Atomic Theory....really??

braithiar · Apr 3, 2010

What part of atomic theory is giving you trouble?

Anne36, LPN · Apr 4, 2010

LOL. What part? Is there an easy part and a hard part? I already took Chem and I dont even know what it is.

tokebi · Apr 4, 2010

I don't know of good websites but I could give it a shot at a summary on the topic, because I do remember the introductory chem texts really suck at this, leaving many students feeling like, "Wha...? What was that all about?" Seriously, the history of the atomic theory becomes really fascinating in more advanced texts.

While this post may not be what you need, oh well, it's good for me to refresh my memory, too. :)

First, forget about the ubiquitous image of atom where there's the proton and the electrons orbiting around it. Not only is it inaccurate but also hampers the deep appreciation that one feels when learning about the gradual and painstaking process of discovery.

So, people used to believe that matter was continuous, but then various experiments with gasses (think Dalton, Avogadro, etc) led to the idea of molecules and atoms making up the macromolecular world we can see. At this point, people believed that the atom was the smallest, indivisible particle.

Then comes the famous JJ Thompson's cathode ray tube. At the time people were puzzled by the ray of light in low-pressure gas tubes when electric currents were applied to the tube. What Thompson did was manipulating the cathode ray with electric and magnetic fields, which bent the ray this way and that. By using some clever mathematics, he came up with the "charge-to-mass ratio" or e/m. What's the significance? This mysterious, negatively charged (since it runs from cathode to anode) particles turned out to have the same quantity, e/m, no matter what kind of metal Thompson used to create the ray. Furthermore, the mass of the particle was a lot less, extremely less than the smallest known atom, hydrogen.

So here's what we have so far: 1. the newly discovered subatomic (gasp!) particle -- electron. Not only is it subatomic particle, its mass is very very small, almost negligible compared to an atom. 2. An atom is neutral -- it has no electric charge. A tiny, negatively-charged electron can be separated from the atom, and so, there must be a large, positively-charged particle making up the rest. In other word, proton.

From this information, people came up with this atomic model: huge proton with tiny specks of electrons embedded in it -- natural assumption if you associate large mass with large size. They called it "plum pudding model." While every textbook never fails to mention this nickname, it always confused me. Does anyone know what the heck plum pudding is? I'd rather call it "raisin muffin model."

But then came the famous Rutherford's scattering experiment. I don't know how your book describes it, but the gist of it is this: the only way you could see the type of deflection that Rutherford saw, is only if there's a very concentrated mass in a small region of the atom, very different picture from the popular notion of plum pudding model at the time. If you have difficult time visualizing the experiment, consider this: Imagine shooting many rounds of BB gun through a semi-hard sponge. This would be what you'd expect if plum pudding model were true, where the large mass of proton is distributed evenly in the atom. Now imagine shooting BB gun again through a very soft sponge with a small but hard pebble in the center. Most of your rounds will tear right through the sponge. But occasionally, you might manage to hit the rock in the center and your bullet will ricochet off. That's the deflection that surprised Rutherford and led him to re-formulate the atomic model.

Uh oh. I didn't expect this to be this long, and I didn't even get to Bohr model... Is this just redundant information? Do you have to do calculations? Did you want info on a specific part? Let know. If this is useful at all, I will continue on to spectra, Bohr model, and quantum model, etc.

braithiar · Apr 4, 2010

LOL. What part? Is there an easy part and a hard part? I already took Chem and I dont even know what it is.

I've had many mind erasing classes. After some I didn't even know who I was anymore :lol2: .

I think people have more trouble with the concepts than the history of it. Besides, it's a big subject and the answer will affect the website I recommend.

tokebi · Apr 4, 2010

I think people have more trouble with the concepts than the history of it. Besides, it's a big subject and the answer will affect the website I recommend.

Absolutely, it's a big subject!

As for its "history," it is my observation that understanding the concept of atomic theory is often difficult because students do not see the theory in the context of what was known at the time, what misconceptions existed at the time. Why was a particular experiment so important? How did it change the atomic theory of the time, etc?

ChervRN · Apr 6, 2010

WOW, Tokebi211....I'm impressed!

but the history is definitely not what's stumping me !

What we're going over now is Electron placement, and the Quantum Mechanical Model of an Atom.

(energy levels, etc)

I get it, yet I don't. Maybe there are just some things that won't ever make sense.

tokebi · Apr 6, 2010

Haha, flutterbye, I think I was quite over-zealous when I first posted! Perhaps I was channeling my own frustrations from years ago when I first embarked on chemistry?

If your problem is with those concepts dealing with quantum mechanics, my goodness, please don't sweat it! When I was doing undergrad, my classmates majoring in physics or chemistry did not really understand it either. Sure, we knew how to do calculations and solve equations, but the ability to connect the technical/mathematical details with concepts was rare.

I think the problem lies in that there's just no comparison in our familiar, macroscopic world. Things just behave very strangely at subatomic level, beyond our comprehension.

I once watched a video on PBS website... one of their NOVA series. It was really good trying to present the strangeness of it all in layman's terms... I can't remember the details though, ugh...