light and the atom created 25mar2005 by dan smith
TRANSCRIPT
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Light and the Atom
Created 25Mar2005by Dan Smith
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Why do we want to look inside an atom?
• We want to know how an atom is structured inside. This will help us understand how different atoms interact to make compounds, and why some atoms are very reactive (like F and K), but other atoms are not reactive at all (like He and Ne).
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How can we look inside an atom to see how it’s put together?
• In order to look inside something very small, one must use a probe that is even smaller.
• Few things are smaller than an atom; only subatomic particles (protons, neutrons, electrons)…and light waves.
• Subatomic particles are too destructive to use, but light can look inside an atom without damaging it.
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Let’s understand the tool we’re using.
• We call light “Electromagnetic Radiation”.
• It’s composed of both an electrical wave and a magnetic wave.
• Visit this website to see the interlocking waves: (click on the red & blue wave)
http://www.monos.leidenuniv.nl/smo/index.html?basics/light.htm
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Wave or particle?
• Just like Dr. Jekyll and Mr. Hyde, light can be thought of both as a wave and as a particle (called a photon or a quantum).
• In its particle nature, light behaves like a bouncing ball – it can reflect off a surface.
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Wave or particle?
• In its wave nature, light can bend, just like when sound waves bend and spread out when passing through an open door. We call this bending refraction and diffraction.
• We’re going to concentrate on light’s wave nature.
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First, the terminology
• Wavelength – the distance from one wave top or crest to the next crest
• The waves of light are so small that they’re measured in nanometers (1 x 10-9 meters)
• The symbol for wavelength is , the Greek letter “lambda”.
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wave crestwave troughnode
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Describing Light Waves
• 3 things are needed to adequately describe a light wave:
– its wavelength () in nanometers
– its frequency () in “waves per second”, also known as Hertz (Hz)
– its speed (c) in meters per second.
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Wave frequency
• Frequency is the number of waves that pass a given point in 1 second. The unit is also known as a Hertz (Hz), or “cycles per second.”
• Your car’s radio dial is marked in kHz (kiloHertz) for ‘am’ stations and MHz (MegaHertz) for ‘fm’ stations.
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More about Frequency
• The symbol for frequency is the Greek
letter “nu” ().
• Frequency and wavelength are inversely related to each other. As the wavelength gets shorter, more waves pass in 1 second (the frequency increases.)
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How are they related?
• Frequency and wavelength are inversely related through the equation:
xc“lambda times nu = c”
“wavelength times frequency = c”
where “c” is the speed of light.
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C - the speed of light
• c is the speed of light, a constant in a vacuum or in air.
• c = 3.00 x 108 meters / second (300,000,000 m/s or 300,000 km/s.)
• This ‘c’ is the same thing in Einstein’s famous E = mc2 equation.
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Let’s try some calculations
• Example 1: What is the frequency of light, if the wavelength is 500 nanometers (nm)?
• Step 1: convert nanometers to meters.
500 nm = 500 x 10-9 m = 5.00 x 10-7 m
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• Step 2: re-arrange the equation
If xc, then c /
• Step 3: insert the numbers
frequency = 3.00 x 108 m/s
5.00 x 10-7 m
= 6.00 x 1014 Hertz
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You try this one…
• What is the frequency of light that has a wavelength of 400. nm?
• Don’t go on until you’ve tried this!
If you answered 7.50 x 1014 Hz, you’re right!
3.00 x 108 m/s / 4.00 x 10-7 m = 7.50 x 1014 Hz
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Finding wavelength from frequency
• Finding the wavelength is similar to finding the frequency:
If xc, then c /
• What is the wavelength of light that has a frequency of 2.5 x 1014 Hz? Go to the next page after you’ve solved the problem.
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If you said 1.2 x 10-6 m, you were right!
Now convert this answer into nanometers.
If you said 1.2 x 10-6 m = 1200. x 10-9 m, and this is 1200. nm, you were right again.
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The energy value of light
• Light is energy, right? So how much energy is there in 1 wave or photon of light?
• The amount of energy depends on the light’s wavelength or frequency.
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Max Planck
• In the early 1900’s a German physicist, Max Planck, determined that each photon or wave of light carries energy.
• Since shorter wavelengths of light have a higher frequency (more waves per second), shorter wavelength light should pack more energy.
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That’s Max…groovy guy, huh?
So…shorter wavelength
= higher frequency…
= higher energy!
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How much energy?
• The energy equation for light is…
Energy = a constant x frequency
or
E = h x
h is a constant called Planck’s Constant.h = 6.6 x 10-34 Joule / Hertz
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Calculating the energy
• Example 2: How much energy is in light that has a wavelength of 450 nm?
• Step 1: Convert 450 nm to meters:
450 nm = 450 x 10-9 m = 4.50 x 10-7 m
• Step 2: Divide the speed of light by
wavelength to get frequency: = c 3.00 x 108 m/s / 4.50 x 10-7 m = 6.67 x 1014 Hz.
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• Step 3: Multiply the frequency (result of step 2) by Planck’s Constant to get the energy: E = h x
(6.67 x 1014 Hz) x (6.6 x 10-34 J/Hz) =
4.40 x 10-19 Joules (in 1 photon
of this color of light).
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Your turn to practice
• You try the following problem:
What is the frequency and energy of yellow light that has a frequency of 510 nm?
Did you get these answers?
= 5.10 x 10-7 meters = 5.88 x 1014 Hz 3.00x108 m/s / 5.10x10-7 m
E = 3.88 x 10-19 J 5.88x1014Hz x 6.6x10-34 J/Hz
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Time for the quiz!
• Rank these 3 different colors of light from the least to the most energy:
• Light A has a wavelength () of 700 nm.
• Light B has a frequency () of 5.00 x 1014 Hz.
• Light C has an energy of 2.5 x 10-19 Joules/photon.
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What’d you get?
• If you ranked them C, A, B (from least to most energy), you’re right on target.
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Shift Gears – The Electromagnetic Spectrum
• Visible light is only a very tiny portion of the entire Electromagnetic Spectrum.
• Visible light has wavelengths from 400 nm (violet light) to 700 nm (red light).
• Most light we can’t even see, but other types of light are useful in chemistry.
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Look at the previous slide again
• Notice how as the wavelength gets longer, the energy decreases.
• The different parts of atoms and molecules can be probed by different types of light.
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Types of light & chemistry
• Low energy radio waves can be used to vibrate the nuclei of atoms.
• If you’ve ever had an MRI (magnetic resonance imaging), radio waves were shot into your body. The atoms of your body responded by giving back a “radar image” of your organs, bones, etc.
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An MRI scanRadio waves in action!
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Microwaves and Infrared
• Microwaves and Infrared (IR) light can be used to examine the chemical bonds between atoms in a molecule.
• Lots of chemical reactions also give off IR light – it’s the same as heat!
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Chemical bonds
A reaction givingoff lots of infraredand visible light.
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Ultraviolet light
• Ultraviolet (UV) light can cause lots of chemical reactions.
• 2 chemical reactions that are harmful are the tanning of your skin and the formation of cataracts in the lenses of your eyes.
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X-rays and Gamma rays
• The last 2 types of light can (again) be used to look into the nuclei of atoms, and to strip electrons from atoms (ionization).
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So just how does light affect the electrons within atoms?
• You’ve seen pictures of atoms that look like this:
• The circles represent “energy levels” within the atom.
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Energy levels
• The energy levels are like floors within a building. Electrons can only exist on the energy levels, but never between them.
When a wave of light hits an atom,an electron can ride the wave up toa higher energy level. In the process, it absorbs the light’s energy. The electron gains energy.
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Losing the energy that was gained
• After a short time, the energy that the electron gained is lost, and the electron falls back down to the lowest energy level that has room for it.
• Exactly how far the electron falls determines the energy (the color) of light that is given off.
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A short fallproduceslow energylight.
A long fallproduceshigherenergylight.
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It’s all in the spacing
• While every element is built basically the same way, the spacing between the energy levels is different for every element.
• Since the spacing is different, each different element will give off its own distinct set of colors when its electrons fall back to lower energy levels.
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Identifying elements by their light
• The science of identifying the chemical elements by the colors of light that they give off is called spectroscopy.
• Spectroscopy can tell us just how the electrons within an atom are arranged.
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Some characteristic colors
• On the following slides are some characteristic emission colors for different elements:
Lithium and Strontiumgive off red flames.
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Copper gives offblue and green.
Potassium gives off alilac flame.
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Try some spectroscopy for yourself
• Go to the following websites to explore the emission spectra of some common elements.
http://jersey.uoregon.edu/vlab/elements/Elements.html
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/linesp16.swf
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Remind me
• Remind me when we’re back in the regular classroom…I’ll show you some first-hand emission colors from different elements.
• In the meantime, have fun with your homework!
• Yeah, that’s all folks.