Download Chapter 27 Early Quantum Theory and Models of the Atom

Chapter 27
Early Quantum Theory and Models of the Atom
Chapter 27
• Discovery and Properties of the Electron
• Planck’s Quantum Hypothesis and Blackbody Radiation
• Photon Theory of Light and Photoelectric Effect
• Energy, Mass, and Momentum of a Photon
• Compton Effect
• Photon Interactions; Pair Production
Chapter 27
• Wave­Particle Duality; the Principle of Complementarity
• Wave Nature of Matter
• Electron Microscopes
• Early Models of the Atom
• Atomic Spectra: Key to the Structure of the Atom
• The Bohr Model
• de Broglie’s Hypothesis Applied to Atoms
Discovery and Properties of the Electron
“Cathode rays” were discovered in late 1800’s and found to be deflected by electric or magnetic fields in ways that implied particles with a negative charge.
Discovery and Properties of the Electron
By accelerating the rays through a known potential and then measuring the radius of their path in a known magnetic field, the charge to mass ratio was measured:
(27­1)
The result is Discovery and Properties of the Electron
The currently accepted value of e is:
Knowing e and the e/m ratio allows the electron’s mass to be calculated. Result is Planck’s Quantum Hypothesis and Blackbody Radiation
All objects emit radiation whose total intensity is proportional to the fourth power of their temperature. This is called thermal radiation; a “blackbody” emits only thermal radiation. The spectrum of blackbody radiation is in this figure for a few temperatures. The frequency of peak intensity increases linearly with temperature.
Planck’s Quantum Hypothesis and Blackbody Radiation
This spectrum could not be reproduced using the physics known in the late 19th ­ century. A solution was proposed by Max Planck in 1900:
The energy of oscillations within atoms cannot have an arbitrary value; it is related to the frequency:
The constant h is now called Planck’s constant.
Planck’s Quantum Hypothesis and Blackbody Radiation
Planck found the value of this constant by fitting blackbody curves for a number of temperatures
Planck’s proposal was that the energy of an oscillation had to be an integral multiple of hf. This is called the quantization of energy.
Photon Theory of Light and the Photoelectric Effect
Einstein suggested that ,given the success of Planck’s theory, light must be emitted in small energy packets:
These tiny packets of energy are like a particle and are called photons. So is light a wave or a particle ?
Photon Theory of Light and the Photoelectric Effect
Evidence for Particle ­The photoelectric effect: When light strikes a metal, electrons are emitted. But, this effect does not occur if the frequency of the light is too low. The kinetic energy of an electron increases with frequency.
Photon Theory of Light and the Photoelectric Effect
If light is a wave, theory predicts:
• Number of electrons and their energy should increase with intensity
• Frequency would not matter
• Also one can get Interference and Diffraction from waves but not from particles
Photon Theory of Light and the Photoelectric Effect
If light is made up of particles, theory predicts:
• Increasing intensity increases number of electrons but not energy
• Above a minimum energy required to break atomic bond, kinetic energy will increase linearly with frequency
• There is a cutoff frequency below which no electrons will be emitted, regardless of intensity
Photon Theory of Light and the Photoelectric Effect
The particle theory assumes that an electron absorbs a single photon with energy hf.
Plotting the kinetic energy vs. frequency:
This shows clear agreement with the photon theory, and not with wave theory.
Energy, Mass, and Momentum of a Photon
Clearly, a photon must travel at the speed of light. Looking at the relativistic equation for momentum, From relativity it is clear that this can only happen if a photon’s rest mass is zero
We already know that the energy is hf; we can put this in the relativistic energy ­ momentum relation and find the momentum:
The Compton Effect
Compton scattered X­rays from different materials. He found that the scattered X­rays had a slightly longer wavelength than the incident ones, and that the wavelength depended on the scattering angle:
The Compton Effect
This is another effect that is correctly predicted by the photon model and not by the wave model.
Photon Interactions and Pair Production
Photons passing through matter can undergo the following interactions:
• Photoelectric effect: photon is completely absorbed, electron is ejected
• Photon may be totally absorbed by electron, but not have enough energy to eject it; the electron moves into an excited state
• The photon can scatter from an atom and lose some energy
• The photon can produce an electron­positron pair.
Photon Interactions and Pair Production
In pair production, energy, electric charge, and momentum must all be conserved.
Energy will be conserved through the mass and kinetic energy of the electron and positron; their opposite charges conserve charge; and the interaction must take place in the electromagnetic field of a nucleus, which can contribute momentum.
Wave­Particle Duality The Principle of Complementarity
We have phenomena such as diffraction and interference that show that light is a wave, and phenomena such as the photoelectric effect and the Compton effect that show that it is a particle. How do we decide which one it actually is?
The question has no answer one way or the other ! We must accept the dual wave­particle nature of light based on the experimental results. So, are what we think of as particles also waves?
Wave­Particle Duality
The Principle of Complementarity
The principle of complementarity states that both the wave and particle aspects of light are fundamental to its nature.
Whether we “see” waves or particles is just our interpretation of how light behaves in various types of measurements – based on an incomplete theory. Wave Nature of Matter
Just as light sometimes behaves as a particle, matter sometimes behaves like a wave.
The wavelength of a particle of matter is also
This wavelength is extraordinarily small for any object that has any significant mass, but becomes more important for very light particles such as an electron. Electrons show the same diffraction from crystals as found for x­rays.
Electron Microscopes
The wavelength of electrons will vary with energy, but is still quite short. This makes electrons useful for imaging – remember that the smallest object that can be resolved is about one wavelength. Electrons used in electron microscopes have wavelengths of about 0.004 nm.
This means it is possible to “see” an individual atom with some electron based techniques
Electron Microscopes
Transmission electron microscope the electrons are first accelerated by a potential difference before encountering the object, and are focused by a magnetic “lens” to form the image.
Electron Microscopes
Scanning electron microscope – the electron beam is scanned back and forth across the object to be imaged
Electron Microscopes
Scanning tunneling microscope – up and down motion of the probe keeps the current constant.
Plotting that motion produces an image of the surface.
Early Models of the Atom
It was known that atoms were electrically neutral, but that they could become charged, implying that there were positive and negative charges and that some of them could be removed.
A popular atomic model was this “plum­pudding” model:
Early Models of the Atom
Rutherford did an experiment ­ now known as Rutherford scattering ­ that showed that the positively charged nucleus must be extremely small compared to the rest of the atom. He scattered alpha particles – helium nuclei – from a metal foil and observed the scattering angle. He found that some of the angles were far larger than the plum­pudding model would allow. The expanation was that the positively charged region had to be tiny compared to the atoms overall size.
Rutherford Scattering Experiment
The only way to account for the large angles was to assume that all the positive charge was contained within a tiny volume – now we know that the radius of the nucleus is 1/10000 that of the atom.
Early Models of the Atom
Therefore, Rutherford’s model of the atom is mostly empty space:
Problem: accelerated electron should radiate all of the time if moving in circular motion.
Atomic Spectra: Key to Atom’s Structure
A very thin gas heated in a discharge tube emits light only at characteristic frequencies.
Atomic Spectra Provided the Key to the Structure of the Atom
An atomic spectrum is a line spectrum – only certain frequencies appear. If white light passes through such a gas, it absorbs at those same frequencies.
Atomic Spectra: Key to Atom’s Structure
The wavelengths of electrons emitted from hydrogen have a regular pattern:
This is called the Balmer series. R is the Rydberg constant:
Atomic Spectra: Key to Atom’s Structure
Other series include the Lyman series:
and the Paschen series:
Atomic Spectra: Key to Atom’s Structure
A portion of the complete spectrum of hydrogen is shown here. The lines cannot be explained by the Rutherford theory without some extra constraints.
The Bohr Model of an Atom
Bohr proposed that the possible energy states for atomic electrons were quantized – only certain values were possible. Then the spectrum could be explained as transitions from one level to another.
The Bohr Model of an Atom
Bohr also found that the angular momentum was quantized in an orbital picture of the atom:
The Bohr Model of an Atom
An electron is held in orbit by the Coulomb force:
The Bohr Atom
Using the Coulomb force, one can easily calculate the radii of the orbits:
The Bohr Model of a Hydrogen Atom
The lowest energy level is called the ground state; the others are excited states.
de Broglie’s Hypothesis Applied to Atoms
De Broglie’s hypothesis is the one associating a wavelength with the momentum of a particle.
He proposed that only those orbits where the wave would be a circular standing wave will occur. This yields the same relation that Bohr had proposed.
These ideas are the start of current understanding of the electronic structure of an atom but did not yet incorporate the idea of electronic wavefunctions. They did have the energy & angular momentum properly quantized – that’s why they work so well.
de Broglie’s Hypothesis Applied to Atoms in Bohr’s Model
These are circular standing waves for n = 2, 3, and 5.
Explains why electrons do not radiate, as one expects from an accelerating charge.
Emission occurs only for a state change, ∆n
Summary of Chapter 27
• Planck’s hypothesis to explain blackbody spectrum: molecular oscillation energies are quantized
• Light can be considered to consist of photons, each of energy
• Photoelectric effect: incident photons knock electrons out of material when incident light is above some threshold frequency
Summary of Chapter 27
• Compton effect and pair production also support photon theory
• Wave­particle duality – both light and matter have both wave and particle properties
• The de Broglie wavelength of an object:
Summary of Chapter 27
• Principle of complementarity: both wave and particle properties are necessary for complete understanding • Rutherford showed that atom has tiny nucleus
• Line spectra are explained by electrons having only certain specific orbits L is quantized
• Ground state has the lowest energy; the others are called excited states