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Light and Electron: Wavelength, Frequency, and Energy

This article discusses the wave and particle models of light, wavelength, frequency, and energy calculations, emission and absorption spectra of hydrogen, and ionization energy calculations. Includes worked examples and relevant formulas.

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Light and Electron: Wavelength, Frequency, and Energy

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  1. Chemical Ideas 6.1Light and the electron.

  2. Sometimes we use the wave model for light … λ (lambda)= wavelength

  3. Wavelength increases, Frequency ? decreases C

  4. c = λν λ = wavelength m lambda c = speed of light = 3.00 x 108 m s-1 ν = frequency Hz ( or s-1 ) nu Use m for λ c λ ν

  5. c = 3.00x108 m s-1 C = λν problems…. Work out λ: 1. If ν is 3.00 m 2. If ν is 30.0 cm 3. If ν is 3.00 mm Work out λ: 4. If ν

  6. Sometimes we use the particle model … Hz h = 6.63 x 10-34 J Hz -1 Planck constant Packages of energy are called photons Light is a stream of photons E = 6.63 x10-34 x 4.5 x 1014 = 3.0 x 10-19 J

  7. Rearranging again…. E h

  8. E = hν problems … • h = 6.63 x 10-34 J Hz -1 • Planck constant 5. If the frequency is 1.089 x 106 Hz, what is the energy of each photon? 6. If E = 3.65 x 10-20 J per photon, what is the frequency (ν) of the radiation?

  9. Emission Spectrum of hydrogen. Black background Coloured lines.

  10. Spectroscopy Sample of hydrogen High voltage Prism

  11. Electrons can only exist in fixed energy levels. Electrons absorb energy and move to a higher energy level. ΔE The electrons drop back to a lower energy level and emit energy. The frequency of the radiation emitted depends on ΔE.

  12. 3  2 4  2 ?

  13. Why no • 7  2 • 8  2 • 9  2? 1 2 3 4 6 7 etc 5

  14. visible ultra violet E increases

  15. 1 1 2 2 3 3 4 4 E increasing Balmer  2 1 Lyman  ? 6

  16. Absorption Spectrum of hydrogen. Continuous spectrum Black lines.

  17. How do the 2 compare?

  18. 1 2 3 4 What happens to electrons? Electrons absorb energy Electrons are excited Electrons move to higher energy level E = h v

  19. Back to the Storyline…. Particles in the chromosphere absorb some of the light Photosphere of hot stars emit visible or UV light

  20. Every element has a different absorption spectrum H He Fe Have a look at assignment 5

  21. Ionisation energy 1 2 3 X (g)  X+(g) + e- Ground state electron? Energy needed to remove one e- from 1 mole of atoms of a gaseous element 4

  22. Convergence limit = 3.27 x 1015 Hz 7. Use ΔE = hν to calculate the energy per photon corresponding to this frequency (in J). h = 6.63 x 10-34 J Hz -1. 8. Ionisation energy has units of kJ mol-1. You have calculated the energy required to ionise a single atom. Work out the ionisation energy for hydrogen. L = 6.02 x 1023 mol-1.

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