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ATOMIC SPECTRA

ATOMIC SPECTRA. Objectives 1. Determine the emission spectrum of Hydrogen and other elements. 2. Calculate the expected wavelengths of H using the Rydberg equation. 3. Determine the composition of unknown solutions using flame tests.

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ATOMIC SPECTRA

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  1. ATOMIC SPECTRA Objectives 1. Determine theemission spectrum of Hydrogen and other elements. 2. Calculate the expected wavelengths of H using the Rydberg equation. 3. Determine the composition of unknown solutions using flame tests. 4. Determine the absorption spectrum of colored solutions and solids. Animation of the dispersion of white light as it travels through a triangular prism.

  2. History of Optics & Light Studies Ibn Alhazen is considered the “Father of Optics.” He wrote the “Book of Optics”, which correctly explained and proved the modern theory of vision. His experiments on optics greatly influenced later scientists. His experiments included ones on lenses, mirrors, refraction, reflection, and the dispersion of light into its constituent colors. He studied the electromagnetic aspects of light, and argued that rays of light are streams of energy particles traveling in straight lines. Ibn Alhazen (965 – 1039) Arab Muslim Scientist “Father of Optics”

  3. Historical Background of Spectroscopy In 1608,Galileo Galilei is credited as the first to turn his telescope to the heavens. He soon discovered craters on our Moon, sun spots, the moons of Jupiter, and that Venus has phases like our Moon. Galileo claimed that his observations only made sense ifall the planets revolved around the Sun (as proposed by Aristarchus and Copernicus) rather than the Earth. Galileo Galilei 1564 - 1642 The Inquisition eventually forced Galileo to publicly recant this conclusion.

  4. A Quantitative Study of Light Sir Isaac Newton was one of the first people to study light scientifically. In 1672, Newton directed a beam of white light through a triangular bar of glass, called a “prism”. He discovered that the light coming out of the prism was separated into bands of colors. The arrangement of colors produced by a prism is called a “spectrum”. Prior to this it was believed that “white light” was equal topurity. Sir Isaac Newton 1643 - 1727

  5. Original Studies Of Light Used Only One Prism . When a narrow band of light from a “white” light source is sent through a prism, a continuous spectrum containing all wavelengths of visible light is formed.

  6. Newton’s Contribution to Spectroscopy Newton contributed more to spectroscopy than scientifically proving that sunlight traveling through a prism was always broken down into the components of the rainbow. In fact, his main contribution was to show that after the sunlight had been broken down into its components by one prism, if a narrow ray of the light from the first prism was passed through another prism there would be no further breakdown.

  7. Classification of Electromagnetic Radiation The color components of light are separated along the visible range of light. The visible range of light (400-700 nm) is merely a small portion of the entire electromagnetic spectrum.

  8. Advancements in the Study of Light Joseph von Fraunhofer is best known for his discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for designing achromatic telescope objectives. At age 11, he was orphaned and forced to apprentice for no pay for a harsh glassmaker named Philipp Anton Weichelsberger. In 1801, the glass shop collapsed and Fraunhofer was buried alive. When Fraunhofer survived the collapse, the court-councilor von Utzschneider, gave him books on mathematics and optics. King Max Joseph also took an interest in him and gave him a present of eighteen ducats. With this money Joseph acquired his own glass grinding machine and bought his release from Weichelsberger. Joseph von Fraunhofer (March 6, 1787 – June 7, 1826) German Optician

  9. Development of the Spectroscope Joseph von Fraunhofer’s initial desire was to create a glass lens that did not produce an image that was fringed with a rainbow of colors. He realized the problem was that the glass lens bent some colors more than others. He began searching for a source of light of a single color. In 1814, he developed a spectroscope to study the spectrum of the light given off by the sun. He was amazed to discover that in the midst of the rainbow of colors was a series of black lines. These dark lines were later determined to be the result of the absorption of selected frequencies of the electromagnetic radiationby an atom or molecule. Joseph von Fraunhofer (March 6, 1787 – June 7, 1826)

  10. Development of Diffraction Gratings Fraunhofer also completed an important theoretical work on diffraction and established the laws of diffraction. One important innovation that Fraunhofer made was to place a diffraction slit in front of the objective of a measuring telescope in order to study the solar spectrum. He later made and used diffraction gratings with up to 10,000 parallel lines per inch. By means of these gratings he was able to measure the minute wavelengths of the different colors of light. (Diffraction gratings will be discussed more later.)

  11. 1855-1860 - Gustav Kirchhoff and Robert Bunsen Gustav Robert Kirchhoff (March 12, 1824 – October 17, 1887) German Physicist Robert Wilhelm Eberhard Bunsen (March 31, 1811 – August 16,1899) German Chemist Bunsen and Kirchhoff further developed the spectroscope by incorporating the Bunsen burner as a source to heat the elements. In 1861, experiments by Kirchhoff and Bunsen demonstrated that each element, when heated to incandescence, gave off a characteristic color of light. When the light was separated into its constituent wavelengths by a prism, each element displayed a unique pattern or emission spectrum.

  12. Emission Spectra Complement Absorption Spectra The emission spectrum seemed to be the complement to the mysterious dark lines (Fraunhofer lines) in the sun's spectrum. This meant that it was now possible to identify the chemical composition of distant objects like the sun and other stars. They concluded that the Fraunhofer lines in the solar spectrum were due to the absorption of light by the atoms of various elements in the sun's atmosphere.

  13. Atomic Spectra Experiment • PART A: Hydrogen emission spectrum. • PART B: Emission spectrum of other elements. • PART C: Flame Tests (organic & inorganic). • PART D: Absorption spectrum of colored solutions and solids.

  14. PART A: Record Hydrogen line spectrum with a Scanning Spectrophotometer. The hydrogen line spectrum contains only a few discrete wavelengths. In the visible region, there are only four wavelengths.

  15. Scanning Spectrophotometer (side view) A light beam enters the spectrophotometer. The focal point of the beam is brought to the slit of the spectrophotometer. The light passing through the slit is reflected off of a collimating mirror and sent to the diffraction grating. The diffraction grating disperses the parallel beams of light into their component wavelengths. Each different wavelength comes off of the grating at a slightly different angle. So the image of the slit is spread out by color similar to a rainbow.

  16. Scanning Spectrophotometer (top view) A hydrogen light source will be viewed using a scanning spectrophotometer. The wavelengths will be calculated for the Balmer and Lyman series and then compared to those generated by the computer attached to the scanning spectrophotometer.

  17. Computer Output from a Scanning Spectrophotometer The peaks on the spectrograph correspond to the energy changes of the electrons for the Hydrogen atom.

  18. Hydrogen Spectrum – The Balmer Series In 1885,Johann Jakob Balmer analyzed the hydrogen spectrum and found that hydrogen emitted four bands of light within the visible spectrum. His empirical formula for the visible spectral lines of the hydrogen atom was later found to be aspecial case of the Rydberg formula, devised by Johannes Rydberg. Johann Jakob Balmer (May 1, 1825 – March 12, 1898) Swiss Mathematician & Honorary Physicist

  19. Quantum Properties of Light E = nh E – the change in Energy n= 1, 2, 3, … h – (Planck’s constant) h = 6.62610-34 Js  - frequency Max Karl Ernst Ludwig Planck (April 23, 1858 – October 4, 1947) German Physicist The Nobel Prize in Physics 1918 for The discovery of energy quanta. The profile of radiation emitted from a black body In 1900, Planck hypothesized that energy was quantized (i.e., energy can be gained or lost only in whole-number multiples of the quantity h.) This hypothesis of quantum properties was later extended by Albert Einsteinto include light. Einstein envisioned light as small discrete particles of energy called photons.

  20. In 1913, Bohr developed a quantum model for the hydrogen atom. • Proposed a model that the electron in a hydrogen atom moves around the nucleus only in certain allowed circular orbits. Niels Henrik David Bohr Oct. 7, 1885 – Nov. 18, 1962 Danish Physicist The Nobel Prize in Physics 1922 for the investigation of the structure of atoms and of the radiation emanating from them. Solar System Model has electrons moving around the nucleus.

  21. Calculations for Energy Levels Where Z = Atomic Number of Element n = Quantum Number & the units are Joules. Energy levels available to the electron in the hydrogen atom.

  22. Calculating the Balmer & Lyman Series As noted earlier, the four bands of light calculated by Balmer could be simply calculated using the Rydberg equation: * Where v = frequency n = the quantum number R =(Rydberg constant) R = 3.29 1015 Hz 1 Hz = 1 s-1 The permitted energy levels of a hydrogen atom. *Write this equation on top of page 9-7.

  23. Recall that Frequency and Wavelength are related where frequency times wavelength equals the speed of light. Wavelength (): Distance between two consecutive peaks [unit: nm] Frequency (): Number of waves per second that pass a given point in space [unit: s-1 (Hertz)] *   = c Where C is the speed of light & C = 2.9979108 m/s *Write this equation on top of page 9-7. Since the speed of light is a constant, as wavelength decreases, then frequency must increase.

  24. PART B: Emission spectrum of other compounds using The STAR Spectrophotometer. • View the line spectrum through the STAR Spectrophotometer • - point arrow towards the light and view to the left. • 2. Verify that the scale is lined-up accurately by looking at the fluorescent light. In addition to other lines, you should see a green doublet for mercury at ~570 nm (the scale on the bottom). 3. Measure the line spectrum of the gas tubes set up in Room 201. 4. Compare your results with literature values.

  25. Atomic Spectra of Noble Gases Helium Neon Argon The Atomic Spectra will be determined for Hydrogen & the Noble Gases by looking at the gas discharge tubes.

  26. PART C: Flame Test (Organic Compounds) Beilstein Test If a clean copper wire is coated with a halogen-containing compound and placed in a flame, the presence of the halogen is revealed by a green to blue color. It is often possible to distinguish between chlorine, bromine and iodine based on the color of the flame.

  27. PART C:Flame tests and identification of an unknown metal. Observe and record the color of the flame for each known sample. Then determine the unknown compound based on the comparison between its flame color and those of the known samples.

  28. Flame Tests • Flame Test:A test used in the identification of certain elements. • It is based on the observation that light emitted by any element gives a unique spectrum when passed through a spectroscope. Flame spectrum for lithium. (Notice the faint bands of color in the spectra.)

  29. PART D: Absorption spectrum of colored solutions and solids. Sample Solution Which color is being transmitted by this sample? Which color is being absorbed by this sample?

  30. Sample Solution For the Solution first we’ll determine where the maximum absorption occurs.

  31. Absorbance Beer's law - the linear relationship between absorbance and concentration of an absorbing species. Second, we’ll produceA Working Curve by plotting the Absorbance vs. the Concentration. From this we can determine the concentration of an unknown sample by knowing the absorption.

  32. Checkout – (All items checked out should be returned) 1-STAR Spectroscope 1-nichrome flame test loop 1-copper Beilstein test loop 1-pc blue cobalt glass (to block yellow Na emission) 8-cuvettes in a test tube rack 2-beral pipettes In Lab Flame test knowns – in hoods Flame test unknowns – vials at bench by blackboard 12M HCl for cleaning loops – in “Beilstein” hood Gas discharge tubes (for viewing by STAR spectroscope) – in 201 Computerized spectrophotometer – 1 setup in 201 Cobalt Chloride solution – at bench by blackboard Colored solids – to be viewed at bench by blackboard Color spectrum vs. wavelength charts & Color Wheels – on desks Spec-20s – on desks All students View Scanning Spectrophotometer for Part A in Room 201. View Gas Discharge tubes for Part B in Room 201.

  33. Hazards 12M HCl – strong acid, corrosive (use solid NaHCO3 on spills) CoCl2 solution - heavy metal, irritant, oxidizer CH2Cl2- halogenated volatile organic solvent Bunsen Burner – open flames Waste Liquid waste in container marked “Atomic Spectra”. Next Week Turn In Colorimetry Formal Report &Atomic Spectra Worksheets. Read Gas Chromatography in the Green Book.

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