Types of Spectra. How do spectrum lines form? 3/30/09. Electron cloud. Atom. Nucleus

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1 The electron should be thought of as a distribution or cloud of probability around the nucleus that on average behave like a point particle on a fixed circular path Types of Spectra How do spectrum lines form? Electron cloud Atom Nucleus 1

2 Nucleus Hydrogen atom energy levels Electrons in atoms are restricted to particular orbits The size of an electron s orbit is related to the energy that binds it to the atom Nature permits atoms only certain amounts of binding energy Thus, an atom can have orbits of only certain sizes Hydrogen atom The electron in a hydrogen atom can occupy any permitted orbit, but not orbits in between 2

3 How do spectrum lines form? A photon can be absorbed and excite the electron to a higher energy orbit An atom cannot be in an excited state forever; must give up the energy it absorbed and emit a photon The energy of the photons, and thus their wavelengths, are determined by the energy difference between the two energy levels Photons (light-waves) are emitted from an atom when an electron moves from a higher energy level to a lower energy level Nucleus Photons (light-waves) can also be absorbed by an atom when an electron moves from a lower energy level to a higher energy level Nucleus 3

4 Differences between energy levels lead to observed spectrum lines Hydrogen Energy Levels Which represent correctly an atom that includes a nucleus with four energy levels that electrons could occupy? In-class Activities: Light and Atoms Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the question is asking, ask me. 4

5 Energy is released from atoms in the form of light when electrons a) move from high energy levels to low energy levels. b) move in their orbit around the nucleus. c) move from low energy levels to high energy levels. d) are emitted by the atom. e) are absorbed by atoms. If an electron in an atom moves from an orbit with an energy of 5 to an orbit with an energy of 10, A. a photon of energy 5 is emitted B. a photon of energy 15 is emitted. C. a photon of energy 5 is absorbed. D. a photon of energy 15 is absorbed. E. None of the above Which drawing (not to scale) represents the process by which an photon with the greatest energy is absorbed? 5

6 Which drawing (not to scale) represents the process by which an photon with the shortest wavelength is emitted? Each chemical element produces its own unique set of spectral lines when excited Unique energy levels = Unique spectrum lines Each element has its own pattern of permitted orbits The permitted orbits depend primarily on the number of protons in the nucleus So, each elements has a unique set of spectrum lines! Permitted orbits of hydrogen (1 proton), helium (2 protons), and boron (5 protons) 6

7 Spectrum Lines = Atomic Fingerprints An atom s spectrum is like a fingerprint......or a unique barcode The Sun s spectrum tells us about its chemical composition Two dark absorption lines appear in the yellow region of the solar spectrum at the wavelengths nm and nm Only sodium can produce this pair of lines So, the sun must contain sodium Sodium lines in the Sun s spectrum All stars produce dark line absorption spectra 7

8 What can we learn by analyzing starlight? A star s chemical composition Why does the Sun shine? A hot plate s coil glows different colors as it heats up If it is not too hot, the coil is deep red As it heats up, it grows brighter and yellower The Sun s surface glows because it is also hot Hot, opaque objects emit blackbody (or thermal) radiation, including stars, hot plates, you The spectrum of this light depends on only one property: temperature Blackbody Curve - a graph of an object s energy output versus wavelength. The WAVELENGTH that the PEAK of this curve occurs at tells us about the object s TEMPERATURE and COLOR. Energy Output UV IR Wavelength 8

9 Two Properties of Blackbody Radiation: Wein s Law The hotter an object is, the shorter is the wavelength of its maximum output Hot objects tend to emit radiation at shorter wavelengths and look bluer than cooler objects Which object is hotter, an object that is emitting mainly red light or mainly blue light? Hot objects emit light that PEAKS at short wavelengths (blue). Cool objects emit light that PEAKS at long wavelengths (red) increasing temperature 9

10 What color is our 5800K Sun? The Sun emits all wavelengths of electromagnetic radiation (light); however, the wavelengths of light it emits most intensely are in the green/yellow part of the spectrum. What color does the Sun appear? WHITE!! A star, like the Sun, which peaks in the middle of the visible part of the spectrum (green/ yellow light) will appear WHITE to the human eye because it is giving off nearly equal amounts of all the visible colors of light. Which star is hotter? a) Capella (yellow) b) Vega (blue) c) Antares (red) 10

11 Stars and Blackbody Radiation Stars emit light that is close to an ideal blackbody Cool objects still produce blackbody radiation The human body has a temperature of 310 K Emits blackbody radiation mostly in the infrared part of the spectrum Infrared security cameras can detect burglars by the radiation they emit Mosquitoes can track you down in total darkness by homing in on your infrared radiation Blackbody radiation per unit area from a human vs. stars The human body emits mostly infrared light Two Properties of Blackbody Radiation: Stefan-Boltzmann Law Hotter objects emit more energy than cooler objects of the same size An object emits energy at a rate proportional to the fourth power of its temperature 11

12 Luminosity is the total energy (light) emitted by an object in each second. It depends on an surface area (A), and its temperature (T 4 ) Luminosity = (Α)T 4 Big and Hot objects have greater luminosity than small cool objects Our Sun What if the Sun became hotter? Our Sun What if the Sun became hotter? What if the Sun became cooler? 12

13 Our Sun What if the Sun became hotter? What if the Sun became cooler? Our Sun What if the Sun became hotter? What if the Sun became cooler? What if the Sun became bigger? Our Sun What if the Sun became hotter? What if the Sun became cooler? What if the Sun became bigger? What if the Sun became smaller? 13

14 V I B G Y O R V I B G Y O R V I B G Y O R V I B G Y O R 3/30/09 1. Which object gives off the greatest amount of Blue light? 2. Which object gives off the greatest amount of Red light? 3. Which object would appear Red? A B 4. Which object would have the lowest temperature? C Try to determine EVERYTHING about how these four stars compare!! Temp, Energy output, Color, size (area).. Energy Output per second visible range Energy Output per second visible range Object A Object B Wavelength Wavelength Energy Output per second visible range Energy Output per second visible range Object C Object D Wavelength Wavelength In-class Activities: Luminosity, Temperature and Size & Blackbody Radiation Work with a partner! Read the instructions and questions carefully. Discuss the concepts and your answers with one another. Take time to understand it now!!!! Come to a consensus answer you both agree on. If you get stuck or are not sure of your answer, ask another group. If you get really stuck or don t understand what the question is asking, ask me. 14