176 2.671 1.570 Dipole moment(D) 9.001 8.593 10.269 6.327 % ionic character 83.9 80.1 82.2 79.3

Good correlation between two methods Calculated from dipole moment Good correlation between two methods nA – nX 

Ionic with covalent character 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 Ionic with covalent character Polar covalent Purely Ionic Purely covalent

Ionic with covalent character  difference in electronegativity values  difference in electronegativity values NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2 Ionic with covalent character Polar covalent Purely Ionic Purely covalent

Ionic with covalent character  extent of polarization of ionic bond  extent of polarization of covalent bond NaCl MgCl2 AlCl3 SiCl4 PCl5 SCl2 Cl2 Ionic with covalent character Polar covalent Purely Ionic Purely covalent

Polarity of Molecules depends on : - Polarity of bonds  nA – nX  or dipole moment Geometry of molecules Symmetrical molecules are usually non-polar due to symmetrical arrangements of dipole moments

Bond polarity Geometry of molecule Polarity of molecule Polar Asymmetrical Polar Polar Symmetrical Non-polar Non-polar Asymmetrical Non-polar Non-polar Symmetrical Non-polar

The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs. Net dipole moment (the vector sum) is zero  Non-polar

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

The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs. Net dipole moment (the vector sum) is zero  Non-polar

The overall dipole moment of a molecule is the vector sum of dipole moments of individual bonds and lone pairs. Net dipole moment (the vector sum) is zero  Non-polar

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

Q.54 +  or

Q.55 Non-polar Polar

Symmetrical  Non-polar Q.55 Symmetrical  Non-polar

Xe F Dipole moments of the two lone pairs point in opposite directions Non-polar

Q.55 Non-polar

Q.55 Non-zero vector sum  Polar molecule

Q.56(a) >

Q.56(b) >

Explain the following phenomena: 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.

Explain the following phenomena: 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.

Explain the following phenomena: (b) Both NBr3 and NF3 are polar but their molecules align differently in a non-uniform electrostatic field.

(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:

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.

Cancelling out of dipole moments Non-polar molecules Tetrahedral Trigonal planar Linear Cancelling out of dipole moments Molecule Shape

Cancelling out of dipole moments Non-polar molecules Octahedral Trigonal bipyramidal Cancelling out of dipole moments Molecule Shape

Polar molecules Tetrahedral Trigonal pyramidal V-shaped ( or bent) Net resultant dipole moment Dipole moment of individual polar bonds Molecule Shape

Use of dipole moments Provide important structural information about molecules

9.1 Incomplete electron transfer in ionic compounds (SB p.250) Example 9-1 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? Compound Theoretical lattice enthalpy (kJ mol-1) Experimental lattice enthalpy (kJ mol-1) X+Br-(s) -665 -670 Y+Br-(s) -758 -890 Answer

9.1 Incomplete electron transfer in ionic compounds (SB p.250) Example 9-1 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.

9.1 Incomplete electron transfer in ionic compounds (SB p.250) Example 9-1 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

9.3 Polarity of covalent bonds (SB p.252) Let's Think 1 Pure ionic bond and pure covalent bond are two extreme bond types. Why? Answer 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. Back

Ionic with covalent character 9.3 Polarity of covalent bonds (SB p.252) Back Check Point 9-3A 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 Ionic with covalent character Polar covalent Purely covalent

9.3 Polarity of covalent bonds (SB p.257) Example 9-3 Explain the variation in dipole moment of the following molecules. Molecule Dipole moment (D) CH4 NH3 0.35 H2O 0.65 HF 1.07 Answer

Example 9-3 9.3 Polarity of covalent bonds (SB p.257) The dipole moment of a molecule is based on two factors: 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 O F 2.1 2.5 3.0 3.5 4.0

Example 9-3 9.3 Polarity of covalent bonds (SB p.257) 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

Example 9-3 Back 9.3 Polarity of covalent bonds (SB p.257) 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.

9.3 Polarity of covalent bonds (SB p.257) Check Point 9-3B 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

Check Point 9-3B Back 9.3 Polarity of covalent bonds (SB p.257) Polar Tetrahedral (c) CHCl3 Trigonal pyramidal (b) NH3 Non-polar Trigonal planar (a) BCl3 Polar or non-polar Structural formula Shape Molecule