READ: Thomson's Plum Pudding Model

READ: Thomson's Plum Pudding Model

Scientific models – (simple representation of a more complex system) are best when demonstrating a single attribute of the desired concept. They can represent things far too large or too small to normally demonstrate. Many models have limitations because many of them cannot be made to scale. If you were to build a model of an atom with a proton the size of a gumball, the closest electron would be about a mile away. Another limitation is that a model may not behave like the real object.

Changing Models of the Atom

In 1897, a scientist named J. J. Thomson conducted some research that suggested that Dalton’s Atomic Theory wasn’t the entire story. As it turns out, Dalton had a lot right, but he was wrong in saying atoms were indivisible or indestructible. As it turns out, atoms are divisible. In fact, atoms are composed of even smaller, more fundamental particles. These particles, called subatomic particles, are particles that are smaller than the atom. 

Thomson’s Plum Pudding Model

In the mid-1800s, scientists found that by forcing electricity through a tube filled with gas, the electricity made the gas glow. Scientists didn’t, however, understand the relationship between chemicals and electricity until a British physicist named J. J. Thomson began experimenting with what is known as a cathode ray tube.

The figure shows a basic diagram of a cathode ray tube like the one J. J. Thomson would have used. A cathode ray tube is a small glass tube with a cathode (a negatively charged metal plate) and an anode (a positively charged metal plate) at opposite ends. By separating the cathode and anode by a short distance, the cathode ray tube can generate what are known as cathode rays – rays of electricity that flow from the cathode to the anode. J. J. Thomson wanted to know what cathode rays were, where cathode rays came from, and whether cathode rays had any mass or charge. 

First, by cutting a small hole in the anode, J. J. Thomson found that he could get some of the cathode rays to flow through the hole in the anode and into the other end of the glass cathode ray tube. Next, Thomson figured out that if he painted a substance known as “phosphor” onto the far end of the cathode ray tube, he could see exactly where the cathode rays hit because the cathode rays made the phosphor glow.

J. J. Thomson must have suspected that cathode rays were charged, because his next step was to place a positively charged metal plate on one side of the cathode ray tube and a negatively charged metal plate on the other side of the cathode ray tube, as shown in Figure 3. The metal plates didn’t actually touch the cathode ray tube, but they were close enough that a remarkable thing happened! The flow of the cathode rays passing through the hole in the anode was bent upwards towards the positive metal plate and away from the negative metal plate. 

Using the “opposite charges attract, like charges repel” rule, J. J. Thomson’s experiment with cathode rays found that the ray moved away from negatively charged plates and toward positively charges plates. What does this say about the charge of the ray? Thomson argued that if the cathode rays were attracted to the positively charged metal plate and repelled from the negatively charged metal plate, they themselves must have a negative charge!

J. J. Thomson then did some rather complex experiments with magnets, and used his results to prove that cathode rays were not only negatively charged, but also had mass. Remember that anything with mass is part of what we call matter. In other words, these cathode rays must be the result of negatively charged “matter” flowing from the cathode to the anode. Thomson suggested that the small, negatively charged particles making up the cathode ray were actually pieces of atoms. He called these pieces “corpuscles,” although today we know them as electrons. Thanks to his clever experiments and careful reasoning, J. J. Thomson is credited with the discovery of the electron.

Now imagine what would happen if atoms were made entirely of electrons. Electrons are negatively charged. Therefore, if atoms were made entirely out of electrons, atoms would be negatively charged themselves… and that would mean all matter was negatively charged as well. Of course, matter isn’t negatively charged. In fact, most matter is what we call neutral – it has no charge at all. If matter is composed of atoms, and atoms are composed of negative electrons, how can matter be neutral? The only possible explanation is that atoms consist of more than just electrons. Atoms must also contain some type of positively charged material that balances the negative charge on the electrons. Negative and positive charges of equal size cancel each other out, just like negative and positive numbers of equal size.

Based on the fact that atoms are neutral, and based on J. J. Thomson’s discovery that atoms contain negative subatomic particles called “electrons,” scientists assumed that atoms must also contain a positive substance. It turned out that this positive substance was another kind of subatomic particle, known as the proton –(a positively charged subatomic particle). Although scientists knew that atoms had to contain positive material, protons weren’t actually discovered, or understood, until quite a bit later.

When Thomson discovered the negative electron, he realized that atoms had to contain positive material as well – otherwise they wouldn’t be neutral overall. As a result, Thomson formulated what’s known as the “plum pudding” model for the atom. According to the “plum pudding” model, the negative electrons were like pieces of fruit and the positive material was like the batter or the pudding. Thomson theorized that the positive material in the atom must form something like the “batter” in a plum pudding, while the negative electrons must be scattered through this “batter.” (If you’ve never seen or tasted a plum pudding, you can think of a chocolate chip cookie instead. In that case, the positive material in the atom would be the “dough” in the chocolate chip cookie, while the negative electrons would be scattered through the dough like chocolate chips.)

Lesson Summary

• Models have limitations, and better models are created as more experimental evidence is found 
  
• Dalton’s Atomic Theory wasn’t entirely correct. It turns out that atoms can be divided into smaller subatomic particles.
  
• According to Thomson’s “plum pudding” model, the negatively charged electrons in an atom are like the pieces of fruit in a plum pudding, while the positively charged material is like the batter.
  

CC-BY-NC-SA Utah State Office of Education. Material adapted from ck12.org

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