So in order for us to start with our understanding of the atom and matter in general we have to go back and I mean way back thousands of years to when scientists looked like this and really their popularity and their success I think was based on who had the biggest beard and the whitest toga and this guy had the biggest beard and the whitest toga of them all at least when it came to matter in the atom because he was the one, Democritus, that coined the term Atomos. And what he theorized, because really science was just based on theory at that point, what he theorized was that all matter was made up of these tiny, indivisible particles, and that these particles were all different, and they retained the properties of the substance that they were meant to describe or be a part of. So things like oil would be really slippery spheres that could slide over one another. Prior to this, things were thought to be made up of the four elements and just varying compositions of those.
So, if we take the term atomos and the idea that we can have these indivisible particles, and we move ahead several thousand years to a fellow by the name of John Dalton, we start to get a better understanding, or a more current understanding, of our model of the atom. And what he came up with was very similar to what Democritus proposed, which was that atoms are tiny indivisible particles, but he did so based on science. observation, experimentation, and what he found was that atoms, when they combine, did so in definite proportions, so that when things combine they always did so in the same ratio. And he hypothesized that these tiny indivisible spheres, which he envisioned look a little bit like this, were the same for all substances that were the same and were different for different substances.
So, for example, he theorized that gold atoms would all be the same as other gold atoms, but they'd be different than copper atoms, but all of the copper atoms would be the same as other copper atoms. So he came up with several tenets of his atomic theory that held for many numbers of years. Now as we continue to fast forward, we can see that Dalton was just the beginning of evidence-based science. That is, more and more scientists were starting to evolve their theories based on experimentation and not just observation. And what he came up with sort of cracked open Dalton's idea of what an atom was.
That is, maybe atoms aren't indivisible. Maybe they are made up of smaller particles. And his experimentation with cathode rays and their response to magnetic field helped him to theorize that perhaps there are these tiny negatively charged particles within the atom.
and that these tiny negatively charged particles, which further became known as electrons, were interspersed amongst a positive matrix that allowed the atom itself to remain neutral, but also allowed the atom to respond to a field illustrating that indeed there is a negative charge within the atom. This became known as the Raisin-Bunn model, and if we take a look at this depiction of the Raisin-Bunn model, we can see that these negative charges appear to be fixed, although Thompson, to his own admission, thought that they started... spin around or move around this particular version of his atom.
It's important to note that while Thompson theorized that there must be some positively charged particle in the atom to offset the negative charges, he never conclusively proved that they were there. That took an individual by the name of Ernest Rutherford and his famous Gold Foil Experiment or Geiger-Marsden Experiment. You might recognize the Geiger name from Geiger Counter fame.
But what they discovered was that these atoms do in fact contain positively charged particles. Now, how did they do that? Well, like many things in science, the discovery of these positively charged particles within the atom came about pretty much by accident.
Rutherford had directed Geiger and Marsden to perform a series of experiments that would confirm some of his ideas about how radioactive matter behaved. And what he had them set up was what became known as the gold foil experiment. That is, he had a piece of radioactive material that was emitting these positive alpha particles.
that should be transmitted through a piece of thin gold foil. Hence the name. Now what Rutherford thought was that these alpha particles would all pass completely through the gold foil. Remember, atoms at the time were thought to be neutral, that they had the negatively charged particles. within that positive, gooey matrix of the Raisenbaum model.
But what was found was that some of these alpha particles, these positively charged particles, were being deflected as they moved through the gold foil, and that some of them, in fact, were being reflected back. Now, what the evidence led to was Rutherford's hypothesis that atoms are made up of mostly empty space. And that there's a tiny, intensely positive nucleus or core that contained these positively charged particles. And so Rutherford's model looked a little bit more like this. Now the atom is starting to look a little bit more like we tend to think.
of the atom now isn't it? But we're not quite there yet. If we believe that these positively charged particles are within the nucleus and that we have these negatively charged particles swirling around the nucleus it would lead to the idea that these these negatively charged particles would be attracted to the positively charged particles and go spiraling into the nucleus. Now since we're all here that clearly doesn't happen. So how is it that the electrons don't go spiraling into the nucleus and destroy all matter in life as we know it?
Well we can thank Niels Bohr for getting that ball rolling. Now Bohr's idea was that electrons could only occupy fixed paths or orbitals around the nucleus. That is they could only be in one place or another and never in between. And he referred to these energies as being quantized or having specific amounts of energy.
Now you can vision it kind of like a set of stairs. This ball representing an electron can only be on one stair or another, never in between. And in that way the electrons could maintain their momentum and not go spiraling into the nucleus if they have to be and can only be found in these specific and set energy levels. And in fact there was evidence for Bohr's model although it really only applied to one electron systems like a hydrogen atom. Now the last piece of the puzzle is what's located in the nucleus.
You see if we step back for a second we can say well if the nucleus does contain these positively charged protons, why don't they repel each other? After all, like charges repel. Now, it wasn't until the 1930s that an individual by the name of James Chadwick ultimately came up with evidence that proved that there are these neutralizing particles, or neutrons, within the atom's nucleus itself. So it has taken literally hundreds, if not thousands of years to take our understanding of the most fundamental unit of matter, the atom, from this to this.
And it would be naive to think that our understanding over the next hundred years or thousand years isn't going to change and evolve as to how we understand the structure. In fact it should be noted this really isn't the most modern interpretation of the atom that we have out there. But in terms of an introductory video I think this definition will suffice.
Thanks for watching.