A Quantum Theory for Atomic Structure

Experimental Proof!

In 1914, James Franck and Gustav Hertz performed an experiment which clearly demonstrated the presence of excited states (as predicted by Bohr) in mercury atoms. Electrons were accelerated towards a positively charged grid mounted in a glass tube filled with mercury vapour. Behind the grid 'A' was a collection plate 'B' held at a more positive voltage than the grid.

While the drop at (1) corresponds to the electron exciting one mercury atom, the drop at (2) corresponds to the electron having sufficient energy to excite two mercury atoms. It is in fact possible to observe up to 10 sequential bumps with equal spacings.

This data provided direct evidence for Bohr's idea that electrons only occupy discrete, quantised energy levels around the nucleus. They won the Nobel Prize in Physics in 1925 for this work.

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