Atomic Theory Class #2

1. Atomic Theory Class #2 OB: students will review the models of the atom through scientific history, learning how ideas progressed and were dismissed as new information was developed. Most famous chemists had remarkable mustaches. That’s the easiest way to keep track of who’s who in science!

2. About 2400 years ago a philosopher named Democritus came up with this idea: If you took anything and cut it in half, and in half, and in half, over and over, sooner or later you would get to a piece of this stuff that could not be cut in half again. That piece was called an ATOMOS (he was Greek) And that’s the start of chemistry. You could have an ATOMOS of a rock, of a fish, of a diamond, etc. The smallest piece of anything, that could not be cut again was to be known as an ATOM of that stuff. A good name, and it stuck. The first of many mustaches in our course!

3. In the early 1800’s a man named John Dalton thought he could add some to this whole chemistry course. It had to be longer than a singe word, really. He worked in his barn and came up with a few good ideas that are still true! He told us1. All substances were made up of atoms. All atoms had their own mass, different from other atoms’ masses. All atoms were built like billiard balls(a game he liked to play)

4. Dalton came up with what is now called the Atomic Theory This is so important I can’t highlight it enough! All elements are composed of individual kinds of atoms. Atoms of one element are identical. Atoms of different elements have different masses. Atoms can physically mix with other atoms. Atoms can also chemically combine with other atoms IN SIMPLE WHOLE NUMBER RATIOS, to form new substances. Chemical reactions change the arrangement of atoms, but they do not become different kinds of atoms. Dalton did not know of protons, neutrons or electrons. He was brilliant, but had little science ability in 1803. You know a lot more about atoms now than he ever did!

5. Around 1897 J. J. Thomson discovers the electron! Look at the hair on that man’s lip! Thomson did a variety of experiments, some using what’s called the cathode ray tube, to detect and measure electrons. He found the first subatomic particle, which was negatively charged. Where was this electron in the atom, he didn’t really know. With just his thoughts, and his wife’s desserts, he proposed that the atom was a positive mush, containing these small negatively charged electrons. It looked sort of like his wife’s (chopped up) plum pudding. (think chocolate chip cookies). The atoms were like a positively charged cake, with negative bits throughout. That kept the atoms neutral.

6. In 1908, my chemical hero, Earnest J. Rutherford discovers the nucleus! - and much more about the atom than previous scientists. Rutherford The Gold Foil ExperimentDiscovers the nucleus! The gold foil experiment was one of thetop 10 experiments of all time, and costhim about fifty bucks. Wow! Try to imagine the honor it would be to get to have dinner with this man.

7. Details of the GOLD FOIL experiment to memorize and share with your friends.

8. What does this gold foil experiment prove? Atoms are mostly empty spacesince most of the alpha particles pass though the foil like it’snot really there. Atoms are neutral (he knew this) so the nucleus must be dense and positively chargedsince the alpha particles which are positive didn’t stick, and they dinged off of something big every once in a while Neutral atoms must therefore have electrons outside the nucleusflying around outside (like planets?) made sense.

9. Problems with this new theory of Rutherford’s So, Earnest, atoms are mostly empty space, and since every thing is made of atoms, everything is mostly not there? Is that what you’re saying? EJ, my man, you think that these negative electrons fly around a positive center, and they never are attracted together? They just keep flying? They never use up their energy? Ever? Really? How far away are these electrons Ernie - and do they just fly willy nilly?

10. Rutherford was right, but he needed some help. In 1913, one of his students, a man with a plural first name, comes to his rescue. Niels Bohr Niels Bohr is able to do some very funky math, (physics level + Nobel Prize style math) and he proves Rutherford is correct. He shows that for hydrogen, if the electron fly at the right speed, and the right distance, it will somehow never run out of energy, and never collapse into the nucleus. And it doesn’t, does it? You already know he’s right even though you can’t do the math. None of the atoms collapses, not a one, and even though he can’t do the math for even 2 electrons, his proof is enough to save the modern model of the atom, central nucleus, flying electrons.

11. Niels Bohr further shows that electrons don’t fly willy nilly, rather they fly in specific orbits. Orbits, as in flying around the sun, in a nice line. And, he showed that each orbit was really also an energy level. The smallest orbit closest to the nucleus was the lowest energy level. And, that electrons could gain energy and jump temporarily to higher orbits (higher energy levels). And they could jump back by giving off the exact amount of energy they absorbed to get excited in the first place. He also showed that each energy level (orbit) was unique, and that it took specific quanta of energy to make these various leaps. Orbits was wrong, but life was easier when it was believed to be true.

12. In the 1920’s the Modern Model of the atom, also known as the Wave-Mechanical Model was developed. This is what we know to be the best model of atoms so far. Electrons do not live in orbits, they do not follow invisible lines round the nucleus. Rather they inhabit “zones” or ORBITALS. The orbitals are more statistical models of the likelihood of finding electrons. Like a cloud diagram. The darker the cloud, the more likely an electron would be found there, the lighter the cloud, the less likely the electrons frequent that area. Electron orbitals are still energy levels, they just aren’t so neat. Electrons are both waves of energy andmechanical bits of charge, at the same time?!

14. Take out Matter HW #2 to review (or collect) Tonight: READ the Atomic Theory Diary