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Development of the atomic theory part 1
- 1. Development of the Atomic Theory Part 1 A story of something you cannot even see.
- 2. Let’s Think About It.
- 3. Democritus 440 B.C. Solid Homogeneous Indestructible Atomos!
- 4. John B. Dalton Late 1700’s All matter is composed of atomsAtoms cannot be made or destroyedAll atoms of the same element are identicalDifferent elements have different types of atomsChemical reactions occur when atoms are rearrangedCompounds are formed from atoms of the constituent elements.
- 5. JJ Thomson 1897 The atom is no longer indestructible! Found the electron! (He called them corpuscles.) Electrons have a negative charge. To keep the atom neutral, the electron were thought to float in some kind of positive jelly. This was known as the Plum-pudding model.
- 6. Resources Gregory L. Curran. Development of the Atomic Model, Lesson 3-2.© Copyright 2004 Fordham Preparatory School, All Rights Reserved.Last Modified February 07, 2008. http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson32.htm Introduction of Matter, Book K. Holt, Rinehart, and Winston, Austin TX, 2002.
- 7. Websites of screencaptures http://v1kram.posterous.com/great-indians http://library.thinkquest.org/28383/nowe_teksty/htmla/2_5a.html http://www.tutorvista.com/content/physics/physics-iv/atoms-and-nuclei/thomsons-model.php http://www.kentchemistry.com/links/AtomicStructure/JJThompson.htm
Notes de l'éditeur
- The following is an adaptation of a science history story as told by Gregory L. Curran from the Fordham Preparatory school and myself.
- To borrow an example from Albert Einstein, imagine if you had never seen a clock or a watch before, and someone gave you an intricate Swiss timepiece. Imagine studying the motion of the hands, but never being allowed to remove the watch face and see the mechanisms which produced the sychronized movements. If you thought about it long enough, you might be able to come up with a model to explain the motion of the hands, but you could never be sure that your model was an accurate depiction of what was going behind the face of the watch. In fact, if someone was to come along with a better explanation for the motion of the hands, you would be forced to update your model. Our atomic model has much in common with the imaginary watch from the above example. We can't base our model on actual observations of atoms, because they are too small to be seen with our most sensitive instruments. Instead, we must come up with a model of an atom that can account for and explain observations that we can actually see. As new observations are made, we are forced to update our model to accommodate them. As a result, our model of the atom has evolved over time, and we must accept the fact that it is likely to change again in the future.
- A very, very long time ago, about 2500 years ago, a man sat and thought about everything. He was known as the philosopher, Democritus. What exactly he thought that day when he thought about matter, I’m not quite sure. But perhaps it went something like this: As Democritus wrote his thoughts down, he stared at his paper and wondered of what it was made. Some people had thought all known stuff was made of earth, water, wind, and fire. He was not quite so sure that this was the case. He looked at his paper and began to tear it in half. Hmmm, it was still paper. He then tore it in half again…still paper. Over and over he tore it until he had but only a strand. Then he tried to tear that. He thought some more: “If I could somehow keep tearing this matter in half would I ever reach the end?” He felt that he could. There had to be a point where one would get to the smallest particle that made up that matter. It would have to be very, very small and indestructible. His Greek word for this was Atomos. Which we now say Atom! Other than extremely small and hard, he also claimed that atoms were made of a single material formed into different shapes and sizes. These atoms would also have to be constantly moving. Different atoms would join together to form different matter.We will find out that this man, who existed long before scientific equipment even came near looking at things near the atomic level, was nearly hitting the nail on the head.
- John Dalton, a chemist and school teacher, was our first noted scientist that performed experiments with different substances. He found that elements would combine in specific proportions, like they had to link up a certain way. It reminded one of different Lego blocks that could connect in certain combinations. Atoms could still not be seen, even with the best of microscopes of the day. Daltons thoughts about what these atoms looked like seemed very similar to Democritus' idea. Dalton theorized based mostly on his experimentation, that all substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. (Guess they didn’t have nuclear power back then.) Now his theory did get very close to what we know today when he stated the rest of his theory which goes like this: Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms to make new substances.
- The Atom has been cracked open like an egg! Well not quite, JJ Thomson found, through his experiments, that the atom was not the smallest particle. Ah…you may be thinking that he found something inside because he had a good microscope. Nope! A hundred years after Dalton… and scientist still could not see the atom, let alone something smaller. But how did he know, you may ask. Well, he experimented with a cathode ray tube. What’s that, you say? Do you remember the big TV’s that were very bulky and the back of the TV stuck out towards the back? Perhaps you still have one we do for …it still works. That picture tube is a cathode ray tube and it works the same way JJ Thomson’s experiment did. On the front of his tube he had a silver screen that would glow when a charge hit it just like yours. Way in the back was a source of energy. The back of the cathode ray tube sort of looks like the bottom of a light bulb whose base fell off with thin wires sticking out. This is where the electrical charge enters. After it enters it flies straight to the silver screen, due to the tube having all the air vacuumed out. Now, inside this tube, just before the middle, were two conductive plates. One could be charged negatively and the other positively. When Thomson allowed these plates to be charged, the electrically charged beam bent towards the positive plate. He knew this because the screen glowed towards the bottom, on the same side as the positive plate. That meant that the charge that was flowing in was a negative one! After several repeated experiments he concluded that there existed. It also worked with magnetic plates of opposite charges. He decide to give these negative particles a name, corpuscles. Later on a Greek name, electron, was used instead. Thomson then theorized that every type of atom had corpuscles, that is, electrons. Now the model of the atom had to change. No longer was it indestructible. He thought. If there are negative particles inside and most matter, which is made of atoms, has no charge then there must be something else inside to balance out the charges. He figured that these negative corpuscles, electrons, floated around in some kind of positive jelly. To help people understand he drew similarities to a popular dessert, plum-pudding. Most people today do not know what that is. So I will give you another dessert to compare it to: a nice round scoop of chocolate chip ice cream with the chips representing the corpuscles, electrons, and the vanilla ice cream can represent the positive jelly. Ok! On that note it is time for a snack! And this concludes part 1 of the development of the atomic theory. Next time we are going to explore Rutherford’s kleenex that bounces back bullets (well he compared his findings to that) and some quantum jumping that will test your sense of reality.