Thomson and Subatomic Discoveries
Let's have a quick recap- atomic theory began in ancient times, when smart people hypothesized that matter is composed of small particles. Since they had little proof and since nobody really cared at the time, atomic theory went to sleep until the 1800s.
Then, John Dalton came up and used some new laws and hit own experiments to come up with his Atomic Theory, which was based on the idea that every element is made up of a unique type of small particles called atoms, and atom can neither be created, destroyed, nor subdivided. Atoms of different elements differ by mass and can combine to form compounds.
And everyone was happy. They said, "yay, we have an atomic theory to explain what matter is made up of! Let's have a party". And they did. But then, those scientists came back to their scientific ways and made some discoveries that would alter the atomic world forever…
Cathode rays
In 1605, a German dude named Otto Von Guericke invented vacuum pumps. Once scientists had their vacuum pumps, whey began to play around with them. People like Michael Faraday, William Crookes and Heinrich Geissler developed and improved an apparatus called a cathode ray tube, which would prove essential to atomic theory developments.
This is a nice picture from Wikipedia of a cathode tube. Here's how it works: you connect a low voltage power supply to one end of the tube- the negatively charged cathode. Then a high voltage power supply is connected to the other end, a positively charged anode. You can put a cool shape in the middle of the tube, but you've got to make sure there's vacuum in the tube.
Here's what people found- the anode side of the tube glowed. Fruthermore, if there was a shape inside, it would cast a shadow. They hypothesized that the cathode had rays (cathode rays) which moved from the cathode to the anode. Cool!
Time
for Thomson to step in
Cathode rays are cool, but nobody really understood what was going on in them (they had their theories, but most were wrong). Until one day a guy named J.J. Thomson stepped up. J.J. conducted 3 important experiments that allowed him to make a shocking discovery:
- J.J. used a magnetic field with a slit and showed that magnetism could bend the cathode rays, whatever those were.
- J.J. then used an electric field to bend the rays. He realized the rays were really negatively charged particles.
- J.J. measured the deflection to find the charge-to-mass ratio of these particles. He then compared the ratio to a Hydrogen ion, and found it to be significantly higher. He also used different materials for the cathodes, and found out that the charge-to-mass ratio was unaffected.
From this, J.J. Thomson concluded that these particles are either very light or very highly charged. He also deduced that the particles are coming from the atoms of the cathode, and are found in atoms of all elements. He called them corpuscles. His discovery was published in 1897, and scored him a Noble prize in 1906 (sweet!).
Corpuscles, which we now call electrons, were a shattering discovery to Dalton's model of the atom- principally, it proved that atoms can be subdivided. Thomson used that idea to come up with his own atomic theory.
The
chocolate-chip atomic model
It was 1904, and our good friend J.J. took the time to publish a new model of the atom. Instead of a uniform sphere, he now described the atom as a collection of electrons surrounded by a positively charged cloud. The electrons were free to rotate inside this cloud in rings or different structures. This concept made atoms look kind of like chocolate-chip cookies.
But this model wouldn't last long either. It was Thomson's own student who would see the cookie crumble…
To be continued in Part 4
