Friday, May 16, 2014

Science Friday: The Bohr Atom (4)

I've been reading through George Gamow's Thirty Years That Shook Physics. This week starts chapter 2, "N. Bohr and Quantum Orbits."  The previous posts were:

1a. Planck's Quantum
1b. Jumping Photons (Einstein and the Photoelectric Effect)
1c. The Compton Effect (Proof of Energy Packets)

Hindsight is 20/20. You can see the stubbornness of scientists, what Kuhn called "normal science," as you look back at history. This chapter starts by recounting J. J. Thomson's stubbornness as the Dane Niels Bohr suggested that his model of the atom wouldn't work given the recent developments of Planck and Einstein.
Thomson's Plum Pudding Model
Thomson had proved the existence of electrons, negatively charged particles that could be dislodged from an atom. But Thomson had suggested a model something like a pudding with raisins stuck in it. When Bohr arrived at Cambridge, Thomson wasn't at all excited about these new ideas. Bohr finally switched to Manchester, where Ernst Rutherford was bombarding atoms with alpha particles (basically, helium atoms without the electrons).

What Rutherford found was that while most alpha particles passed right through foil, some actually were deflected at significant angles. His conclusion was that most of an atom must be empty, with the positive charge in an atom concentrated in a very small space in the middle (the nucleus).

Rutherford proposed a model of the atom that was much like the solar system, a heavy nucleus in the center with orbiting electrons.
The Rutherford Model of the Atom
But Rutherford's model couldn't explain why the energy of the electrons didn't gradually dissipate. That would take Bohr...
Thomson on left, Rutherford on right
Next Week: Bohr's contribution

No comments: