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

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

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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