Quantum Theory and the Nuclear Atom

Quantum Theory and the Nuclear Atom.

There are two ways of finding what atoms or other small particles are like. One is to fire something even smaller at them and see how they break up or how the projectile bounces off of them. The other is to shake them about (giving energy) and seeing what comes out. 

PROBING THE ATOM WITH ALPHA PARTICLES

The initial method was used for a productive experiment in 1909. Alpha particles of a radioactive source were fired at a thin film of metal atoms. This was called the Geiger-Marsden experiment; some particles bounced back at angles, which meant that they had hit something smaller and with mass. From this Rutherford ( a physicist) worked out in 1911 that an atom has a positive nucleus surrounded by negative electrons. He suggested that electrons could be orbiting about the nucleus like planets around the sun. However, such an atom would not be stable; an orbiting electron, like an orbiting planet, has an acceleration directed towards the attracting object. An accelerating electron continuously radiates electromagnetic waves, so should lose energy and spiral into the nucleus.

Rutherford's model was saved in 1913 by Danish physicist Niels Bohr. He used the unique quantum ideas of energy, saying that the electron could have only certain 'allowed' energy states; with definite energy gaps between them corresponding to definite orbits. So electrons could not lose energy continuously and spiral into the nucleus. Electrons could only move between the orbits by gaining or losing definite, set quanta of energy. 

This did indeed seem a very far-fetched idea at the time, but Bohr backed it up with calculations of how much energy an atom could gain/lose and matched this with the energy of the light quanta it had emitted. 

Bohr did not explain why the electron couldn't fall into the nucleus. This had to wait until a later version of quantum theory.

"A physicist is just an atom's way of looking at itself."  

- Niels Bohr.