Atomic Structure and Function 25 September 2003 Professor Taylor PhysicsChemistry Astronomy Astronomy Geology GeologyBiology Ecology Ecology

Atomic Structure and Atomic Structure and FunctionFunction

25 September 200325 September 2003Professor TaylorProfessor Taylor

PhysicsPhysics

ChemistryChemistry

AstronomyAstronomy

GeologyGeology

BiologyBiology

EcologyEcology

Atomic Structure and Atomic Structure and Function:Function:OutlineOutline

Early history in 18Early history in 18thth and 19 and 19thth centuries centuries Discovery of the electron (1897)Discovery of the electron (1897) Discovery of the nucleus (1911)Discovery of the nucleus (1911) Atomic structure: consternationAtomic structure: consternation Atomic structure: the Bohr atom (1913)Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Atomic Structure: Theory of Quantum

Mechanics (1930’s)Mechanics (1930’s) Applications of our understandingApplications of our understanding Discovery of the elementsDiscovery of the elements

Atomic Structure and Atomic Structure and Function:Function:

Take Home MessageTake Home Message Science as a evolutionary, incremental process Science as a evolutionary, incremental process

over decades (e.g., exploring the structure of over decades (e.g., exploring the structure of atom)atom)

Role of hypothesis development and testing in Role of hypothesis development and testing in the sciences – scientific methodthe sciences – scientific method

Demystification of physics and chemistry: Demystification of physics and chemistry: structure of the atom is a simple conceptstructure of the atom is a simple concept

Understanding of atomic structure underpins Understanding of atomic structure underpins future course topics (e.g., chemical reactions, future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental geology, biochemistry, ecology, environmental sciences)sciences)

Applications – some examplesApplications – some examples

Early History in 18Early History in 18thth and and 1919thth Century Century

Atomic Theory of DaltonAtomic Theory of Dalton Atoms as indivisible “marbles”Atoms as indivisible “marbles” Element (shape and size the same for a given Element (shape and size the same for a given

element)element) Molecules/compounds (e.g., water or HMolecules/compounds (e.g., water or H22O) combine O) combine

in predictable ways and ratiosin predictable ways and ratios Atoms as basis for understanding chemistry Atoms as basis for understanding chemistry

but without electrons, protons and neutrons but without electrons, protons and neutrons (analogy to a marble)(analogy to a marble)

Elements – 60 known (e.g., hydrogen, gold, Elements – 60 known (e.g., hydrogen, gold, sodium)sodium)


Early history in 18Early history in 18thth and 19 and 19thth centuries centuries Discovery of the electron (1897)Discovery of the electron (1897) Discovery of the nucleus (1911)Discovery of the nucleus (1911) Atomic structure with consternationAtomic structure with consternation Atomic structure: the Bohr atom (1913)Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Atomic Structure: Theory of Quantum

Mechanics (1930’s)Mechanics (1930’s) Applications of our understandingApplications of our understanding

Thompson’s Discovery of Thompson’s Discovery of the Electronthe Electron

Idea that there may be some smaller Idea that there may be some smaller components that comprise atomscomponents that comprise atoms

HypothesisHypothesis: Atoms consist of : Atoms consist of subcomponents, one of which is subcomponents, one of which is negatively charged and very small in negatively charged and very small in massmass

Experiment to test hypothesisExperiment to test hypothesis

Thompson’s ExperimentThompson’s Experiment

Thompson’s ExperimentThompson’s Experiment

Thompson’s Discovery of Thompson’s Discovery of the Electronthe Electron

Idea that there may be some smaller Idea that there may be some smaller components that comprise atomscomponents that comprise atoms

Hypothesis: Atoms consist of some Hypothesis: Atoms consist of some subcomponents, one of which is subcomponents, one of which is negatively charged and very small in negatively charged and very small in massmass

Hypothesis: accept the hypothesisHypothesis: accept the hypothesis




Rutherford’s Discovery of a Rutherford’s Discovery of a NucleusNucleus

Idea that there may be some smaller Idea that there may be some smaller components that comprise atoms, components that comprise atoms, including negatively and positively including negatively and positively charges componentscharges components

HypothesisHypothesis: Atoms consist of multiple : Atoms consist of multiple subcomponents, some negative and subcomponents, some negative and some positivesome positive

Experiment to test hypothesisExperiment to test hypothesis Alpha particles (+), gold foil, “bullets”, Alpha particles (+), gold foil, “bullets”,

and “tracks”and “tracks”

Rutherford’s Experiment:Rutherford’s Experiment:

ExplanationExplanation

Observations, Hypothesis Observations, Hypothesis and Conclusionand Conclusion

Observations:Observations: Almost all alpha’s passed through foil Almost all alpha’s passed through foil

unaffectedunaffected Very small number of alpha’s deflected @ Very small number of alpha’s deflected @

small anglesmall angle 1/1000 deflected at large angle (struck 1/1000 deflected at large angle (struck

“head on”)“head on”) HypothesisHypothesis: Atoms consist of some : Atoms consist of some

positive subcomponentspositive subcomponents Accept the hypothesisAccept the hypothesis

New Model of AtomNew Model of Atom

Small, dense center, positively charged – Small, dense center, positively charged – called the nucleus (later to be called the called the nucleus (later to be called the proton)proton)

Small, light (w/o mass), negative Small, light (w/o mass), negative charged subcomponent circles the charged subcomponent circles the nucleus (electron)nucleus (electron)

Later additions (1932)Later additions (1932) NeutronsNeutrons ProtonsProtons Charge (neutrality calls for + and – charges)Charge (neutrality calls for + and – charges)




Atomic Structure with Atomic Structure with ConsternationConsternation

Consternation with Rutherford ModelConsternation with Rutherford Model Electrons moving, so must give off Electrons moving, so must give off

energyenergy 22ndnd Law of Thermodynamics Law of Thermodynamics Electrons should collapseElectrons should collapse Yet atoms are billions of years oldYet atoms are billions of years old

Conclusions: model violated Conclusions: model violated fundamental laws of physicsfundamental laws of physics




Atomic Structure: Bohr Atomic Structure: Bohr AtomAtom

Observations (1913)Observations (1913) Heat hydrogen gas and light is emitted Heat hydrogen gas and light is emitted

as a as a discretediscrete wavelength (not wavelength (not continuous spectrum)continuous spectrum)

Other gases behave the same in Other gases behave the same in producing discrete wavelength, but producing discrete wavelength, but each gas unique in wavelengths each gas unique in wavelengths producedproduced


HypothesisHypothesis Electrons circle the nucleus at specific Electrons circle the nucleus at specific

distances from the nucleus in defined distances from the nucleus in defined orbitsorbits

DiagramDiagram


Explanation of light in discrete Explanation of light in discrete wavelengthswavelengths As “excited”/heated electrons drop As “excited”/heated electrons drop

back from orbit, a packet of energy is back from orbit, a packet of energy is lost as a function of distance from the lost as a function of distance from the nucleusnucleus

Packet of energy is called photon (light)Packet of energy is called photon (light)


Key aspects of the Bohr atomKey aspects of the Bohr atom No space between the orbits for electrons to be No space between the orbits for electrons to be

so “jump” has to be “all or none” – so “jump” has to be “all or none” – quantumquantum levels of electronslevels of electrons

Excited electrons in outer orbit has 2 options:Excited electrons in outer orbit has 2 options: Drop down to next lower orbit and emit a packet of Drop down to next lower orbit and emit a packet of

energy (light) unique for that distanceenergy (light) unique for that distance Drop back to even lower level (ground state) and emit Drop back to even lower level (ground state) and emit

packet of energy (light) unique for that that distancepacket of energy (light) unique for that that distance Energy release is unique for each element and Energy release is unique for each element and

is a “is a “windowwindow” on atomic structure (lab ” on atomic structure (lab exercise)exercise)

Ideas verified two decades later using Ideas verified two decades later using quantum mechanicsquantum mechanics

Bohr AtomBohr Atom


Early history in 18Early history in 18thth and 19 and 19thth centuries centuries Discovery of the electron (1897)Discovery of the electron (1897) Discovery of the nucleus (1911)Discovery of the nucleus (1911) Atomic structure: consternationAtomic structure: consternation Atomic structure: the Bohr atom (1913)Atomic structure: the Bohr atom (1913) Atomic Structure: Atomic Structure:

Consternation with the Bohr Atom (theory)Consternation with the Bohr Atom (theory) Theory of Quantum Mechanics (1930’s)Theory of Quantum Mechanics (1930’s)

Applications of our understandingApplications of our understanding Discovery of the elementsDiscovery of the elements

Theory of Quantum Theory of Quantum MechanicsMechanics

ObservationsObservations Behavior of electrons in heated hydrogen gas were Behavior of electrons in heated hydrogen gas were

consistent with Bohr Model (orbits, etc.)consistent with Bohr Model (orbits, etc.) Behavior of other heavier gases could not be Behavior of other heavier gases could not be

explained by Bohr Modelexplained by Bohr Model Investigators (Heisenberg, Shrodinger): Investigators (Heisenberg, Shrodinger):

wave-particle duality of lightwave-particle duality of light Key: Integrated mechanisms of waves and Key: Integrated mechanisms of waves and

particles, focusing on “fuzzy electron particles, focusing on “fuzzy electron clouds”/clouds”/waveswaves

HypothesisHypothesis: new model of atomic structure : new model of atomic structure and function: quantum mechanics and function: quantum mechanics theorytheory of of the atomthe atom Incorporation of know factsIncorporation of know facts Prediction of new properties yet to be discoveredPrediction of new properties yet to be discovered

Quantum MechanicsQuantum Mechanics

Quantum MechanicsQuantum Mechanics

Quantum Mechanics ModelQuantum Mechanics Model

Key PrinciplesKey Principles Electron behaves as a Electron behaves as a wavewave not a particle (Bohr) not a particle (Bohr) Energy of electron behaves as a “spread-out Energy of electron behaves as a “spread-out

wave” in three dimensions - not discrete bundle wave” in three dimensions - not discrete bundle of energy (Bohr)of energy (Bohr)

Description is based on quantum numbersDescription is based on quantum numbers Distance from nucleusDistance from nucleus Energy level and sublevelEnergy level and sublevel Orientation of electrons in spaceOrientation of electrons in space Direction of electron spinDirection of electron spin

Quantum Mechanics Model: Quantum Mechanics Model: Principle Energy LevelsPrinciple Energy Levels

Main energy level of electronsMain energy level of electrons Distance from nucleusDistance from nucleus

n = 1 (lowest energy level)n = 1 (lowest energy level) n = 2n = 2 n = 3n = 3 n = 4 (highest energy level)n = 4 (highest energy level) etc.etc.

Higher the number, the higher the Higher the number, the higher the energyenergy

Quantum Mechanics Model: Quantum Mechanics Model:

Sub-Energy LevelsSub-Energy Levels

Electrons in “fuzzy” probability Electrons in “fuzzy” probability distribution called an “orbital”distribution called an “orbital” e.g., n = 1 has 1 sub-energy level/orbital (2 e.g., n = 1 has 1 sub-energy level/orbital (2

electron orientations)electron orientations) Heisenberg Uncertainty PrincipleHeisenberg Uncertainty Principle

Electrons in an orbital can only exist in Electrons in an orbital can only exist in pairs (2) and the pair spins in opposite pairs (2) and the pair spins in opposite directions directions Pauli Exclusion PrinciplePauli Exclusion Principle

Pauli’s Exclusion PrinciplePauli’s Exclusion Principle

Quantum Mechanics Model: Quantum Mechanics Model:

Electron ConfigurationElectron Configuration

Electrons always adopt most energetically Electrons always adopt most energetically stable energy levels/orbitals consistent stable energy levels/orbitals consistent with Laws of Thermodynamics and with Laws of Thermodynamics and previous conditions (above)previous conditions (above) If n = 1, two electrons in 1st orbital (If n = 1, two electrons in 1st orbital (ss orbital) orbital) If n = 2, six electrons in 3 orbitals (If n = 2, six electrons in 3 orbitals (pp orbitals) orbitals)

Electron Configuration in p Electron Configuration in p OrbitalOrbital




Applications of Our Applications of Our Understanding of Atomic Understanding of Atomic

StructureStructure SpectroscopySpectroscopy: quantum differences in light : quantum differences in light

emitted or absorbed result in unique emitted or absorbed result in unique signature for each element (“fingerprint”)signature for each element (“fingerprint”)

LASER (LASER (LLight ight AAmplification by mplification by SStimulated timulated EEmission of mission of RRadiation)adiation)

Binding of elements to create compoundsBinding of elements to create compounds Neurotransmitters Neurotransmitters in the brainin the brain Allergy reactionsAllergy reactions Etc.Etc.

Atomic Structure and Atomic Structure and Function:Function:

Take Home MessageTake Home Message Science as a evolutionary, incremental process Science as a evolutionary, incremental process

over decades (e.g., exploring the structure of over decades (e.g., exploring the structure of atom)atom)

Role of hypothesis development and testing in Role of hypothesis development and testing in the sciences – scientific method and progressthe sciences – scientific method and progress Consternation, rejection, new models & new Consternation, rejection, new models & new

hypotheseshypotheses Demystification of physics and chemistry: Demystification of physics and chemistry:

structure of the atom is a simple conceptstructure of the atom is a simple concept Understanding of atomic structure underpins Understanding of atomic structure underpins

future course topics (e.g., chemical reactions, future course topics (e.g., chemical reactions, geology, biochemistry, ecology, environmental geology, biochemistry, ecology, environmental sciences)sciences)

Applications – some examplesApplications – some examples

Atomic StructureAtomic Structure


Early history in 18Early history in 18thth and 19 and 19thth centuries centuries Discovery of the electron (1897)Discovery of the electron (1897) Discovery of the nucleus (1911)Discovery of the nucleus (1911) Atomic structure: consternationAtomic structure: consternation Atomic structure: the Bohr atom (1913)Atomic structure: the Bohr atom (1913) Atomic Structure: Theory of Quantum Atomic Structure: Theory of Quantum

Mechanics (1930’s)Mechanics (1930’s) Applications of our understandingApplications of our understanding Discovery of the elementsDiscovery of the elements

Documents

Atomic Structure and Function 25 September 2003 Professor Taylor PhysicsChemistry Astronomy Astronomy Geology GeologyBiology Ecology Ecology