The ELECTRON: Wave Particle Duality“No familiar conceptions can be wovenaround the electron. Something unknown isdoing we don’t know what.” -Sir Arthur Eddington The Nature of the Physical World (1934)
The Electron• Discovered by JJ Thomson in 1897.• Used Cathode Ray tubes.• Electron beam appeared regardless of material used Must be a part of all atoms.• Repeled by a negatively charged plate, attracted to positive Must have negative charge.• Did not appreciably affect atoms overall mass must be virtually weightless.
The Problems With the Rutherford Model• Where are the electrons really at?• Electrons outside the nucleus are attracted to the protons in the nucleus – What keeps the atom from collapsing?• How can different elements that have similar composition, have such different properties?
The Missing Key• The key piece of indirect evidence that led to the advancement of atomic theory came from Robert Bunsens flame tests.
The Observation that different elements gave off uniquelycolored flames led chemists to take a closer look at light.
Light as Waves• Waves carry energy.• Conventional wisdom holds that light is a transfer of energy through the wavelike vibration of the universes electric and magnetic fields. – Electromagnetic Radiation
Electromagnetic radiation propagates throughspace as a wave moving at the speed of light. c = c = speed of light, a constant (3.00 x 108 m/s) = frequency, in units of hertz (hz, sec-1) = wavelength, in meters
The energy (E ) of electromagnetic radiationis directly proportional to the frequency ()of the radiation. E = h E= Energy, in units of Joules (kg·m2/s2) h= Planck’s constant (6.626 x 10-34 J·s) = frequency, in units of hertz (hz, sec-1)
Long Wavelength Wavelength Table =Low Frequency = Low ENERGY Short Wavelength = High Frequency = *Wavelength and Frequency areHigh ENERGY Indirectly Proportional
Answering the Dilemma of the “Collapsing” Atom• Treat electrons as waves• As the electron moves toward the nucleus, the wavelength shortens• Shorter wavelength = higher energy• Higher energy = greater distance from the nucleus
The Wave-like Electron The electron propagates through space as an energy wave. To understand the atom, one must understand the behavior of electromagnetic waves.Louis deBroglie
COLOR•Waves of light can have a wide range of frequenciesand wavelengths.•The energy of a beam of light of light correspondsto defined colors.•Visible Light falls somewhere between Red (LowEnergy) andViolet (High Energy)•This range of frequencies and wavelengths isknown as the electromagnetic spectrum
White Light•The electromagneticspectrum is continuous- thereis no part of it that does notcorrespond to a uniquewavelength or frequency oflight.•White Light (such as sunlight)contains light at allwavelengths of the spectrum.•The different frequencies oflight can be separated whenpassed through a prism. •Known as an emission spectrum.
Under the Wave Model of theElectron one would expect to seea continuous emission spectra (all colors) because the electron’swavelength gradually changes as it moves towards or away from the nucleus.
Spectroscopic analysis of the hydrogenspectrum… …produces a “bright line” spectrum
PROBLEM!• When you filter the light emitted from heated materials through a prism you DO NOTproduce a continuous spectrum.• Heated objects only produce light at specific frequencies.
Emission SpectraThe specific frequencies of light emitted are unique to specific elements. -Therefore emission spetcra can be used as an identification tool.
Quantum Theory• The observation that light was not emitted in a continuous spectra (but rather at specific frequencies) led German Physicist Max Planck to the surprising conclusion that matter can only gain or loose energy in small specific amounts.• Quantum:Specific amount of energy that can be gained or lost by an atom. (You can only have whole quantum numbers)• Photon:Packet of electromagnetic radiation that carries one quantum of energy.• When you add energy and excite atoms (such as in a neon sign) photons are absorbed- they can then be released emitting light of a particular corresponding frequency.
Bohr Model (1913)• Electrons orbit the nucleus in circular paths of fixed energy. – Called energy levels, electron shells, or quantum shells (all terms for the same thing).• Electrons can jump from energy level to energy level.• Electrons absorb or emit light energy when they jump from one energy level to another.
Energy Levels• The energy levels are like the rungs of a ladder but are not equally spaced. – It requires different amounts of energy to be absorbed or emitted for the electron to jump betweendifferent levels.• Energy emitted by the electron as it leaps from the higher to the lower energy level is proportional to the frequency of the light wave. – Frequency define the color of visible light.
Reexamining the Quantum• A quantum of energy is the amount of energy required to move an electron from one energy level to another.• Photons are bundles of light energy that is emitted by electrons as they go from higher energy levels to lower levels.
Energy of a PhotonEnergy of the emitted photon = Difference in energy between two states E=h E=energy =frequency h=Plank’s constant 6.7x10-34Js
Emission Spectrum• Since the energy of a photon released corresponds to the energy difference particular jump made, you get photons of different energies (and hence frequency). – Frequency defines the color of light. – Explains unique emission spectrum
Hydrogen• Bohr developed his model by working with hydrogen.• His model explained the emission spectrum of the hydrogen atom but did not always explain those of other elements.
So Neither Model is Perfect...• How can you have you cake and eat it too? – Answer: Combine the models.• The electron can situationally act as either a wave or a particle. – We call this “Electron Duality”
Wave-Particle DualityJJ Thomson won the Nobel prize for firstdescribingthe electron as a particle.His son, George Thomson won the Nobel prize fordescribing the wave-like nature of the electron. The electron is a particle! The electron is an energy wave!