The Atom in History
Most people think that the atom is a very boring thing to study. However, that can’t possibly be true – after all, lots of people have studied it for thousands of years. It must be exciting! Let’s take a look at what we’ll discuss here:
Let’s get started!
The Greeks and their imaginary atom:
Even though the Greeks had very little in the way of high technology, they still felt that they could use the power of their brains to figure out what matter was made up of at the smallest levels. As a result, lots of them talked about it a lot.
Democritus was one of these guys. He came up with a model of the atom that said:
Because Aristotle disagreed with him and everybody thought that Aristotle was a big hotshot, practically nobody paid attention to Democritus. The moral of the story: Don’t mess with Aristotle. Or something like that.
Some ideas that nobody had ever thought of before:
For a really long time, nobody really thought that Aristotle was wrong. Eventually, however, with the advance of science, people started to rethink their devotion to the dead Greek guy. Here are some of the discoveries that changed this:
In the 1800’s, some English guy named John Dalton came up with his own idea of what atoms were like. His theory included the following ideas:
people seemed pretty happy with
Thomsen and his cathode ray tube:
Since every chemistry textbook in the world shows a picture of the cathode ray tube experiment, I’m not going to reproduce it – I suggest you turn to it, though, since it might help with my explanation.
Anyway, one day Thomsen was goofing around the lab with these cathode ray tubes he found somewhere. What he found was that when he connected these big long hollow tubes to batteries, a beam of light would go from one end to another. Since he had a lot of time on his hands, he decided to figure out what the deal was with the light. After all, if there was nothing in the tube to start with, where’d the light come from? He figured, it must come from the electrodes – since the electrodes were made of atoms, the atoms must somehow be coming apart.
Among other things, Thomsen got a magnet and held it near the beam. When he did this, he found that the beam would bend toward the positive side of the magnet and away from the negative side. From this, he figured that the beam must contain very small particles from the atom and that they must have negative charge.
This led directly to his “plum pudding” model of the atom, named after a dessert that nobody can eat without throwing up. Think of a chocolate chip cookie, instead. His idea was that the dough in the chocolate chip cookie made up most of the atom and that it had positive charge. The chips represented the little tiny bits of negative charge that made up the light he was messing around with – unlike the dough, they could leave the atom if you gave them a shove (with a battery, for example).
For this discovery, Thomsen is forever known. This, despite the fact that his model was almost instantly disproved.
When he did this (again, you can find MUCH better pictures of this in your textbook than I can make), he found that most of the particles went right through the foil, while some of them either passed through or bounced off at irregular angles. Why is this?
idea was to come up with a model of the atom in which most of the atom is empty
space with electrons floating around in it.
The protons, however, are all concentrated in the middle of the atom
(called the nucleus) – according to this model, the positively charged alpha particles
would go straight through the atom most of the time and only be deflected on
the rare occasions when they passed very close to the tiny nucleus. For this discovery, we will always know
Random interlude: Chadwick and the discovery of the neutron
In 1932, James Chadwick discovered the neutron (which has no charge at all) by doing some really complicated experiments that I don’t understand even a little bit. Don’t worry, though – your teacher probably doesn’t understand it either (unless they’re a nuclear scientist or something), so if you just remember that Chadwick discovered it, you’re probably fine.
Neils Bohr and the planetary model:
tends to be the case with models of the atom, nobody really bought into
His idea was that electrons traveled only in certain circular paths around the nucleus, much as the planets circle the sun. When energy is added to the electrons, the electrons jump from their normal orbit (called the “ground state” orbital) to a higher energy orbital farther from the nucleus (called the “excited state” orbital). Since the world likes to exist at low energy more than at high energy, the electrons eventually return to their ground state orbitals. When this happens, the energy that they absorbed is given off as light. Since the color of light is very closely related to its energy, you only see very particular colors of light being given off by the very particular energy differences between the ground state and the excited state.
His idea further went on to say that the energies of the orbitals were different for every atom. As a result, the colors of light given off by every element is unique, which allows us to identify them via the magic of spectroscopy (specifically, this phenomenon is called atomic emission spectroscopy).
Like all models of the atom, this was overturned in about 15 minutes by a bunch of guys who invented something called quantum mechanics. However, don’t feel sorry for the planetary model of the atom – it still has a healthy and thriving life in elementary school textbooks (which for some reason refuse to acknowledge the existence of quantum mechanics).
© 2007 Ian Guch – All Rights Reserved