Let’s start by going on a journey through time. And possibly space, because what I’m going to talk about took place in a particular locale. Anyway, we’re in the year 1909. Atoms are pretty much the bomb, but no one really knows that much about them. They know that there are positive and negative charges in them, because Ben Franklin said so (simplification), but the prevailing theory at the time was what was called the “plum-pudding” model: the charges were distributes more or less evenly throughout the structure. If you’ve made it through some science today, you know that they were dead wrong, and it’s ok to laugh at them, because it’s fun to laugh at people when they’re wrong. But, since no one had eyes good enough to see how atoms were composed, they were just basing it on what amounted to a wild-ass guess.
Enter Hans Geiger and Ernest Marsden. In Ernest Rutherford’s laboratory (Ernest was just a great name to have then. There was tremendous importance placed upon being Ernest) they used a radioactive isotope to shoot alpha particles (which are just Helium nuclei—so only positive charges, not electrons) at a really, really, really thin sheet of gold foil. Why gold, you ask? Well, no one had come up with a way of making aluminium cheap and malleable like we have today. That wouldn’t come about for a long time, and they used tin before that, which was rather difficult to work with (but the name “tin foil” has obviously stuck). Either way, the gold foil was a good idea because gold is pretty damn dense. And, what Geiger and Marsden noticed was that not all the alpha particles were making it through the gold.
This, of course, surprised absolutely no one, because if you shoot particles at something, you don’t expect them all to make it through. If you throw sand at a screen door, some of it will be stopped, even though all of it could technically make it through the spaces. This is the same idea. However, what Geiger and Marsden also noticed that garnered much more attention is that they saw alpha particles at all sorts of angular offsets to the foil—even alpha particles coming nearly right back toward the source.
It should be noted that in 1909 there were no computers. While this seems obvious, it also means that there was no computerized processing of data. When I did this experiment myself in a lab, I could just run it and walk away while the computer collected the data for me. Geiger and Marsden had to sit and stare at a Zinc-Sulfide screen, and watch for the tiny blips when an alpha particle collided with it. Physics was so exciting back in the day.
Anyway, this was crazy. Rutherford quickly realized that there was no way the particles could be deflected as much as they were without some force acting on them. The only force that could be the culprit was the coulomb force—the force that repels two like charged bodies, such as the positively-charged alpha particles, and the positively-charged protons in the atom. But, the only way this could make sense is if all the protons were clustered together, and not distributed like the Thompson model of the atom (which I previously referred to as the “plum-pudding” model, because that’s a more fun way to say it).
This revelation lead to a revolution in atomic physics, which coalesced into the Bohr model of the atom (also technically wrong, but it’s the one you learn in school because real atoms are kind of crazy and weird), and with that the realization that the atom has two distinct parts: the nucleus (dense, heavy, and positively charged), the the electron cloud (99% of the volume of the atom, but almost no mass. Just a few tiny electrons whizzing about, doing their thing, and occasionally acting really weird and confusing quantum physicists). Why is this important? Because pretty much all modern technology wouldn’t exist without this understanding of the atom. Also, because it’s awesome.