light and electronic transitions. the big questions what is light? how is light emitted? what do...
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Light and Electronic Transitions
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The Big Questions
What is light? How is light emitted? What do electrons have to do with light? What are emission spectra? How do flame tests help identify metals?
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The Electromagnetic Spectrum
All light is part of the EM spectrum. Most is invisible:
gamma, X-rays, UV, IR, microwaves, radio waves Visible light: wavelength (w.l.) from 400 to 700
nm.
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The EM Spectrum
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EM Radiation
Light is a carrier of energy. Energy is proportional to frequency. Frequency is inversely proportional to
wavelength. Longer wavelength = lower frequency = lower
energy. Shorter wavelength = higher frequency = greater
energy.
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Electrons and Quanta
Ground state – the lowest energy position an e- can occupy.
Excited state – a temporary high-energy position.
Quantum (pl. quanta) – the amount of energy needed to move an e- to a higher energy level.
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Electrons and Quanta
If an atom absorbs exactly 1 quantum of energy, an electron can be boosted from a ground state to an excited state. The electron is only in the excited state for a
very short period of time. Soon the e- returns to its ground state and
emits the quantum of energy as light. In some cases the emitted light is in the visible
spectrum.
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Light and Electrons
Ground state (G.S.) electron
Excited state (E.S.) electron
1 quantum
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Light and Electrons
Ground state (G.S.) electron
Excited state (E.S.) electron 1 quantum
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Light and Electrons
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Emission Spectrum
Emission spectrum – wavelengths of light given off by an element when it is excited (usu. by heat). Every element has unique emission spectrum.
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Emission Spectra
Hydrogen
Helium
Carbon
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Flame Tests
Flame test – used to ID some metals in compounds. Each metal gives a
flame a characteristic color.
Can identify metals based on flame colors.