Chemical bonding 2 by aditya abeysinghe


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Chemical bonding 2 by aditya abeysinghe

  1. 1. Continued from ChemicalBonding-1 by Aditya AbeysingheFor more information about thebasics of bonding please visit theabove mentioned presentationChemical Bonding-2 by AdityaAbeysinghe1
  2. 2. Chemical Bonding-2 by AdityaAbeysingheChemical Bonding-2 by AdityaAbeysinghe2
  3. 3. 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 thesubstanceChemical Bonding-2 by AdityaAbeysinghe3
  4. 4. 1. Size of the substance-When the size of the substance increases, it is moretendentious 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.Chemical Bonding-2 by AdityaAbeysinghe4
  5. 5. 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:Chemical Bonding-2 by AdityaAbeysinghe5
  6. 6. Chemical Bonding-2 by AdityaAbeysinghe6Molecule Dipole Moment (D)H2 0HF 1.78HCl 1.07HBr 0.79HI 0.38ClF 0.88BrF 1.29BrCl 0.52H2O 1.85H2S 0.95CO2 0NH3 1.47NF3 0.23NCl3 0.39CH4 0
  7. 7. 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 forcesChemical Bonding-2 by AdityaAbeysinghe7
  8. 8. Dipole-dipole forcesThe interactions between atoms through theiroppositely charged poles are called dipole-dipoleforces.Chemical Bonding-2 by AdityaAbeysinghe8δ+ δ-δ + represents the positive pole of the atomwhile δ - represents the negative pole of theatomBroken lines showing theintermolecular attractions-dipole-dipole forcesDue 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
  9. 9. Dipole-ion forcesThe forces between cations and anions due to thepresence of an electric field between them can besummarized as follows:Chemical Bonding-2 by AdityaAbeysinghe9+ -
  10. 10. These bonds are important in forming hydratesand ammoniates of compounds.E.g.: [Mg.6H2O]2+ , [Ca.6NH3]2+Chemical Bonding-2 by AdityaAbeysinghe10
  11. 11. Ion-induced dipole forcesA 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 AdityaAbeysinghe11δ+I II-Iodideion Iodine (I2 )moleculeI3- CompoundAnother example- Li+ nG , where G is a noble gas and n= 1 or 2
  12. 12. 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)Chemical Bonding-2 by AdityaAbeysinghe12HO -2δDipole(Water)Electrically neutralsubstance laterinduced andseparated into polesby the inducingaction ofwater(dipole)Induced dipole
  13. 13. 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.Chemical Bonding-2 by AdityaAbeysinghe13DipoleInduced dipole-1 Induced dipole- 2Induceddipole-Induceddipolebond
  14. 14. 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.Chemical Bonding-2 by AdityaAbeysinghe14
  15. 15. 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 ).Chemical Bonding-2 by AdityaAbeysinghe15
  16. 16. 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.Chemical Bonding-2 by AdityaAbeysinghe16
  17. 17. Chemical Bonding-2 by AdityaAbeysinghe17CClHCH3ODue to the presence of hydrogen bonds, there’s an increase in the melting and boilingpoints of elements where hydrogen bonds are presentBy energy of bonds,London Forces < Hydrogen bonds < Primary bonds(ionic, metallic, and covalent bonds)
  18. 18. 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.Chemical Bonding-2 by AdityaAbeysinghe18
  19. 19. ΔHD (Single Bond) < ΔHD (Double bond) < ΔHD (Triple bond)Heat needed to break the bonds increasesBecause double bonds and triple bonds tend topull atoms close together,Single bond > Double bond > Triple bondIntermolecular distance increasesChemical Bonding-2 by AdityaAbeysinghe19
  20. 20. 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.Chemical Bonding-2 by AdityaAbeysinghe20
  21. 21. Chemical Bonding-2 by AdityaAbeysinghe21SOResonanceStructure 1ResonanceStructure 2ResonanceStructure 3OSThis intermediate form betweendifferent contributing structures iscalled a resonancehybrid(true structure)
  22. 22. The actual or the true structure is always stable than the resonance structureand the energy of the actual structure is thus lower than the resonancestructuresChemical Bonding-2 by AdityaAbeysinghe22ΔERE – Resonanceenergy differenceΔH /KJmol-1(ResonanceStructures)(Resonance Hybrid(True Structure))
  23. 23. Although sulfur trioxide is an overall neutralmolecule, many ions too show resonance.The charge distribution of such an actual structure is asfollows:Chemical Bonding-2 by AdityaAbeysinghe23- 2/3*The three -2/3 charges on the three oxygen atoms add up to the total of -2 charge on theoverall molecule.