Ernest Rutherford and the Discovery of the Nucleus

In 1911, Ernest Rutherford was experimenting with alpha particles and observing their ability to pass through certain materials to a greater or lesser extent, when he observed that some of the alpha particles bounced off of the surface rather than passing through or being absorbed.

He reconfigured his apparatus to look at this phenomenon carefully. He set up something like the diagram below.

In this diagram, the alpha source is encased in a lead box with a small opening, allowing only those alpha particles going in a certain direction to enter the rest of the experiment. Essentially this created an alpha particle “gun.”

The “target” was made of a single sheet of gold leaf – a foil so thin that 100,000 payers would be about 1 inch thick!. At this thickness, Rutherford estimated that the foil was only a few atoms thick. It is probably closer to 100 atoms thick, but that's still pretty darn thin!

Around the target, Rutherford placed a phosphorescent screen. This is a thin sheet of material that scintillated (glowed briefly) when struck by a small particle like an alpha particle.

The understanding of atomic structure at the time (Thomson's Plum Pudding Model) implied that the alpha particles should go directly through the foil, hitting the screen directly behind the target. This was expected because alpha particles are much heavier than electrons (at that time the only known sub-atomic piece) and the rest of the atom was assumed to be an evenly distributed mush of positive charge.

However, Rutherford and his assistants found that, although the vast majority of the particles did go directly through the gold, a very small number of them were deflected at sizable angles and a few were even reflected backwards toward the source.

Rutherford concluded that there must be something in the atom that was positively charged (in order to deflect the path of the positive alpha particles), very small (since virtually all of the alpha particles missed it) and very heavy (in order to deflect the fast moving alpha particles, which were 1000's of times heavier than electrons). He called this “thing” the nucleus (Latin for little nut) and reasoned that the nucleus of the atom held all of the positive charge of the atom and virtually all of the mass.

What he imagined happening was this,

in which most of the atom is empty space through which alpha particles pass unaffected, while those that get too close to the center are deflected.

It is important that you understand that the preceding diagram is NOT TO SCALE! It is impossible to make a diagram that would be to scale. Put into simple terms if the nucleus were drawn to the size of a small pea, the atom would need to be drawn as large as a professional football stadium (not field, mind you, the entire stadium).