later contributors to atomic theory pg. 90-94 2 nd note taking sheet ©2011 university of illinois...
TRANSCRIPT
Later Contributors to Atomic Theory
Pg. 90-94
2nd Note Taking Sheet
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Interaction of Light and Matter
• In order to understand the contributions of the next scientist, it is important to understand the characteristics of light and how it can interact with matter.
• Other names for light are radiant energy or electromagnetic radiation (emr for short).
• In the early 1900s there were observable phenomena involving light and its interaction with matter that could not be explained.
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Electromagnetic Radiation
• Light consists of an oscillating electric field at right angles to an oscillating magnetic field, thus its name (emr).
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Characteristics of Light (p.92-93)
• In order to understand the contributions of the next scientist, it is important to understand the characteristics of light and how it can interact with matter.
• At this time in history scientists thought of light as waves that propagated (moved) outward perpendicular from the source.
• In a vacuum scientists knew that light in a vacuum traveled at 2.998 x 108 m/s.
• This maximum limit on the speed of light is a universal constant represented by the letter “c”.
• All types of light travel at this speed in a vacuum.• If light travels through a denser material it will slow down
and different energies of light will be bent differently.
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Characteristics of Light Continued• The electromagnetic radiation spectrum is
all the possible energies of light.– Note that humans can see only a very small portion of
emr called the visible range.
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Characteristics of Light ContinuedWavelength
• Light of a certain energy has a characteristic frequency and wavelength.
• A wavelength is the distance from
peak to peak or
trough to trough.
• It is a length
measurement.
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Characteristics of Light ContinuedFrequency
• The frequency of light is the number of wavelengths that can pass through a point in a second.
• Frequency has units of 1/s or s-1 or Hertz (Hz)
• The higher the
frequency the
higher the energy
of the light©2011 University of Illinois Board of
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Characteristics of Light ContinuedSpeed
• In a vacuum all light travels at 2.998 x 108 m/s
• The speed of light, its frequency and wavelength are all related by the equation:
C = λ ∙ νWhere λ (“lambda”) is wavelength and
ν (“nu”) is the frequency
• Note that if frequency becomes greater the wavelength becomes smaller.
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Electromagnetic Radiation Spectrum
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Characteristics of Light ContinuedEnergy
• In 1901 Max Planck found that atoms can only adsorb and emit energy in distinct quantities; this showed that energy is quantized. He also determined that the energy of the light is given by the equation
E = h ∙ νWhere E is energy (J)
h is Planck's constant = 6.626 x 10-34 J∙sV is the frequency in Hz or 1/s or s-1 or cycles/sec
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Inexplicable New Evidence
• Photoelectric effect occurs when light hits a piece of metal and the metal ejects an electron.
• The energy of the light had to be at least a certain minimum value that was different for different metals.
• Black body radiation.
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Albert Einstein (1905)• To explain the photoelectric effect, in 1905
Einstein suggested that light can behave like particles as well as waves.
• The way in which you consider light depends on the phenomenon you are observing. This is known as the dual nature of light.
• Light can be considered to be little discrete packets of energy called photons.
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Hydrogen Line Emission Spectrum
• Scientist were very surprised that they didn’t get a continuous spectrum for the light emitted by hydrogen.©2011 University of Illinois Board of Trustees •
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There is a fingerprint line emission spectrum for all the elements.
Use the spectroscope to see the emission spectrum of other elements.
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Neils Bohr (1913)• Explained the
unexpected result of a hydrogen line emission spectrum.
• Proposed a model of the atom in which the electrons have quantized energy.
• Electrons of an atom could only be certain allowed distances from the nucleus which corresponded to specific energy values.
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Bohr Continued
• When electrons are in their lowest energy state, Bohr called this their ground state.
• He said the when electrons are in a higher energy orbit, they are in the excited state.
• Energy is absorbed to excite an electron and released when an electron goes back to its ground state.
• Energy is released in the form of light.
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Hydrogen Line Emission Spectrum
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Louis DeBroglie (1923)
• In writing his doctoral thesis Louis DeBroglie suggested that electrons could behave like waves as well as particles.
• In fact he stated that all matter has a wave nature given by the formula
λ = h/mv Where m is the mass and v
is the speed.
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Louis DeBroglie Continued
• This meant that an electron “orbiting” the nucleus can be thought of as a wave. Each electron moves with a characteristic energy.
• The energy of the electron depends on the wavelength.
• All matter has a characteristic wave called a matter wave – even you!
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Erwin Schrodinger (1926)
• Schrodinger developed a wave theory about how electrons can exist in atoms.
• His wave theory explains the different energy states of an electron and is based on electrons behaving as waves.
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Warner Heisenberg (1927)
• Realized that if electrons behave as waves their existence is more spread out and it is impossible to know exactly where the electron is or how fast it is moving.
• This is a statement of the uncertainty principle.
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