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Chapter 7 Beyond Rutherford to “The Most Successful Theory of the 20 th Century”. View Rutherford’s experiment. http://www.learnerstv.com/animation/chemistry/ruther14.swf. Rutherford’s Atomic Model (“planetary model”). e-. Orbiting electron (fixed radius). Empty space. Nucleus - PowerPoint PPT Presentation
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Chapter 7
Beyond Rutherford to“The Most Successful Theory of the
20th Century”
http://www.learnerstv.com/animation/chemistry/ruther14.swf
View Rutherford’s experiment
Rutherford’s Atomic Model(“planetary model”)
e-
NucleusDiameter = 10-15 m
Empty spaceOrbiting electron(fixed radius)
Diameter of atom = 10-10 m
Problem with Rutherford’s Model !! It did not obey the classical laws of physicsIt did not obey the classical laws of physics
But atoms don’t collapse, yet Rutherford’s experiment But atoms don’t collapse, yet Rutherford’s experiment showed that electrons can be located a distance away showed that electrons can be located a distance away from the nucleus.from the nucleus.
So, the model of the So, the model of the ______________ behavior is flawed. behavior is flawed.
According to Newton’s According to Newton’s classical laws, electrons classical laws, electrons orbiting the nucleus should orbiting the nucleus should radiate energy, slow down, radiate energy, slow down, and be pulled into the and be pulled into the nucleus & collapse the atomnucleus & collapse the atom
Collision of Ideas
Dalton
Thomson
Rutherford
Newton
Maxwell
Plank
Einstein
MatterMatter
LightLight
?Bohr &de Broglie
What is the nature of light?
Isaac Newton: “Light is a particle”
Newton’sprism
By the 17th century, light was found toBy the 17th century, light was found to travel in straight linestravel in straight lines reflect & refractreflect & refract transmit energy from one place to anothertransmit energy from one place to another
The The WAVE THEORYWAVE THEORY, advocated by, advocated by
Robert Hooke
Christian Huygens argued thatargued that light is a wavelight is a wave..
The The PARTICLEPARTICLE THEORYTHEORY, advocated by, advocated by Isaac Newton and and Pierre Laplace,, argued thatargued that light was made up of a stream of tiny particles (“corpuscles”).
Two Competing Theories
The The theory of lighttheory of light
The The theory of lighttheory of light
“white light” light energy composed of a continuous spectrum of visible electromagnetic radiation
ultraviolet
infrared
Wavelength = distance between wave crests (m)
Frequency = cycles per second (Hz)
11 or
Basics of wave theory
Electromagnetic Wave Theory (1865)Electromagnetic Wave Theory (1865)
Electromagnetic waves have a variety of wavelengths, Electromagnetic waves have a variety of wavelengths, but all travel at the speed of lightbut all travel at the speed of light,,
Based on conservation of energy, Maxwell derived theBased on conservation of energy, Maxwell derived the wave equation,wave equation,
Based on experiments ofBased on experiments of Michael Faraday
Theory developed byTheory developed by James Clerk Maxwell
cc ==
cc == 2.9982.998 ×× 101088 m/s m/s
Electromagnetic Spectrum
1020 Hz 1014 Hz 1010 Hz
10-6 nm 108 nm
higher energy lower energy
ROY G BIVROY G BIVlow energy low energy high energy high energy
Colors in the visible spectrum:Colors in the visible spectrum: RRed, ed, OOrange, range, YYellow, ellow, GGreen, reen, BBlue, lue, IIndigo & ndigo & VVioletiolet
Problems with the Wave Theory of Light
1.1. Blackbody RadiationBlackbody Radiation
2.2. The Photoelectric EffectThe Photoelectric Effect
3.3. Emission Spectra of AtomsEmission Spectra of Atoms
By the mid-1800s, the wave theory became predominant, By the mid-1800s, the wave theory became predominant, but……but……
When light interacted with matter, the wave theory failed. When light interacted with matter, the wave theory failed.
The important examples are:The important examples are:
Problem #1. Blackbody Radiation
blackbodyblackbody““object that absorbs all theobject that absorbs all thecolors in the spectrum”colors in the spectrum”
When heated to a high enough temperature, the blackbody When heated to a high enough temperature, the blackbody radiates white light. radiates white light.
The wave theory predicts a continuous spectrum of emitted light, The wave theory predicts a continuous spectrum of emitted light, but the but the theory fails to match experimenttheory fails to match experiment..
Blackbody Simulation
actualspectrum
Planck’s Quantum Theory
Measured blackbody radiation did not produce a Measured blackbody radiation did not produce a continuous spectrum, as wave theory predictedcontinuous spectrum, as wave theory predicted
In 1900, German Physicist Max Planck proposed a new In 1900, German Physicist Max Planck proposed a new quantum theory of light:quantum theory of light:
Light is taken up and given off by a blackbody not as a Light is taken up and given off by a blackbody not as a continuous wave, but in little continuous wave, but in little “packets” of light energy “packets” of light energy of of specific valuesspecific values
Planck called these packets “Planck called these packets “quantaquanta” (singular is ” (singular is quantum) of energyquantum) of energy
Quantum Theory of Light and Quantum Physics
Plank’s quantum theory of light was Plank’s quantum theory of light was a historical turning point in physics, a historical turning point in physics, transitioning classical physics from transitioning classical physics from the 18the 18thth and 19 and 19thth centuries to the centuries to the quantum physics of the 20quantum physics of the 20thth centurycentury. .
Problem #2Photoelectric
Effect
Animation
Problem #2. Photoelectric Effect Imagine shining light of various wavelengths (energies) on the surfaces of different metalsImagine shining light of various wavelengths (energies) on the surfaces of different metals
Only light energies Only light energies above a certain thresholdabove a certain threshold cause electrons to be ejected from the metal surface cause electrons to be ejected from the metal surface This conflicts with predictions of the wave theoryThis conflicts with predictions of the wave theory
Animation
Einstein’s Photons In 1905, a Swiss patent clerk proposed that light In 1905, a Swiss patent clerk proposed that light
consists of particles called consists of particles called photonsphotons.. As Planck proposed, Einstein’s photons have a As Planck proposed, Einstein’s photons have a
certain quanta of energy (based on wavelength)certain quanta of energy (based on wavelength) His model of light solved the problem of the His model of light solved the problem of the
photoelectric effect.photoelectric effect.
Duality of LightDuality of Light
Wave behaviorWave behavior
Particle behaviorParticle behavior
Solar Sail (based on Einstein’s photon theory) - Light reflecting off a mirror imparts momentum - Yet light has no mass (experiment by Compton in 1923)
Cosmos 1 concept
Energy of Photons
At a specific frequency (or wavelength) At a specific frequency (or wavelength) photonsphotons possess a specific quantity of energy (possess a specific quantity of energy (EE ))
Planck’s constantPlanck’s constant
EE == hh
hh == 6.6266.626 xx 1010-34-34 J·s J·s
EE == hh c/c/
Question: Is 400 nm light (violet light) more or less energetic than 750 nm light (red light)?
Concept Check
The energy required to dislodge electrons from sodium metal via the photoelectric effect is 275 kJ/mol.
What wavelength (in nm) has sufficient energy per photon to dislodge an electron from the surface of sodium?
sodium
Concept Check
Which photons have the highest energy?
A) Cell phone operating at 1900 MHz
B) A laser pointer using 635 nm light
Problem #3. Atomic Line Spectra
Periodic Table of Line SpectraPeriodic Table of Line Spectra
Flame testshttp://college.cengage.com/chemistry/general/ebbing/general_chem/9e/assets/instructors/protected/videos.html#Chapter 7
Problem #3. Atomic Line Spectra
Periodic Table of Line SpectraPeriodic Table of Line Spectra
Emission spectra for pure elements
Fireworks
Niels Bohr (1885-1962) Danish physicist who worked with J.J. Thomson at Danish physicist who worked with J.J. Thomson at
Cambridge University in 1911. He didn’t agree with Cambridge University in 1911. He didn’t agree with Thomson’s atomic model, Thomson’s atomic model, so worked for Rutherford in 1912.so worked for Rutherford in 1912.
In 1912, in a bold step, he suggested that the In 1912, in a bold step, he suggested that the classical classical laws of physics cannot be appliedlaws of physics cannot be applied to matter as small as to matter as small as atoms and electrons. Instead, atoms and electrons. Instead, new laws are needednew laws are needed
Bohr sought to solve the problem with Rutherford’s Bohr sought to solve the problem with Rutherford’s atomic model and explain the phenomenon of atomic atomic model and explain the phenomenon of atomic spectra, by spectra, by applying the quantum theory of light to applying the quantum theory of light to atoms and electronsatoms and electrons
Bohr’s Quantum Atomic Model
Postulated that the Postulated that the energy of the energy of the electron must be quantizedelectron must be quantized. Only . Only certain electron energies are possible.certain electron energies are possible.
Orbit radii (energy levels) correspond Orbit radii (energy levels) correspond to definite energiesto definite energies
Energy is emitted or absorbed by the Energy is emitted or absorbed by the electron only as the electron changes electron only as the electron changes from one allowed energy level to from one allowed energy level to anotheranother
nn = = energy level number or principal quantum number
Why does an electron possess energy? Why does an electron possess energy? 1)1) 2)2)
How do quantized energy levels explain spectral lines?
Atoms “place” electrons in lowest possible energy levels Atoms “place” electrons in lowest possible energy levels ((“ground state”“ground state”))
When electrons are provided with enough energy, they When electrons are provided with enough energy, they “jump” to higher energy levels, where they are unstable “jump” to higher energy levels, where they are unstable ((“excited state”“excited state”))
The electrons then fall back down to the lower possible The electrons then fall back down to the lower possible energy levels, releasing absorbed energy as a photon of lightenergy levels, releasing absorbed energy as a photon of light
We see these photons as the spectral lines emitted by excited We see these photons as the spectral lines emitted by excited atomsatoms
Stairstep Stairstep
analogyanalogy
Energy of H electron = E = -RH/n2
n = 1, 2, 3, … ∞RH = 2.179 x 10-18 J
energylevels
“quantum jump”∆E4→2 = E2 - E4 = h4→2
H emission spectrum
A “quantum jump”
EmissionEmission ∆E = E2 - E4 = h4→2
AbsorptionAbsorption ∆E = E4 – E2 = h2→4
Simulations of Bohr Model
Visible emission spectral lines of hydrogenVisible emission spectral lines of hydrogen
Success & Limitation of Bohr’s Quantum Model
Explained the existence of spectral linesExplained the existence of spectral lines Solved the problem with Rutherford’s model of the Solved the problem with Rutherford’s model of the
hydrogen atomhydrogen atom But, the mathematics only worked for atoms with 1 But, the mathematics only worked for atoms with 1
electron!electron!
How can this model be made to How can this model be made to work for all elements?work for all elements?
de Broglie’s Novel NotionLight was “known” (thought) to be a wave, but Light was “known” (thought) to be a wave, but
Einstein showed that it also acts particle-like.Einstein showed that it also acts particle-like.
Electrons were “known” to be particles mass & charge.Electrons were “known” to be particles mass & charge.
French physicist:French physicist:
What if ……What if ……
1923
electrons behaved as waves also
Diffraction pattern obtained by firing
a beam of electrons through
a crystal.
Dr. Quantum videoDr. Quantum video
Werner Heisenberg
In 1927, German physicist, proposed that the In 1927, German physicist, proposed that the dual nature of dual nature of the electronthe electron places limitations on how precisely we can know places limitations on how precisely we can know both the location and speed of the electronboth the location and speed of the electron
Instead, we can only describe electron behavior in terms of Instead, we can only describe electron behavior in terms of probabilityprobability
The Uncertainty Principle
speedspeed
positionposition
Heisenberg’sUncertainty Principle
Wave behavior limits what can be known!Wave behavior limits what can be known!
What if the particle has a small mass?What if the particle has a small mass?
What if the electron’s position is known very precisely?What if the electron’s position is known very precisely?
What if the electron’s speed is known very precisely?What if the electron’s speed is known very precisely?
(±x)(±v(±x)(±vxx))
Can the electron’s orbit be precisely defined?
± position ± speed
hh44mm
Erwin Schrodinger
In 1926, Austrian physicist, proposed an equation that In 1926, Austrian physicist, proposed an equation that incorporates both the wave and particle behavior of the incorporates both the wave and particle behavior of the electronelectron
When applied to hydrogen’s 1 electron atom, solutions provide When applied to hydrogen’s 1 electron atom, solutions provide the the most probable locationmost probable location of finding the electron in the first of finding the electron in the first energy levelenergy level
Can be applied to more complex atoms too!Can be applied to more complex atoms too!
Wave Equation &
Wave Mechanics
• Extremely small mass• Located outside the nucleus• Moving at very high speeds• Have specific energy levels• Standing wave behavior
• Extremely small mass• Located outside the nucleus• Moving at very high speeds• Have specific energy levels• Standing wave behavior
Electron Characteristics
A baseball behaves as a particle and follows a predictable path.
BUT
An electron behaves as a wave, and its path cannot be predicted.
All we can do is to calculate the probability of the electron following a specific path.
Baseball v. Electron
What if a baseball behaved like an electron?
Characteristic wavelength ()• baseball 10-34 m• electron 0.1 nm
So, all we can predict is…..
== hh /(/(mumu))mass speed
“deterministic” “probabilistic”
Bohr Model v. Quantum Mechanics
Energy
Electron
Position/Path
Elements
Bohr Quantum Mechanics
The electron's movement cannot be known precisely.
We can only map the probability of finding the electron at various locations outside the nucleus.
The probability map is called an orbital.
The orbital is calculated to confine 99% of electron’s range.
Energy of the electron is quantized into sublevels.
Quantum Mechanics Model
Quantum Mechanics ModelDescribes the energy, arrangement and space occupied
by electrons in atoms
Quantum
Mechanics
Electron’s energy is quantized
Mathematics of waves to define orbitals(wave mechanics)
“Most Successful Theory of the 20th Century”
Dalton
Thomson
Rutherford
Newton
Maxwell
Plank
Einstein
Matter
Light
Schrödinger
Heisenberg
WaveMechanics
Quantum
MechanicsBohr &de Broglie