1. Chemistry 40S Unit 2 – Atomic Structure Lesson 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’s theory of light was favoured over Huygens’ particle theory of light for over 100 years.  1807: Thomas Young & Augustin Fresnel  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.  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 –10 m  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