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In the fifth century B.C., a guy named Leucippus (loo-KIP-us) suggested that all matter -- everything -- was made up of a few simple building blocks. He managed to convince a number of people. One of them was a guy named Democritus (duh-MOCK-ri-tus).
Democritus took the theory even further. He decided that everything was made up of teeny tiny particles. All of these particles were made of the same stuff, but they came in different sizes and shapes. The size and shape of the particles determined what the material was like. For example, water had particles of a certain size and shape, but wood particles had a different size and shape. Democritus called these particles "atoms."
This theory is frighteningly close to the truth (as we know it now).
The one slip-up Democritus made was in the name. "Atom" means "uncuttable." Democritus thought that atoms couldn't be split into smaller parts. He was wrong. But everything else was very close to being right -- which is very impressive for a guy who didn't have any electron microscopes or cyclotrons or. . .well, any anything.
Less than a hundred years later, Aristotle came up with a new theory. He didn't believe in atoms. And everyone knew that Aristotle was a very smart guy, so they believed him. Too bad he was wrong.
Fast forward about 2,000 years. People are starting to think about atoms again. A Croatian scientist (well, he was from there, even though it wasn't called Croatia yet) named Rudjer Boscovich suggests that Democritus was right, except for one thing. He says that atoms can be split into smaller parts. Yay!
In 1803, a British scientist named John Dalton published his atomic theory. He said that everything is made up of atoms, and that different elements have different kinds of atoms. What makes elements different is the size, shape, and mass of their atoms. (Sound familiar?)
So far, so good. But what are atoms like? How are they structured?
Dalton thought that atoms were solid spheres -- like a bunch of marbles all stacked together. The first person to come up with a new theory was a British physicist named J. J. Thomson. He discovered the electron, and figured out that it was part of an atom. Electrons (as you may know) have an electric charge. We say that it's a negative charge. So Thomson decided that an atom was a positively-charged ball with lots of negatively-charged electrons stuck into it.
Rutherford wasn't so sure. He'd been working with alpha particles, which have a positive charge. (We know now that alpha particles are two protons and two neutrons stuck together -- in other words, a helium atom with the two electrons removed.) Rutherford set up a piece of gold foil and shot alpha particles into it.
I want to pause here a minute and talk about gold foil. It's really cool. I've used it in a physics lab. Don't think of the aluminum foil in your kitchen. Gold foil is very, very thin. You have to handle it with tweezers. If you pick it up with your fingers, you'll never get it off your finger tip. It's just too thin. And it floats like a feather -- no, lighter than a feather. See, a sheet of gold foil is only a few atoms thick. Seriously. And that's why scientists like to use it.
We should also talk about alpha particles. They're very, very small. If you tossed a grain of sand at a sheet of gold foil, it'd bounce off. So would a dust speck. But alpha particles are much, much smaller. They're part of an atom, remember? So they're smaller than an atom is. That's important.
Anyway, Rutherford shot alpha particles into gold foil to see how they bounced off. Well, most of them didn't. Most of them went right through the foil as if it weren't there. This told Rutherford that most of a gold atom (or any other atom; he experimented with other metal foils, too) is empty space. Some of the alpha particles did bounce off though. So Rutherford knew that in the middle of each atom was a small, positively-charged nucleus. The electrons, he figured, were grouped around it.
Rutherford was basically correct. His theory has only been fine-tuned twice. Just two years after he published his model of the atom, a Danish physicist named Niels Bohr (neels bore) fixed it up a little. He said that electrons orbited the nucleus along certain paths. This is pretty much the model that your parents, grandparents, or I might have been taught in high school.
Which is sad, because it was already being replaced. Experiments in the 1930s and 40s (no, I'm not that old) led to a new model. This one involves what's called an "electron cloud." The idea is that we can't ever be sure where the electrons are. They're somewhere in the cloud. The darker parts of the cloud are places where an electron is more likely to be. The lighter parts are places where one is less likely to be. But it might be there just the same.
Now back to Rutherford. You know how the British government can honor a person by making him a knight (or a lady, for a woman)? Well, Rutherford's work was so important, that he was given an even greater honor. (He was British, because New Zealand was part of Britain back then.) He was made a baron. Since he was born in the town of Nelson, he became Baron Rutherford of Nelson. And so we're supposed to refer to him as Lord Rutherford. Pretty cool, huh?
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