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Light and the
Quantum Mechanical Model of the Atom
Light
• The study of light led to the development of the quantum mechanical model.
• Light is a kind of electromagnetic radiation.
• Electromagnetic radiation includes many kinds of waves
• All move at 3.00x108 m/s or 3.00x1010 cm/s (abbreviated: c)
Parts of a wave
Wavelength
AmplitudeOrigin
Crest
Trough
Parts of Wave
• Origin - the base line of the energy.• Crest - high point on a wave• Trough - Low point on a wave• Amplitude - distance from origin to
crest• Wavelength - distance from crest to
crest
• Wavelength - is abbreviated Greek letter lambda)
Frequency• The number of waves that pass a
given point per second.• Units are cycles/sec or hertz (Hz)• Abbreviated the Greek letter
nu)
c =
Frequency and Wavelength
• Are inversely related• As one goes up the other goes down.
• Different frequencies of light are different colors of light.
• There is a wide variety of frequencies
• The whole range is called a spectrum
Radiowaves
Microwaves
Infrared
Ultra-violet
X-Rays
GammaRays
Low energy
High energy
Low Frequency
High Frequency
Long Wavelength
Short WavelengthVisible Light
Electromagnetic Spectrum
Energy and Frequency
• E = (h)(• E is the energy of the photon• is the frequency• h is Planck’s constant • h = 6.626 x 10 -34 Joules*sec.
The Math You WILL need to Know for this Chapter
Only 2 equations:• c = • E = h
• c is always 3.00 x 108 m/s
• h is always 6.626 x 10-34 J•s
Examples
• What is the frequency of red light with a wavelength of 4.2 x 10-5 cm?
• What is the wavelength of The River 105.9, which broadcasts at a frequency of 640 kHz?
• What is the energy of a photon of each of the above?
Atomic Spectra How light & color tells us about
atoms
• It is made up of all the colors of the visible spectrum.
• Passing it through a prism separates it.
• This is called a continuous spectrum • because you see all the
colors of the visible spectrum blurred together (ROYGBIV)
White Light
If the light is not white• By heating or electrifying a
sample of atoms scientists can get it to give off colors.
• Passing this light through a prism does something different
• You get colors of light that relate to specific frequencies and therefore specific energies
• This is called …
Atomic Emission Spectrum or Line Spectrum
• Each element gives off a unique set of colors.
• This info can be used to help identify atoms in substances• Ex: stars
http://jersey.uoregon.edu/elements/Elements.html
An explanation of Atomic Spectra
How it all begins…• In the atom all
electrons start in the lowest energy level they are normally found in.
• This is called the ground state.
Hydrogen atom
• Let’s take a closer look at a hydrogen atom
Changing the energy• Adding energy can move the electron up
to higher energy levels
• The more energy added, the more energy levels the e- moves
• As the electron falls back to its original ground state it gives the absorbed energy back as light we can see
• The further the e- fall, the more energy, and the higher the frequency of light.
Max Planck
• 1900 – from Germany
• Tried to explain why metals changed color the way they did when they are heated
• They only show certain colors, not all the colors in the visible spectrum. WHY?
• The energy is changed in a specific amount called a quantum
Light as a Particle
• Energy is quantized.
• These smallest pieces of light are called photons.
So…
What is light then??
• Light is a wave we can measure its wave length and determine its frequency and it behaves as a wave
• Light is also a particle it comes in chunks called photons• Albert Einstein came up with this idea
Photoelectric Effect
• Metals eject electrons when light of a specific frequency (called a threshold frequency) shines on them.
• The light has to have enough energy and if the light is the wrong frequency then it won’t work at all (no matter how bright the light)• Uses of this: solar cells, camera flash, &
automatic doors• FYI: Explaining this effect is how Einstein won his
Nobel Prize
More obvious with the very small
• To measure where a electron is, we use light.
• But the light moves the electron• And hitting the electron changes the
frequency of the light.
Matter is also a Wave• BUT…• This does not apply to objects bigger
than an atom b/c their wavelengths are too small• Ex: A baseball has a wavelength of
about 10-32 m when moving 30 m/s
• An electron at the same speed has a wavelength of 10-3 cm
• This length is big enough to measure.
• We called these “matter waves”
The physics of the very small
• Quantum mechanics explains how the very small behaves.
• Quantum mechanics is based on probability because
Heisenberg Uncertainty Principle
• It is impossible to know exactly the speed and position of a particle.
• The better we know one, the less we know the other.
• The act of measuring changes the properties.
Moving Electron
Photon
Before
Electronchanges velocity
Photon changes wavelength
After