Discoveries: The Nuclear Model

At the end of the 19th century, Thomson's plum-pudding model was the predominant view of the structure of the atom. However, Ernest Rutherford was determined to explore the limits of this model, subjecting it to scientific scrutiny to see if it would hold up. To discuss his techniques, we need to talk a little about radiation.

Radiation

The word "radiation" is very fraught; when many people hear the word, they think of nuclear explosions, dangerous x-rays, or maybe UV light from the sun giving them a sunburn. However, to scientists, "radiation" simply means any instance of a particle or a bit of light moving away from a source. Thus, the light that hits your retina and allows you to see is a form of radiation. So are the microwaves that warm your leftovers, and the radio waves that are used for cell phones, wifi, and myriad other technologies. Some forms of radiation are harmless, some are safe when encountered with caution, and some are extremely dangerous.

Cathode rays were one of the first forms of radiation known to exists (besides some forms of light). At the end of the 19th century, many new forms were discovered rapidly. Wilhelm Roentgen discovered X-rays in 1895, and Paul Villard discovered gamma rays in 1900; these are both types of high-energy light, similar to the light we see, just packing a more energetic punch.

Villard's gamma rays were found to be emitted naturally from certain radioactive materials. Other forms of radioactivity were also known: Thomson had discovered beta radiation a few years earlier, which consists of electrons ejected from atoms at high speed (much like cathode rays, but naturally occurring). And Rutherford had discovered alpha radiation in 1899, in which positive ions were also ejected from atoms at high speed. It was this alpha radiation that Rutherford would use to test Thomson's plum pudding model.

The Gold Foil Experiment

To test J.J. Thomson’s plum pudding model, Rutherford directed a beam of alpha particles at a thin sheet of gold foil. If Thomson’s model were correct, the alpha particles should pass right through the foil. The much lighter protons and electrons would not be heavy enough to deflect the more massive and fast moving alpha particles. There would, of course, be some scattering, but all of the alpha particles should emerge from the gold foil along a path only slightly wider than the original beam, as shown in the diagram.

The detector, mounted on a track indicated by the dotted circle, should show many alpha particle when placed directly behind the gold foil, but as it moves around the circle out of the direct path of the beam, the rate of detection should drop to zero not very far to either side – that is, if the plum pudding model is correct.

What Rutherford was doing was a basic, but extremely important, feature of the scientific method. He started with an idea: the plum pudding model of the atom. He used that idea to make a hypothesis: the alpha particles should pass through the foil and hit the detector with very little deflection. Then he tested his hypothesis with an experiment. And what did he find?

When the experiment was conducted, most of the alpha particles did, in fact, pass directly through the foil. But a few were deflected to the side and, much to Rutherford’s surprised, every so often an alpha particle would bounce off the foil completely. Rutherford would later write that his surprise was a great as if he had fired an artillery shell at a piece of tissue paper and it had bounced back and hit him. Based on his calculations, the result should have been impossible.

Rutherford repeated his experiment many times (an important aspect of the scientific method) with many different kinds of metal foils and the result was always the same. Most of the alpha particles passed through easily, but a few were deflected at wild angles. He checked and rechecked his calculations, and was forced to conclude that Thomson's model was incorrect.

Rutherford's Model

Rutherford proposed that the atom was mostly occupied by the electrons, thus most of the alpha particles passed through the foil unperturbed. The protons were concentrated at the center of the atom – in a nucleus. The occasional alpha particle that came near the nucleus was deflected. A rare direct hit would cause the alpha particle to bounce off the foil. Because the nucleus, like the alpha particles, was positively charged, an alpha particle only had to come near it to be deflected. Neutrons had not yet been discovered, so Rutherford did not include them among the particles he thought were to be found in the nucleus.

Rutherford's discovery was far from the final word on atomic structure. As we will see later in the lesson, the decades following his discovery of the nucleus saw chemists and physicists develop new, powerful, mind-bogglingly strange theories about electrons, as well as a much deeper knowledge of the structure of the nucleus. However, for the level of detail covered in this course, the nuclear model was good enough to explain lots of behavior. So, having arrived here, we will now spend some time discussing ions and isotopes (mentioned earlier) in greater detail. We will then return to the topic of subatomic structure to learn about the discoveries of Bohr and Schrodinger, which will be very important in future lessons. We will also revisit the definition of the mole with the added context of what we have learned so far this lesson.

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