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Electron Arrangement and Spectroscopy: Understanding Atomic Structure

This lecture covers the arrangement of electrons in an atom and how spectroscopy is used to study atomic structure. Topics include energy levels, line spectra, and the Bohr model.

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Electron Arrangement and Spectroscopy: Understanding Atomic Structure

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  1. Chapter 2Atoms and Elements Electron Arrangement and Periodic Law LecturePLUS Timberlake

  2. What had we learned so far? • Atomic Structure – • Nucleus • Electrons • Essential question: how are those electrons surrounding nucleus arranged?

  3. Characteristics of Electrons • Extremely small mass • Located outside the nucleus • Moving at extremely high speeds in a sphere • Have specific energy levels LecturePLUS Timberlake

  4. Energy of Electrons • When atoms are heated, bright lines appear called line spectra • Electrons in atoms arranged in discrete levels. • An electron absorbs energy to “jump” to a higher energy level. • When an electron falls to a lower energy level, energy is emitted. LecturePLUS Timberlake

  5. Experimental Evidence Discharging Tubes The light coming out of the excited atomic entities is very specific to particular element! Results are quite reproducible. The Flame Test

  6. Experimental Evidence There has been no radioactive decay going on. Hence the nucleus does not change when the atomic entity gets excited either by electricity or heat. So the colored light must have come from those electrons. Light emitted from excited atomic entities is the tool used to probe how electrons are arranged.

  7. Loss and Gain of Energy Loss G a I n LecturePLUS Timberlake

  8. What is light? Lights, both visible and invisible to human eyes, are electromagnetic waves. Time Out! Before we go any further, what is a Wave?

  9. What is a wave? A wave is a means to transfer energy from point A to point B. Waves in water Sound waves Typical mechanical waves such as those in water and sound waves DO need medium in which they propagate. Water and air are the prerequisites for waves to travel.

  10. Waves – in more the abstract form Wavelength - distance from crest to crest abbreviated Greek letter, l, pronounced “lambda”. Also can be defined as how far the wave travels in a cycle. Note: great link to an online simulation of waves. http://phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.html -

  11. Waves – in more the abstract form • Frequency – the number of complete waves passing any given point per second. • SI unit for frequency is Hertz (Hz), or cycles/sec. • Abbreviated Greek letter, n, pronounced “nu”. The graph shows that the top wave passes any given point 4 complete wave forms every second; the middle one 2 complete wave forms; and the bottom one 1 complete wave form.

  12. High Energy Low Energy Low Frequency High Frequency X-Rays Radiowaves Microwaves Ultra-violet GammaRays Infrared . Long Wavelength Short Wavelength Visible Light

  13. ---------------- > decreasing energy --------------------- ----------------> decreasing frequency ----------------> ---------------> increasing wavelength ---------------->

  14. Types of Spectra • Continuous– all wavelengths within a given range are included. • Electromagnetic – all electromagnetic radiation arranged according to increasing or decreasing wavelength. • unit for wavelength ranges from meters to nanometers • unit for frequency is hertz (Hz) (# waves per second)

  15. Types of Spectra • Visible spectrum - light you can see (ROY-G-BIV) • Red has the longest wavelength and the smallest frequency. • Violet has the shortest wavelength and the greatest frequency. • Bright Line spectrum (emission spectrum) • Bands of colored light emitted by excited electrons when they return to the ground state.

  16. Passing Light Through a Prism White light is made up of all the colors of the visible spectrum. Passing it through a prism separates the colors in white light.

  17. If the light is not white, By heating a gas with electricity we can get it to give off colors. Passing this light through a prism does something different.

  18. If the light is not white, Each element gives off its own characteristic colors. Can be used to identify the atom. This is how we know what stars are made of.

  19. Spectroscopy • Emission spectraof a substance is studied to determine its identity. • Spectroscope– instrument that separates light into a spectrum. • Spectral lines – represent wavelength of light emitted when excited electrons fall back to the ground state.

  20. How Does a Spectroscope Work?

  21. Emission Spectrum (Line Spectrum)

  22. Emission Spectrum

  23. Do you get it? Answer with: 1) Energy absorbed 2) Energy emitted 3) No change in energy A. What energy change takes place when an electron in a hydrogen atom moves from the first (n=1) to the second shell (n=2)? B. What energy change takes place when the electron moves from the third shell to the second shell? LecturePLUS Timberlake

  24. Answers A. 1) Energy absorbed B. 2) Energy emitted LecturePLUS Timberlake

  25. Bohr Model • First model of the electron structure • Gives levels where an electron is most likely to be found • Incorrect today, but a key in understanding the atom LecturePLUS Timberlake

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