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NIS – CHEMISTRY

NIS – CHEMISTRY. Lecture 22 Bohr Atomic Model Ozgur Unal. Ladder and the Rungs. While using the ladder, i s i t possible to stand between the rungs ? What is your energy when you stand at the lowest rung (minimum energy)? How does your energy change as you climb up the ladder?.

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NIS – CHEMISTRY

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  1. NIS – CHEMISTRY Lecture 22 BohrAtomic Model OzgurUnal

  2. LadderandtheRungs While using the ladder, isit possible to stand between the rungs? What is your energy when you stand at the lowest rung (minimum energy)? How does your energy change as you climb up the ladder?

  3. Bohr’s Model of the Atom Dualwave-particle model of photonaccountedforseveralpreviouslyunexplainablephenomena. Scientistsstilldid not fullyunderstandtherelationshipbetweenatomicstructure, electronsandatomicspectra. NielsBohr, proposed a quantum model fortheHydrogen atom in 1913. His model correctlypredictedthe frequencies of thelines in H atomic emissionspectra.

  4. Bohr’s Model of the Atom Bohrproposedthat H atom can onlyhavecertainallowableenergystates. Thelowestallowableenergystate of an atom is calleditsgroundstate. When an atomsgainsenergy, it is saidto be in an excitedstate. • Bohralsorelatedthe H atom’senergystatestotheelectronwithinthe atom. • He suggestedthattheelectron in a H atom movesaroundthenucleus in onlycertainallowedcircularorbits. • Thesmallertheelectron’sorbit, thelowertheatom’senergystateorenergylevel. • H atom can haveseveralexcitedstates, although it has only 1 electron.

  5. Bohr’s Model of the Atom Bohralsoassigned a quantumnumber, n, toeachenergylevel. He calculatedtheradius of eachorbit. CheckoutTable 5.1

  6. Bohrsuggestedthat H atom is in • thegroundstate, alsocalledfirst • energylevel, whenitselectron is in • the n=1 orbit. • Ifenergy is addedtothe H atom, • theelectronmovesto a higherenergy • level, such as n=2 or n=3 andso on. • When a H atom is in an excitedstate, theelectron can dropfromthehigherenergylevelto a lowerenergylevelbyemitting a photon. • Theenergy of thephotonemitted is foundby: • ΔE = Ehigherenergyorbit - Elowerenergyorbit = Ephoton = hν • www.upscale.utoronto.ca/PVB/Harrison/BohrNodel/Flash/BohrModel.html Bohr’s Model of the Atom

  7. Bohr’s Model of the Atom Since onlycertaintransitionsarepossiblebetweentehenergylevels, onlycertainfrequencies of light can be emittedby H atom. Thisexplainsthe H atomicspectrum. Somephotonsemitted thiswayarevisibleto humaneye, someare not.

  8. Bohr’s Model of the Atom Bohr’satomic model can explain H atom and H-likeatoms, such as He+1, Li+2, Be+3etc. Bohr’s model cannotaccountforelementswithmorethanoneelectrons. Indeed, Bohr’s model is fundamentallyincorrect. Electrons be havedifferently in atoms as wewillsee in thenextlecture. Furthermodifications of his atomic model leadtoquantummechanical model of atoms.

  9. NIS – CHEMISTRY Lecture 23 TheQuantumMechanical Model of the Atom OzgurUnal

  10. Electrons as Waves • Bohr’satomic model wasfundamentallyincorrect. • Onereason is that, electronsbehavelikewaves in atoms. • Similartothebladesoccupyingthewholearea, electrons in atomsappeartofilltheentirevolume in an atom. • In 1924, FrenchphysicistsLouis de Brogliesuggestedthatelectronshavecharacteristicssimilartowaves. • Photonshavebothwavelikeandparticlelikeproperties, so do electrons! What information can you tell about the blades? Is it possible to tell where the blades are? Do blades appear to fill the entire area?

  11. de BroglieEquation de Brogliealsosuggestedthat not onlyelectrons but allparticlesshowwaveproperties. He derivedthefollowingequation: λ = h / m v λ representswavelength, h is Planck’sconstant, m is themass of theparticle vrepresentsspeed • Wavesareassociatedwithallmovingparticles, even a moving car has a wavelength! • Since thewavelengths of largeobjectsaresosmall, it is impossibletodetectthesewaves.

  12. TheHeisenbergUncertaintyPrinciple AnotherreasonwhyBohr’s model wasfundamentallyincorrectwasfoundbyWernerHeisenberg. Heisenbergprovedthat it is fundamentallyimpossibletoknowpreciselythepositionandthevelocity (momentum) of a particle at thesame time, therewillalways be an uncertainty in themeasurements. This is Heisenberguncertaintyprinciple. • Imagine a blindfoldedstudenttryingtolocate • a balloon in a room. • If s/he touchestheball, theballgainsvelocity. • Therefore, whiledeterminingtheposition, the • information of velocitycannot be precisely • determined.

  13. TheHeisenbergUncertaintyPrinciple Similarly, theinteractionbetweenphotonsandelectronsmakes it impossibletodetermine thepositionand momentum at thesame time. • Forelectronsmovingaround 6 x 106 m/s, theuncertainty in position is muchmorethanthe size of an atom. • However, we can findtheprobabilitiesthattells us howlikely an electron can be found in a givenvolume of an atom.

  14. ShrödingerWaveEquation AustrianphysicistErwinSchrödingerfoundoutanequationtowhichparticlesobey. Schrödingerequationtreatselectrons as a waveandthisequation can be appliedtoelectrons of elementsotherthan H. Atomic model in whichelectrons aretreated as waves is calledthewave mechanical model of the atom, orthe quantummechanical model of the atom.

  15. TheQuantumMechanical Model of the Atom Accordingtoquantummechanical model of atoms, electronshaveprobablelocations. Theseprobablelocationsarearoundthenucleusandcalledatomicorbitals. An atomicorbitalis like a fuzzycloud in whichthedensity at a givenpoint is proportionaltotheprobability of findingtheelectron at thatpoint.

  16. NIS – CHEMISTRY Lecture 24 Hydrogen’sAtomicOrbitals OzgurUnal

  17. Hydrogen’sAtomicOrbitals Is it possible to find the electron very close to the nucleus? How probable is it? Is it possible to find an electron very far from the nucleus? How probable is it? How do you think we can determine the size of an atom? • Scientists defined the size of an atom by drawing an orbital surface that contains 90% of the electron’s total probability distribution. • Figure 5.15

  18. PrincipalQuantumNumber RememberBohr’satomic model assignsquantumnumberstoenergylevels. Similarly, QuantumMechanical (QM) model assignsfourquantumnumberstoatomicorbitals. Thefirstone is principalquantumnumber (n) andindicatestherelative size andtheenergy of atomicorbitals. As n increases, theorbitalbecomeslargerandtheatom’senergyincreases. Eachenergylevel is calledprincipalenergylevel. Upto 7 energylevelshavebeendetectedfor H atom.

  19. EnergySublevels Principalenerglevelscontainenergysublevels. n = 1 has 1 energysublevel n = 2 has 2 energysublevels n = 3 has 3 energysublevels n = 4 has 4 energysublevels Sublevelsarelabeled as s, p, d and f. • 1s • 2s and 2p • 3s, 3p and 3d • 4s, 4p, 4d and 4f

  20. EnergySublevels All s orbitalsarespherical. • All p orbitalsaredumbbellshaped. • Thereare 3 dumbbellshaped • p orbitals: px, py, andpz.

  21. EnergySublevels Each d sublevelrelatesto 5 orbitals of equalenergy. Four of the d orbitalshaveidenticalshapes but differentorientationsalongthe x, y and z axes. Thefifth d orbital is shaped andorienteddifferently. • Thereare 7 f orbitals. • Each f orbital has complexmultilobedshapes. • CheckoutFigure 5.2

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