Question

why did some particles pass through the foil undeflected.

why did some particles pass through the foil and get partially deflected

why did some particles hit the foil and deflect backwards

Answer 1

In Rutherford's experiment three things happened:

• some alpha particles that were deflected

• some alpha particles came back at him

some alpha particles went straight through the gold foil.

The particles that completely went through passed through the empty space in the atom. The particles that were deflected to the sides were deflated by a passing electron that moved the alpha particle out of the way. The particles that were bounced back hit the nucleus in the center that it bounced off.

The solution for above three scattering principle is, as you probably know, was the nucleus. It was already well established that the atom had a radius of about 10¯⁸ cm. The Thomson model of the atom spread the entire mass of the atom throughout that space. What Rutherford did was put most of the mass of the atom at the center of the atom, in a space much, much smaller that the atom itself -- this is the nucleus.

For the purposes of his 1911 paper, he considered the nucleus to act as a point:

"We shall suppose that for distances less that 10¯¹² cm the central charge and also the charge on the alpha particle may be supposed to be concentrated at a point."

Rutherford never used the word "nucleus" in his paper. His phrase was "charge concentration." In 1912, in a book he published, he devotes a few pages to the nuclear model and uses the word nucleus once.

So, how does the nucleus account for the three major findings by Geiger and Marsden

1) The nucleus is so small that the odds are overwhelmingly in favor of a given alpha particle motoring right on through the gold foil as if nothing were there. It turns out that the atom is a very empty place, indeed!

2) Some alphas, by pure random chance, will pass near some gold atom nuclei during their passage through the foil and will be slightly deflected. By pure chance, some or all of the small deflections will add up and shove the alpha particle off a straight-line path. Those alphas will emerge slightly deviated (say one or two degrees) from a straight-line path. (It might be helpful to remember that the gold nucleus and the alpha particle are both positively charged, so they will repel each other as they come close together.)

3) A very, very few alphas, by pure, random chance, will hit a nucleus almost head-on. The alpha, traveling at 10% the speed of light, penetrates the atom and gets very close to the nucleus. However, the repulsion between the alpha and the atom nucleus is so great that the atom flings the alpha back out, and it does so in a hyperbolic path. Depending on various factors, this occasionally results in the alpha being turned around 90° or more. The very heavy nucleus recoils a bit from the impact, but essentially goes nowhere.