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Chemistry 40S

Chemistry 40S. Unit 2 – Atomic Structure Lecture 1. Electromagnetic Radiation (EMR). 1678: Christian Huygens  light was in the form of waves 1704: Sir Isaac Netwon  light has a particulate nature T his helped explain his observations from his experiments in optics

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Chemistry 40S

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  1. Chemistry 40S Unit 2 – Atomic Structure Lecture 1

  2. Electromagnetic Radiation (EMR) • 1678: Christian Huygens light was in the form of waves • 1704: Sir Isaac Netwon light has a particulate nature • This helped explain his observations from his experiments in optics • Netwon’stheory of light was favoured over Huygens’ particle theory of light for over 100 years. • 1807: Thomas Young & AugustinFresnel performed experiments where light was shone through slits • Observed an interference pattern that could only result if light had a wave-like nature

  3. Electromagnetic Radiation (EMR) • 1860: James Clerk Maxwell mathematically demonstrated that light was composed of both waves of electrical and magnetic energy or electromagnetic radiation • He proposed that both fields moved perpendicular to each other as shown in the diagram below:

  4. Describing Waves • Wavelength (λ)is the distance from one crest to the next crest or trough to trough • Usually measured in metres • Frequency (ν)is the number of wavelengths, or wave cycles, that pass a point per unit time • Measured in cycles per second, or the SI unit hertz (Hz) • Can also be represented by the unit s-1, or the reciprocal of seconds.

  5. The Relationship between Wavelength and Frequency

  6. The Relationship between Wavelength and Frequency • Wavelength and frequency are inversely related • This is logical because the larger the wavelength the longer the wave will take to move past a point

  7. Frequency, Wavelength & Energy • 1900: Max Planck heated objects until they glowed, then studied the light given off by the glowing objects • Discovered that the frequency of the light given off was directly related to the amount of energy released • Wavelength is inversely related to frequency, wavelength will also be inversely related to the energy of the light • The greater the wavelength, the lower the energy • Example: • Radio waves have a wavelength of about 2 m and X-rays have a wavelength of about 1.25 x 10–10m • X-rays have a greater amount of energy than radio waves • Radio waves are all around us and are not harmful to us, but exposure to X-rays is limited because their high energy is damaging.

  8. The Electromagnetic Spectrum • Sunlight + prism = rainbow of colours • Each colour represents a different frequency or wavelength • This is referred to as a continuous spectrum because there are no breaks between the different wavelengths

  9. The Electromagnetic Spectrum • The visible wavelengths make up a small portion of the total spectrum

  10. Line Spectra • When an electric current passed through hydrogen gas in a tube the gas glows • If the light produced by the glowing gas is focused through a slit is passed through a prism, a spectrum with distinct lines is produced • Emission spectrum, since it is the separate wavelengths of light emitted by the gas

  11. Line Spectra • The emission spectrum is also known as a line spectrum because the light separates into discrete wavelengths of light that appear as lines of colour on a screen or photographic plate • Unlike a continuous spectrum, the colours in a line spectrum do not blend into each other • Each coloured line corresponds to an exact wavelength or frequency of light. Example: Line Spectra of Hydrogen

  12. Spectroscopy… • The process of measuring the emission spectra of substances goes by several names: • Spectroscopy, spectrophotometry and spectrometry • This is a useful process as the spectrum for each element is unique, like a fingerprint and as a result may be used to identify substances

  13. Flame Tests • When some elements are burned, they give off a distinctive colour of light • This is due to the emission of light predominantly in that wavelength in their line spectrum • Preliminary evidence of the presence of a metal is often the colour that results when it is placed in a flame • This is called a Flame test

  14. Applications of Line Spectra • Astronomers use line spectra to determine the elements in various light sources, such as stars and nebulae • Astronomers don’t just study visible light emissions. • Many astronomers use radio telescopes to find distant objects • The aurora borealis, or northern lights, are a result of ionized gases becoming excited • This excitation leads to the release of light which you can see in the northern sky.

  15. Applications of Line Spectra • Another popular application of the spectra produces by elements is in the manufacturing of fireworks • Distinctive colours are produced by metal ions in flame tests • These metal salts are used in fireworks to produce the distinctive colours of light. fireworks shell that contains copper and strontium for a burst that will have both red and green.

  16. Fireworks

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