Discoveries: Electrons and Thomson's Model

Reading the previous section, you may have been wondering, "how do we know all of this?" After all, even the most powerful modern microscopes only barely allow us to visualize single atoms, and we're talking now about nuclei that are tens of thousands of times smaller than that. It's absolutely impossible to directly observe anything at the subatomic level, and it probably always will be. So, scientists have had to piece together our understanding of the atom through inference and clever experiment.

The story of the discovery of atomic structure is complex, fascinating, and long. While you will probably not understand all the fine details, hopefully these next few sections will give you an appreciation for the ingenuity and creativity involved.

Dalton's Model

When John Dalton (along with contemporaries like Amadeo Avogadro) initially postulated the existence of atoms, he made no claims about their internal structure. As best we can tell, he and other chemists envisioned them as much like billiard balls: perfect spheres, moving through space, bumping into each other and bouncing off, though sometimes mysteriously binding to each other.

While he did introduce a variety of symbols for the different elements, these symbols didn't make any claims about the differences between them. Dalton was not aware of the existence of protons, neutrons, or electrons (collectively called subatomic particles). And the term itself - "atom" - literally means "uncuttable," because most scientists at the time believed there was nothing smaller than an atom.

Cathode Rays

The discovery of electrons (the first subatomic particle to be identified) was a result of the invention of the cathode ray tube, or CRT. A CRT is a glass tube from which all of the air has been removed. Small amounts of various pure gases can then be added. Inside the tube are two metal plates connected to a high voltage power supply, making one plate positive (the anode) and one plate negative (the cathode). Often the anode is coated with zinc sulfide, a fluorescent material.

Experimenters found that when the power supply was activated, the fluorescent coating on the anode would glow. Also, if a (very) small amount of a gas like neon was added to the tube, the gas would glow in a thin line between the cathode and anode. Scientists, especially British physicist JJ Thomson, postulated that this effect was due to negatively charged particles being ejected from the cathode, traveling through the tube (as "cathode rays") and striking the anode.

This interpretation was confirmed by further manipulations, involving placing charged plates outside of a CRT. In the experiment shown here, the stream of particles is attracted to a positively charged plate placed near the top of the CRT, and away from the negatively charged plate at the bottom. This confirmed the negative charge of the particles. Also, by carefully adjusting the charges on the external plates, Thomson was able to determine the ratio of their mass and charge.

These and other experiments showed conclusively that 1) cathode rays consisted of negatively charged particles and 2) that these particles were far smaller than atoms, and that they always had the same mass-charge ratio. Because all the materials involved were made of atoms, the only possible conclusion was that atoms could, in fact, be broken down into smaller particles. These particles were given the name "electrons." The glow observed in the tube was due to these electrons striking gas atoms at very high velocity.

Thomson's Model

It would take longer for protons to be discovered (see below), so for a time, all Thomson had to go on was the existence of electrons and their negative charge. Because he knew atoms were neutral overall, there had to be positive material inside them as well.

Thomson proposed a model in which electrons were embedded in a matrix of positively charged material. This came to be known as the "plum pudding" model for its resemblance to a common British dessert.

Later studies, of course, would reveal that Thomson's model was incorrect, starting with the discovery of protons.

Positive Rays

When a hole was made in the cathode of a CRT, the gas behind it began to glow as well, indicating another kind of particle. Experiments with this stream of particles showed that they were positively charged (they deflected away from positive plates and toward negative ones) and much heavier than cathode rays. Moreover, their mass/charge ratio changed when the gas in the tube changed. Something very different than before was going on.

The positive rays turned out to be positive ions that were formed when the electrons that made up the cathode rays struck the atoms of gas in the tube. When this happened, an electrons could be knocked off the atom, giving it a positive charge. These glowing, positively charged ions were then attracted to the negatively charged cathode. When a hole was made in the cathode, their momentum would carry some of the glowing ions past the cathode, forming the positive rays.

The lightest of the positive rays were formed when the gas in the tube was hydrogen. The researcher Ernest Rutherford decided that these particles were the smallest positively charge particles possible and named them protons, successfully computing their mass (to a first approximation).

One very important discovery involving these "positive rays" was an observation made when the CRT contained neon gas. It was found that the beam of positive rays (made from neon atoms stripped of an electron) split into two small beams when placed in an electric field. The only possible explanation for this was that different neon atoms had different masses. This was the first hint at the existence of subatomic particles beyond protons and electrons (i.e. neutrons), though the actual discovery of the neutron would not come for another few decades.

The ability of the CRT to separate particles of different mass and charge led to the development of the mass spectrometer. When complex molecules are hit with a beam of electrons, they are broken into a variety of fragments that have different masses and charges. These can be separated in the same way that the two types of neon atoms were separated. The tool of mass spectrometry allowed scientists to determine the masses of atoms to a much higher degree of precision than had once been possible, as well as other features of their charges.

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