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Unit 3 Narrative – Atomic Theory Introduction Modern atomic theory is among the most successful theories in science. The knowledge gained while studying the atom has allowed us to develop incredible technology in the last 100 years when compared to the previous 2000. However, did you know the idea of the atom is not modern? Utilize the following video: https://www.youtube.com/watch?v=thnDxFdkzZs. The credit belongs to the ancient Greeks, often one of (1)____________ or (2)___________ who thought (they performed no scientific tests to prove their idea) that matter was not infinitely divisible. In fact, the word “atom” derives from the Greek work (3)_________ which means (4)____________. The modern idea of what composes an atom is a couple of millennia away but it had to start somewhere. What they did suggest, though, was that the atoms of water were distinctly different than the atoms of iron. So the idea that the atoms themselves are responsible, or at least have an influence, on the behavior of a substance is a very old idea. Essentially, this is what an atom would look like according to the ancient Greeks. There is not much to it because it must be, at the very least, indivisible. In historical terms, this version of the atom did not last very long because someone came next and ruined everything (strictly speaking ruined everything from an atomic theory sense because it would take roughly 2300 years for the idea to return to the mainstream). The man responsible is none other than Aristotle. He had his own ideas as to what constituted matter. Without getting into his ideas, partly because they are quite confusing and also scientifically inaccurate, it is suffice to say that his popularity and fame allowed his ideas to spread further than Democritus and Leuccipus. The atom disappeared in two senses: one is that Aristotle’s ideas did not include the existence of them and two is that it took forever for someone to challenge Aristotle’s. Essentially all that is left of atomic theory is a blank spot. Like that one on the left…that took over 23 centuries to fill! So who came to the rescue of atomic theory? None other than the Englishman (5)________ ___________. He compiled the ideas put forth by his contemporaries into one theory and it is because of this that he is known as the (6)___________ of Atomic Theory. Essentially all of atomic theory is back to where it stood before Aristotle. The atom is back but there is not a whole lot to its structure but there is more known about its behavior, at least in the sense that they were reacting and combining in whole-number ratios. It seems Democritus and Leuccipus were correct about the indivisible nature of atoms but we know this is not quite accurate. But who started to figure out the smaller parts of the atom? Another Englishman, this time a physicist, was working with (7)__________ ____ tubes. His name was (8)__. __. ___________ and he was able to determine the (9)______ of the cathode rays. Now chemists had developed rough estimates for the mass of known elements and the smallest, hydrogen, was 1. Turns out that Thomson’s calculations determined the mass of the cathode ray was roughly 1000x (10)___________ than hydrogen! He called these small particles of matter (11)___________ which today we call the (12)____________. His discovery means the model of the atom must change.
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The model he developed was called the (13)_____ _________ model but it is probably easier to envision a chocolate chip cookie. The negatively charged electrons float around within a mass of positive charge. It looked a bit like the picture on the right. This model clearly is missing common features of atomic models you are used to seeing and the discoveries of those features would come quickly. It would take one of the most famous experiments in atomic science to dismantle Thomson’s plum pudding model. Conducted by (14)________ __________ it was called the (15)_______ _______ _____________. If Thomson’s model was correct, Rutherford expected the alpha particles that were aimed at the gold foil would simply (16)_______ through unaffected; however, it turned out that some of the particles were deflected at various angles and occasionally straight (17)___________ towards the alpha particle source. His conclusion was that the major of the (18)__________ charge was located in the very center of the atom which he called the (19)__________. Since so many particles passed through the gold foil, this means that the majority of the atom is (20)_________ space. Further testing would discover the particle responsible for the positive charge in the atom which we call the (21)__________. Rutherford’s model shows the nucleus which we are accustomed to seeing in diagrams. The next step was trying to figure out what the electrons were doing. Did they simply move around the nucleus in a random fashion or was there some sort of order present? This presented a more complicated question and the first to tackle it with some success was (22)_________ _______. He analyzed the results of Rutherford’s experiments and utilized new mathematical techniques developed by other scientists at the time to predict the (23)__________ of the electrons. His model looks like the (24)_______ system and is sometimes referred to that type of model or planetary model. This representation of the atom is probably the one most familiar because of its simplicity…but unfortunately that just is not what the atom looks like as the electron behavior is not described properly. Describing it properly was not easy and explaining it is not easy as well. It begins with Werner Heisenberg who stated that it is impossible to know both the position of a particle (electron in this case) and where it is headed [this is incredibly simplified]. You may have heard of the Uncertainty Principle or Heisenberg before but the application of the principle (and the work of other scientists like Erwin Schrodinger – of the famous cat problem) means we have to discard Bohr’s model for another model or theory altogether. The replacement is named (25)___________ theory. The first word pretty much means “minimal or very small” but it is an incredibly successful theory. In essence, the idea that we can know exactly where an electron is has been replaced with the idea that the electron is (26)____________ where we say it is. The region where the probability of finding an electron is very high is known as an (27)_________. There are four categories of them known by the letters (28)__, __, __, and __. Instead of referring to this model as the “quantum” model it is commonly called the (29) [electron] ________ model (maybe because drawing all those orbitals would be painful).
