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Atomic Models

Teacher/Instructor Jonathan Osbourne
Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

Atomic model theories have developed over the years. In 450 B.C., Democritus introduced the name atomos, which means "uncuttable" in Greek. Democritus stated that when dividing matter down further and further, we will reach the point where it cannot be divided any further. The current atomic model that we use is called Quantum Mechanics, which was proposed by Erwin Schrodinger and Werner Heisenberg. This model depicts an electron cloud in which electrons are spread out and surrounding the nucleus.

So let's talk about atomic models. The history of our study of the atom. Alright it all begins years and years and years ago, in 450 BC.

Now, there are some things that happened before that, where people were thinking about it but in 450 BC or there about, Democritus who was a Greek scholar, introduced the name, atomos. Which meant in Greek, uncutable. And what the idea was, was that if you try to divide matter down further and further and further, you will eventually get to a point where you can't chop it up anymore, uncutable. And those uncutable things he calls atomos and we still call them atoms today. Alright.

Now in 1661, much much much later than that, Robert Boyle told us that matter is composed of various combinations of corpuscles. So this is known as the corpuscle view of matter and it was used later by Newton to discuss light. And so he had his corpuscular view of light. Alright.

So then in 1803, John Dolton through some experiments, published the law of multiple proportion. And so what this said, was that if you do chemical reactions, you do them in a certain way, then you always get the same ratio. So for example if you start off with a gas, that's uniform carbon dioxide and you try to break off the carbon and the oxygen, you always end up with two oxygens for every one carbon. And that was a very very very important idea because it showed us that there were these things called elements that combined together to form matter. Alright.

A little bit later in 1869, we had a very very very important discovery Dmitri Mendeleev introduced us to the periodic table. The periodic table was an arrangement of all of these kind of basic elements that John Dolton had told us about. But it was an arrangement in which the properties of the elements, could kind of be understood. So you have things like sodium and lithium and potassium that behaved very similarly to each other. So you put them on top of each other. And we ended up fitting these nice periodic structures that you know about today. Alright.

Then in 1888, we had this wonderful discovery by Johannes Rydberg who was able to show that the emission spectrum of hydrogen has wavelengths that follow a very very very simple formula and this will come up later because it's really going to be where the structure of the atom came from, alright.

In 1897, we had another extremely important discovery. JJ Thompson discovers the electron. Now that was huge because before that atoms, I mean, remember that word up here, uncutable, you can't chop them up any further, JJ Thompson showed you could because an electron was a part of all of the atoms. Now electrons are negatively charged, but the atoms themselves have no charge at all. They're neutral. So what JJ Thompson proposed was that atoms consist of a positive jelly like this, it's just kind of all spread out with the electrons just kind of thrown in there, in the mix to make it a neutral atom. This was called the plum pudding model. Maybe you can see why. Alright.

So then we go into the 20th century. In 1909, the experimentalists Geiger and Marsden under the, under the guise of Rutherford were able to show that this plum pudding model that JJ Thompson had brought up cannot be true. So what they did is they took a gold foil and they shot alpha particles at it. These alpha particles are very very very small. Many of them just went straight through and that of course is what you would expect if you didn't have a foil there. But they had a foil there and they just went straight through. But then every once in a while you'd have an alpha who came and bounced right back off. And that's very strange. I mean if it was the plum pudding model, that'd be like taking a handgun, firing it at a pillow and having the bullet come back at you. It's not going to happen. What if it does happen? It means there's a rock in the pillow. And so that's the idea. Geiger, Marsden and Rutherford were able to show that the majority of the mass of the atom was in this really really really small area, about 100,000 times smaller in radius than the atom itself and that that's where all the positive charge resides. And that's called the nucleus. And so their view of the atom was a nucleus in the center and then the electrons doing something. Maybe to know how the electrons made the atoms so big. But they knew that the majority of the mass of the atom was in the nucleus really really really small. Alright.

So then 1913, Neils Bohr comes along. And he takes the results of Geirger, Marsden and Rutherford and he uses them to make the first quantitative model of the atom and this is called the solar system model by some people, it's really called the Bohr model because Bohr's the one who gave it to us. But what it does is it features quantized orbitals. It looks like the solar system. So he has it like this. We've got the nucleus in the middle and then the electron just kind of orbit like planets around the nucleus. Now his big quantitative contribution was in saying that these orbits have to have certain radii. So it could be at a0, which is the standard Bohr radius or it could be at 4 Bohr radii but it can't be at 2. It can't be at one and a half. It's quantized. It must be at this certain radii and that's it. And so that was his contribution because what he did was explain Rydberg's results from like 30 years earlier. So, that was a huge huge breakthrough but it wasn't correct. And it was, it was shown that while it works for hydrogen it didn't work for helium or any of the more complicated atoms.

So then in 1924 comes Louis De Broglie. And what he started talking about was that electrons behave like waves. He said in fact that all matter at some level or another behaves like waves. And that changed the whole idea because you can't have waves going around like a planet. Waves are not concrete sitting there. They're spread out. And that actually paved the way for the next and probably most important well, still the model of the atom. Erwin Schrodinger and Werner Heisenburg came along in 1926 and they independently, independently totally separate came up with the idea of quantum mechanics which took De Broglie's suggestion just head on and said "okay. You want electrons to be waves? They're waves. Let's see how they'll behave in the vicinity of a positive charge." And so what they ended up with was orbitals. So instead of this type of orbit, you have orbitals. You have the electrons spread out in something we call an electron cloud. So the electrons aren't really small. If you leave them alone, they are the size of the atom. The nucleus is real small and then you've got these electrons that are just kind of spread out. They're really really really tiny mass and if you observe them they'll look very very very tiny but if you just kind of leave them alone, they spread out and they make this electron cloud. And that's the current model of the atom and it works for all of the atoms but it's really really really complicated. but we can show that it gives the right experimental answers.

So, till we come up with something else, that's atomic models.