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Atomic and Nuclear Physics

Spectra and Energy Levels in Atoms

Balmer measured the wavelengths of visible light emitted by hydrogen atoms. He found that they could be described by the following formula (now called the Balmer series).

l is wavelength, R is a constant (the Rydberg constant) and n = 3, 4, 5, etc.

In order to explain the Balmer series, Bohr made the following suggestions

1. Electrons in atoms can only have certain "allowed" energies.
2. An electron can be excited from one energy level to another by a collision with another particle or by absorbing a quantum of electro-magnetic radiation.
3. When an electron falls from one energy level to a lower level, it emits one quantum of electro-magnetic radiation.
4. The energy possessed by the quantum, is the difference between the initial and final energy levels of the electron.

hf = Efinal – Einitial

Energy transitions in atoms are often represented by diagrams like the one below. The lowest energy is called the "ground state".

 

 

© David Hoult 2008