today is monday, september 29 th, 2014 pre-class: [choose one] what is white light? how are...
TRANSCRIPT
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Today is Monday,September 29th, 2014
Pre-Class: [choose one]
What is white light?How are fireworks made to be
different colors?How are neon signs made to be
different colors?
In This Lesson:Atomic Emissions
(Lesson 2 of 4)
Stuff You Need:Calculator
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Today’s Agenda
• Atomic Emissions• Flame Tests• The Light Spectrum• Calculations
• Where is this in my book?– P. 138 and following…
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By the end of this lesson…
• You should be able to explain what happens when energy is applied to an atom.
• You should be able to describe and calculate the relationships between wavelength, frequency, and energy.
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Emission Spectra
• Fireworks are exciting because of:– The noise they make.– The variety of color
they display.• We’re going to focus
on the color.
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Identification
• The multicolored lights created by fireworks occur because of the different elements that comprise the powder in fireworks.
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• Scientists have found that each element, when heated, gives off its own specific set of colors.
• The element’s colors are its “fingerprints” and can be used to identify the element.
Identification
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Element ColorsElement Flame ColorSodium Yellow
Potassium VioletRubidium Pinkish-RedCalcium Orange-Red
Strontium RedBarium GreenCopper Blue-Green
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Cesium
• Blue
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Calcium
• Deep Orange
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Sodium
• Orange
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Potassium
• Violet
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Copper
• Jade Green
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Flame Tests
Strontium Sodium Lithium Potassium Copper
Many elements give off characteristic light which can be used to help identify them.
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Electron Energy State• Electrons absorb energy from the flame.• When a certain amount is reached (a
quantum), they jump to a higher energy level: the “excited state.”
• Eventually, the electrons lose the energy in the form of light and fall back to the lowest, most stable energy level: the “ground state.”
Atomic Emission Spectrum of Barium using a Spectrometer
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Electromagnetic Spectrum
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Lyman, Balmer, Paschen Series• Electrons moving down to n=1
emit light along a series of frequencies in the ultraviolet range.– The Lyman series of emissions.
• Electrons moving down to n=2 emit light along a series of frequencies in the visible range.– The Balmer series of emissions.
• Electrons moving down to n=3 emit light along a series of frequencies in the infrared range.– The Paschen series of emissions.
http://1.bp.blogspot.com/_nxSb3loAy3A/TGedfwHgN8I/AAAAAAAAAN8/JhEss5aeCE8/s1600/h-atom.gif
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Lyman, Balmer, Paschen Series
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Sources of Energy
• Where do electrons get energy to “jump” to the next higher energy level?– Collisions from other particles– Heat – Electricity– Light
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Loss of Light?
• As we learned, when electrons fall back to the ground state, they release energy in the form of light.
• It’s complicated, but light can behave as a wave or a particle.
• As a particle, a “unit” of light is called a photon.– Additionally, a quantum (plural: quanta) is the
amount of energy needed to move an electron into an excited state.
– A quantum of light is called a photon.
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Particle-Wave Duality
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Wave Statistics
• Amplitude: The “height” of the wave from zero to crest (peak).
• Wavelength: Distance between peaks in nanometers (nm) or meters (m).– Given by Greek letter λ (lambda).
• Frequency: The number of cycles (wave peaks) that occur in a unit of time (per second or Hertz; Hz)– Given by Greek letter v (nu).
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Wave Equation
• There is a relationship between wavelength and frequency.
• Wavelength times frequency always equals the speed of light, given by c and equal to 2.998 x 108 m/s.• c = λv
• Sample Problem:– Wave Statistics Worksheet: #9
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Wavelength, Frequency, and EnergyLong
Wavelength=
Low Frequency=
Low Energy
Short Wavelength
=High Frequency
=High Energy
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How to remember?
• How can you remember “high frequency = high energy?”
• Imagine riding a bike over the wave peaks!
Takes less energy to do these hills…
…than to do these hills.-Litz, 2014
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Planck’s Constant• In addition to the speed of light
constant c, there is also Planck’s Constant, named for the particularly dour-looking Max Planck.
• Planck’s Constant, given by h, relates the energy of one photon and the frequency of the corresponding wave.
• Energy (E) is in Joules (J).• E = hν (for one photon)• h = 6.626 x 10-34 Js
• Sample Problem:– Wave Statistics Worksheet: #1
http://adam.humanisti.sk/wp-content/2007/10/max_planck.jpg
Max Planck
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Summary
• Electrons can move between energy levels.– Ground state: stable state; an electron is at the
lowest energy level.– Excited state: unstable state; an electron is at a
higher energy level.– Quantum: the amount of energy needed to move
an electron from the ground to excited state.– Photon: a quantum of light.
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Summary
• Wavelength and frequency are inversely related:– When wavelength increases, frequency decreases.
• Frequency and energy are directly related:– When frequency increases, energy increases.
• We only see a small part of all possible wavelengths/frequencies.– The visible spectrum.
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Summary
• Variables:• λ (lambda) – measure of wavelength.• v (nu) – measure of frequency in Hz (cycles/sec).• c – speed of electromagnetic waves
• 2.998 x 108 m/s in a vacuum.
• h – Planck’s Constant• 6.626 x 10-34 Js
• Equations:• c = λv• E = hv
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Other Equations?
• Related equations not covered directly in this course:• E=mc2
• Energy = mass * speed of light 2
• De Broglie’s Equation: λ = h/mv• Allows us to relate Planck’s Constant, mass, and velocity
to wavelength.– Also illustrates the particle-wave duality of matter.
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Summary: Emissions in Real Life
• The reason most streetlights look a little “orange” is because they pass an electric current through sodium vapor.– Remember how sodium burns in orange color?
• Compare LED light to Na vapor:
http://ledlightreviews.files.wordpress.com/2009/08/led-vs-hps-betaled1.jpg?w=460&h=179
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So now then…
• Let’s try some flame tests!• At each of your lab tables is one of seven
different kinds of salt solutions.– This isn’t table salt.
• In the salt solution is a wooden splint that has been soaking in it overnight.
• You should take out your Bunsen burner (if it’s not already out) and light it.
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Flame Tests
• I will turn off the lights.• At that point, each group will put ONE of the
splints into the flame and record the color that is emitted.
• The lights will come back on, and groups will rotate clockwise until all solutions have been tested.
• There will be time for answering the questions that follow.
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What NOT to do…
• Don’t let the splint burn.• Don’t place more than one splint into the
flame.
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Closure
• Which has higher energy, long or short wavelength?– Short wavelength (high frequency).
• Exactly what is burning?– The various salt solutions (NOT the splint)
• Did the electrons get closer to the nucleus or further away?– Further
• What could we say happened to the electrons in terms of their Principal Quantum Number?– They briefly entered a higher energy shell (or principal
quantum number) before falling back into their ground levels.