mystery # 3: atomic spectra · 2015-03-18 · mystery # 3: atomic spectra •narrow bands of colors...
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Mystery # 3: Atomic Spectra
• Narrow bands of colors of light are emitted when gases are excited by high voltage. The colors are characteristic and reproducible for different elements.
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High voltage tube
containing H2(g)
Generated light is dispersed into its
component λ.
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• All elements display a characteristic emission spectra that we can use to fingerprint all elements.
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• Flame tests cause a similar electron excitement characteristic to each element. The color helps us identify the elements.
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Hydrogen Helium Lithium Sodium Potassium
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Chemistry of Fireworks
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Mystery # 3: Atomic Spectra
• How could scientists account for the lines if atomic theory and classical physics were correct?
• How does the emission spectrum relate the atomic and electronic structure of the H atom?
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1. The H atom has only certain energy levels called stationary states.
◦ Each state is associated with a fixed circular orbit of the electron around the nucleus.
◦ The higher the energy level, the farther the orbit is from the nucleus.
◦ When the H electron is in the first orbit, the atom is in its lowest energy state, called the ground state.
Bohr Model of the Hydrogen Atom
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2. The atom does not radiate energy while in one of its stationary states.
3. The atom changes to another stationary state only by absorbing or emitting a photon.
◦ The energy of the photon equals the difference between the energies of the two energy states.
◦ When the electron is in any orbit higher than n = 1, the atom is in an excited state.
Bohr Model of the Hydrogen Atom
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Early Explanation
• In 1885, Rydberg developed a generalized but empirical mathematical relationship between ν and n for all the lines of hydrogen emission spectrum.
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= R Rydberg equation - 1
l
1
n22
1
n12
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Bohr Model of the Hydrogen Atom
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Question arising from Bohr’s Model
• Why couldn’t it explain the emission spectra of other atoms?
• Why is the electron in a Bohr atom restricted to orbiting the nucleus at certain fixed distances?
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