The Atom - Part One

Everybody knows what an atom looks like, right? Wrong! Apart from the latest fuzzy electron microscope images of Uranium, no-one has ever seen an atom.

All we had were "models" of it, "models" as in a realistic idea of what we thought atoms would look like.

JJ Thomson discovered the electron, and his model of the atom looked rather like a currant bun:



He thought electrons were "embedded" inside a large positively charged ball.

Then Earnest Rutherford came along and fired helium nuclei (alpha particles - two protons and two neutrons, although they were only known as alpha particles at the time, with a double positive charge) at a thin film of heavy atoms. Thus was born particle physics.

To his consternation, he found that every so often, an alpha particle came straight back at him.

If you do the statistics on the Thomson model, this was absolutely impossible.

Rutherford was quoted as saying: "It was as if you had fired a bullet from a gun into a layer of tissue paper, and the bullet came right back at you" (sic).

The only way to explain this phenomenon was to conjecture that the Thomson model was wrong, and, that somehow, virtually all the mass and all of the positive charge of the atom was concentrated into a very tiny space, with electrons "circling" around it, rather like a minute solar system.

That way, if an alpha particle hit the heavy atom "head on", the electrostatic repulsion would send it back on virtually the same path.

This was born the Rutherford Atom, as depicted in the top picture.

There was a snag, however, with this model. Maxwell's equations, formulated years before, predicted that accelerated charged particles would radiate electromagnetic energy. And according to standard mechanics, all things in circular motion were in constant acceleration.

Thus if the Rutherford model was correct, the electrons would radiate, lose energy, and spiral inward toward the nucleus, and the atom would collapse in something under 1/1000,000,000,000,000 of a second.

Clearly, this was not the case.

Then along cam Niels Bohr, with another model......but even he was wrong, even though his idea DID "explain it away"