Today is Thursday, March (!) 1 st, 2018

Transcription

1 In This Lesson: Atomic Emissions (Lesson 2 of 4) Stuff You Need: Calculator Today is Thursday, March (!) 1 st, 2018 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?

2 Today s Agenda Atomic Emissions Flame Tests The Light Spectrum Calculations Where is this in my book? P. 138 and following

3 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.

4 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.

5 Identification The multicolored lights created by fireworks occur because of the different elements that comprise the powder in fireworks. Fireworks Filmed with a Drone video

6 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

7 Element Colors Element Sodium Potassium Rubidium Calcium Strontium Barium Copper Flame Color Orange Violet Pinkish-Red Orange-Red Red Green Blue-Green

8 Blue Cesium

9 Deep Orange Calcium

10 Orange Sodium

11 Violet Potassium

12 Jade Green Copper

13 Flame Tests Many elements give off characteristic light which can be used to help identify them. Strontium Sodium Lithium Potassium Copper

14 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

15 Electromagnetic Spectrum

16 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.

17 Lyman, Balmer, Paschen Series

18 Sources of Energy Where do electrons get energy to jump to the next higher energy level? Collisions from other particles Heat Electricity Light

19 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.

20 Particle-Wave Duality

21 Or this Why didn t the photon pack any luggage? Because it was traveling light!

22 Wave Statistics Amplitude: The height of the wave from zero to crest (peak). Indicates brightness. Wavelength: Distance between peaks in nanometers (nm) or meters (m). Given by Greek letter λ (lambda). Indicates color. 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). Indicates energy.

23 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 x 10 8 m/s. c = λv Sample Problem: Wave Statistics Worksheet: #9

24 Wave Statistics Worksheet #9 The wavelength of green light from a traffic signal is centered at 5.20 x 10-5 cm. Calculate the frequency. c = λv 5.20 x 10-5 cm = 5.2 x 10-7 m x 10 8 m/s = (5.2 x 10-7 m) * (v) v = 5.76 x Hz

25 Wavelength, Frequency, and Energy Long Wavelength = Low Frequency = Low Energy Short Wavelength = High Frequency = High Energy

26 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

27 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 = x J s Sample Problem: Wave Statistics Worksheet: #1 Max Planck

28 Wave Statistics Worksheet #1 A photon has a frequency ( ) of 2.68 x 10 6 Hz. Calculate its energy. E = hv E = (6.626 x J s) * (2.68 x 10 6 Hz) E = 1.78 x J

29 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.

30 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.

31 Summary Variables: λ (lambda) measure of wavelength. v (nu) measure of frequency in Hz (cycles/sec). c speed of electromagnetic waves x 10 8 m/s in a vacuum. h Planck s Constant x J s Equations: c = λv E = hv Heads Up! The back of your periodic table lists the constants and the formulas needed to solve these problems. Just know how to use them.

32 Other Equations? Related equations not covered directly in this course: E=mc 2 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.

33 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:

34 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.

35 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.

36 What NOT to do Don t let the splint burn. Don t place more than one splint into the flame.

37 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.