chemical bonding 2 by aditya abeysinghe

Continued from ChemicalBonding-1 by Aditya Abeysinghe

For more information about thebasics of bonding please visit theabove mentioned presentation

Chemical Bonding-2 by AdityaAbeysinghe

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PolarityPolarity is the measurement which depicts thetendency of an atom’s, of a molecule’s or of an ion’sto separate into poles.(Here poles are the two ends of the bond in whichmolecules are bonded or in which atoms are selfpolarized as the positive and the negative poles.)There are two factors that affect the polarity of asubstance:1. The size of the substance2. The distribution of the electrons around the

substance


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1. Size of the substance-When the size of the substance increases, it is more

tendentious to be separated into poles. Thus the polarity ofsuch a substance is higher than that of a smaller substance.(When the size of a substance increases, the electrons in theoutermost shell show less attraction to the nucleus than in asmaller atom)2. The distribution of electrons-

When there’s an asymmetric electron distributionaround the nucleus of a substance its polarity is high ascompared to that in a substance with a symmetrical electrondistribution (Here asymmetric means that the resultant of theforces directed towards the nucleus of a substance is not zero)E.g.: Ammonia is asymmetrical due to the presence of a lonepair of electron whereas methane is symmetrical as theresultant forces of any 3 bonds will be balanced by the fourthforce to make the molecule symmetrical.


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Dipole Moment (μ)Dipole moment is the product of the charge of a poleand the distance between the two charges.The SI unit of the dipole moment is the coulomb meter.However, since the measurements of these types are sosmall, the dipole moment is usually calculated by a unitcalled Debye1 Debye (D) = 3.34 × 10-30 Cm . (Here Cm meansCoulomb meter)Dipole moment is a key factor in deciding about thevariation of the intermolecular attractions.Dipole moments of some compounds are as follows:


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Molecule Dipole Moment (D)

H2 0

HF 1.78

HCl 1.07

HBr 0.79

HI 0.38

ClF 0.88

BrF 1.29

BrCl 0.52

H2O 1.85

H2S 0.95

CO2 0

NH3 1.47

NF3 0.23

NCl3 0.39

CH4 0

Intermolecular forces of attractionThe term intermolecular refers to attractionsbetween molecules.There are different types of such attractions:1. Dipole-Dipole forces2. Dipole-ion forces3. Ion-induced dipole forces4. Dipole-induced dipole forces5. Induced dipole-Induced dipole forces6. Hydrogen bonds7. Vanderwall’s forces


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Dipole-dipole forcesThe interactions between atoms through theiroppositely charged poles are called dipole-dipoleforces.


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δ+ δ-δ + represents the positive pole of the atomwhile δ - represents the negative pole of theatom

Broken lines showing theintermolecular attractions-dipole-dipole forces

Due to this bond single elementscannot escape from the solution inwhich they exist. Thus, meltingpoints of such substances are high.

These attractive forces are weakerthan the full charges carried by ionsin ionic crystals

Dipole-ion forcesThe forces between cations and anions due to thepresence of an electric field between them can besummarized as follows:


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+ -

These bonds are important in forming hydratesand ammoniates of compounds.E.g.: [Mg.6H2O]2+ , [Ca.6NH3]2+


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Ion-induced dipole forces

A nonpolar molecule or atom may be polarized byan electron cloud of an ion and the induced dipolemay make a bond with the ion. Such a bond iscalled an ion-induced dipole force.

Chemical Bonding-2 by AdityaAbeysinghe 11

δ+

I II-

Iodideion Iodine (I2 )

molecule

I3- Compound

Another example- Li+ nG , where G is a noble gas and n= 1 or 2

Dipole- Induced dipole bondAn atom or a molecule which is nonpolar may beinduced by a field of a permanent dipole and theformation of a bond between the permanentdipole and the induced dipole is called a dipole-induced dipole bond.E.g.: Noble gas hydrates (G.xH2O ; maximum x=6)


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H

O -2δ

Dipole(Water)

Electrically neutralsubstance laterinduced andseparated into polesby the inducingaction ofwater(dipole)

Induced dipole

Induced dipole-Induced dipole bondsAlthough the electron cloud of a noble gas may besymmetrically arranged, it can be asymmetrical quicklydue to the movement of electrons.Therefore, noble gases and nonpolar molecules act astemporary induced dipoles. Self-induced dipoles polarizemore non-polar atoms or molecules. Such a bond is calledan induced dipole-induced dipole bond.


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DipoleInduced dipole-1 Induced dipole- 2

Induceddipole-Induceddipolebond

London forcesLondon forces are a type of force that can be foundin both polar and nonpolar molecules.A nonpolar atom/ molecule may sometimesbecome polar due to the constant motion of itselectrons. This might even cause uneven chargedistribution.When this occurs, the molecule/atom has atemporary dipole.This dipole can then cause a second atom beattached to the opposite pole of the first molecule.


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London dispersion forces are the weakest of all the electronic forcesthat act between atoms or between molecules.Due to these forces nonpolar substances such as noble gases andhalogens condense into liquids and then freeze into solids when thetemperature is lowered sufficientlyLondon force are the cause as to why liquids are composed ofmolecules with no permanent dipole attraction and have low boilingpoints compared to their molecular masses.Solids which have London dispersion attraction have rather softcrystals, are easily deformed, and vaporize easily.Because of the low intermolecular forces, the melting points are alsolow and evaporation takes place so easily that it may occur at roomtemperatureE.g.: Iodine crystals , Moth Balls (Para dichlorobenzene andnaphthalene ).


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Hydrogen BondsA proton or hydrogen nucleus has a highconcentration of positive charge when ahydrogen atom is bonded to a highlyelectronegative atom. Its positive charge willhave an attraction for neighboring electron pairs.This special kind of dipole attraction is calledhydrogen bond.


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C

Cl

H

CH3

O

Due to the presence of hydrogen bonds, there’s an increase in the melting and boilingpoints of elements where hydrogen bonds are present

By energy of bonds,

London Forces < Hydrogen bonds < Primary bonds (ionic,metallic, and covalent bonds)

Double and triple bondsTo achieve the octet structure some atoms toshare two or even three pairs of electrons.Sharing two pairs- Double bond (CO2 )Sharing three pairs- Triple bond (C2H2)

In double and triple bonds the attractive forcebetween the nuclei and the shared electrons isgreater as compared to the bonds where singlebonds are formed.


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ΔHD (Single Bond) < ΔHD (Double bond) < ΔHD (Triple bond)

Heat needed to break the bonds increases

Because double bonds and triple bonds tend topull atoms close together,

Single bond > Double bond > Triple bond

Intermolecular distance increases


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Resonance StructuresIt’s not always possible to represent the bondingstructure of a molecule by either the Lewis dotstructure or the line drawing structure.This is because data about the bond strengths andbond distance reveal that most molecules have ahybrid(resonant) structure.Thus many molecules are represented as a series oftransitions of bond structures, not as an individualcomponent.E.g.: Sulfur trioxide and Benzene.


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S

O

ResonanceStructure 1



O

SThis intermediate form betweendifferent contributing structures is

called a resonancehybrid(true structure)

The actual or the true structure is always stable than the resonance structureand the energy of the actual structure is thus lower than the resonancestructures


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ΔERE – Resonanceenergy difference

ΔH /KJmol-1

(ResonanceStructures)

(Resonance Hybrid(True Structure))

Although sulfur trioxide is an overall neutral molecule,many ions too show resonance.The charge distribution of such an actual structure is asfollows:


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- 2/3

*The three -2/3 charges on the three oxygen atoms add up to the total of -2 charge on theoverall molecule.

chemical bonding 2 by aditya abeysinghe

Education

dipoledipole forces2

dipoleinduced dipole

dipole moment dipole

dipole forcesdue

permanent dipole

dipole moments

ioninduced dipole forces4

dipoleion forces3