In desperation (see previous posts), I go directly to the article Periodic Table of the Elements. No answer there, either, but there is a link to "Atomic Radius." There, I am interested to learn that atoms "do behave as if they were spheres with a radius of 30–300 pm" (though their actual shape and size is less definite), with pm being picometers, that is, one one millionth of a millionth of a meter.
Here, of course, is the test of my earlier calculation. I was off by a factor of one million ... well, actually, by 10,000 to 100,000 times ... my estimate was about that much too large - which, actually, isn't too bad!
Wednesday, October 17, 2007
Avogardo
Although the article is not quite explicit about it, it seems that the final clue was detected by the Italian, Avogardo, (see Atomic Theory article) in the form of his observation that the volume of a gas is independent of its mass, and is, instead, a function of the number of molecules it contains.
But, is this the final clue? Now I'm not so sure. It did provide conclussive evidence regarding the relative mass of different atoms. How do we know, though, how many atoms are in a given amount of an element?
But, is this the final clue? Now I'm not so sure. It did provide conclussive evidence regarding the relative mass of different atoms. How do we know, though, how many atoms are in a given amount of an element?
atomic theory
In time, the article on chemistry links to Atomic Theory, and there we start to get answers to our question, in the ideal form: the history of our knowledge of the subject.
As I suspected, a real sense of atomic mass began to emerge from Lavoisier's experiments. Let's see, in the show on PBS, he ran water, or steam, through a heated iron gun barrel, and collected a gas on the other end. Since the iron combined with the oxygen in the steam, the gas that emerged was hydrogen. He then weighed the gun barrel, and found its weight to have increased. He weighed the hydrogen, and found that its weight, combined with the weight added to the barrel, equalled the weight of the water that had passed through the barrel.
Where it went from there, exactly, I'm not sure, but Lavoisier and those who proceeded from his work could tell that substances were being divided and recombined in certain proportions, which gave them evidence about the composition of the substances.
I must read further. Absolutely do link to these articles, as they are enthralling. (Note: they could use a bit of proof-reading.)
As I suspected, a real sense of atomic mass began to emerge from Lavoisier's experiments. Let's see, in the show on PBS, he ran water, or steam, through a heated iron gun barrel, and collected a gas on the other end. Since the iron combined with the oxygen in the steam, the gas that emerged was hydrogen. He then weighed the gun barrel, and found its weight to have increased. He weighed the hydrogen, and found that its weight, combined with the weight added to the barrel, equalled the weight of the water that had passed through the barrel.
Where it went from there, exactly, I'm not sure, but Lavoisier and those who proceeded from his work could tell that substances were being divided and recombined in certain proportions, which gave them evidence about the composition of the substances.
I must read further. Absolutely do link to these articles, as they are enthralling. (Note: they could use a bit of proof-reading.)
a clue: gas laws
If I go to the history of chemistry, via Wikipedia, I learn - among many very, very interesting things - that, starting in about the 1660s (in Europe), gas pressure was carefully observed, and then in the early 1700s it was explained as the kinetic energy of gas molecules.
Read the details at Wikipedia under Boyle's Law.
The kinetic theory must mean that gas pressure is caused by the cumulative impact of numerous molecules colliding with a vessel wall, or the surface of an object. If we know the total mass of the gas in a vessel, for instance, we may be able to calculate a ratio of the number of collisions that mass would need to make and the velocity of each collision, to produce the observed pressure.
This doesn't tell us how many atoms are in a given amount of gas ... it just hints at the number, or at the idea that there are many atoms in a volume of gas.
Read the details at Wikipedia under Boyle's Law.
The kinetic theory must mean that gas pressure is caused by the cumulative impact of numerous molecules colliding with a vessel wall, or the surface of an object. If we know the total mass of the gas in a vessel, for instance, we may be able to calculate a ratio of the number of collisions that mass would need to make and the velocity of each collision, to produce the observed pressure.
This doesn't tell us how many atoms are in a given amount of gas ... it just hints at the number, or at the idea that there are many atoms in a volume of gas.
first yahoo physics listing
No link directly to something on the history of physics, which is what I'm looking for now. (See next post for link to site.)
Now, its clear, I think, that Mendelev had a definite sense of atomic weights, and that means he knew, much more definitely than I do now (as represented by my calculations in the previous post) how many atoms are in a given mass. How did he know that?
Incidentally, I just read that the Planck Length is one billion-trillion-trillionth of a centimeter. How does that compare with my calculation? Let's say our stone is a cube, 100 centimeters to a side. Let's say each pebble is one centimeter across. One face of the cube, then, is occupied by 100x100=10,000 pebbles. A pebble, then is 100 grains of sand across, and a grain of sand is 100 particles of powder across, and a particle of powder is 100 atoms across, so the size of an atom (its width) is 100(grains)x100(particles)x100(atoms)=10,000x100=100,000 times smaller than a pebble, which is 1/100,000th of a centimeter ... much, much larger than the Planck Length. (Of course, I have not the slightest idea how accurate my estimate is.)
Now, its clear, I think, that Mendelev had a definite sense of atomic weights, and that means he knew, much more definitely than I do now (as represented by my calculations in the previous post) how many atoms are in a given mass. How did he know that?
Incidentally, I just read that the Planck Length is one billion-trillion-trillionth of a centimeter. How does that compare with my calculation? Let's say our stone is a cube, 100 centimeters to a side. Let's say each pebble is one centimeter across. One face of the cube, then, is occupied by 100x100=10,000 pebbles. A pebble, then is 100 grains of sand across, and a grain of sand is 100 particles of powder across, and a particle of powder is 100 atoms across, so the size of an atom (its width) is 100(grains)x100(particles)x100(atoms)=10,000x100=100,000 times smaller than a pebble, which is 1/100,000th of a centimeter ... much, much larger than the Planck Length. (Of course, I have not the slightest idea how accurate my estimate is.)
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