The Chemistry Tutorial - Atomic History
The next part of The Chemistry Tutorial is Atomic History. This does not help one learn Chemistry, per se, but it does help to provide a background and understanding of a number of important scientists who learned what we currently know about Chemistry. We will start off with the earliest, and proceed chronologically.
Note: Throughout this topic, many different theories will be presented by many different people. They will seem to contradict each other; that is because they do. The people who lived later in time tend to correct the theories and assumptions of the people who lived before them.
Democritus - lived around 400BC in Greece, during the Greek Golden AgeCreating the name "atomus", meaning indivisible
Describing the world in terms of four elements: Earth, Water, Air, and Fire.
Democritus made two contributions to Chemistry, which was almost non-existant in his day.
John Dalton - lived during the late 1600's in England
Dalton went slightly further than Democritus did, and proposed that there were "atoms". He said that these were spheres, because a sphere was the "divine shape". Remember that this was during the 17th century, and that the Church still had a lot of power. "Atoms" were indivisible, he claimed.
Sir William Crookes - lived during the early 1800's in England
Crookes is most well-known for the Crooke's tube. This was a sealed glass tube which had two metal electrodes inside it, with their ends sticking out of the tube. When a electrical current is applied to them, a fluorescent light appears to be shining out of on the side opposite the negative electrode. He then built another tube with a Maltese cross in it, to see if these lights travel in straight lines. They did, and what could be seen was the outline of a Maltese cross with light completely surrounding it.
Joseph John Thomson - lived during the late 1800's in England
JJ Thomson started where Crooks left off. Knowing that a Crookes tube produced light on one side, Thomson wanted to know if these particles apparently traveling through the tube had mass, which could be demonstrated by something moving along. He put a little car on bars, where it could move easily, and then applied an electrical current to the electrodes. Sure enough, the car moved, proving that these particles did have mass. He called them "corpuscles", which he claimed were in everything, but they were later renamed to what we now know as electrons.
Next, JJ Thomson said that atoms were neutral, and that, if they have the negatively-charged electrons in them, which he had already said everything had, then there must be a positively-charged part as well to the atom. He kept the atom as a sphere, and said that there were electrons all around the sphere, and that there was other "stuff" everywhere else in the atom. This other "stuff" was the positively-charged material. This model of the atom was called the "Plum-Pudding model", because it resembled the Plum Pudding that they commonly ate.
Third, Thomson measured the charge-to-mass ratio of the electron, and computed it to be 1.759 * 1011 C/kg. The unit on this is coulumbs per kilogram. Although this may not seem like much, it was very important in determining the mass of the electron.
Ernest Rutherford - lived during the late 1800's and early 1900's in England
Rutherford is known for disproving Thomson's "plum-pudding" model of the atom. This was done in the following way. He took a radioactive element, and put it in a lead box with a very small hole in it. Around that box he put a phosphorescent screen, which would pick up the α (alpha) particles leaving the radioactive element. α particles are much heavier than electrons, and move very quickly. At first, he saw a very big glow directly opposite from the hole in the lead box. However, as he extended the screen farther and farther, he saw small bits of light at various places, and not directly opposite from the hole. This lead him to believe that the α particles were being bounced off of something, since that was the only possible way for them to move somewhere not directly across from the hole in the lead box. The only way in which they could bounce off of something is if that something has a very large mass. And this mass would have to be compacted together very tightly, because most of the α particles went passed and did not hit anything. This proved that Thomson's model of the atom was wrong, and Rutherford proposed a new model of the atom: instead of having all of the positively-charged material all around the atom, it was all in one tiny speck.
Robert Millikan - lived during the late 1800's and early 1900's in America
Millikan was known for determining the charge of an electron, and proving that all electrons have the exact same charge. He did this by taking a can with two metal plates on it, the top one having a positive charge, and the bottom one having a negative charge. He then sprayed oil over this can, causing them to gain electrons through the air, and got many small drops of oil above the can. One of them would fall through a hole in the top, and the top plate would be turned on, pulling the electron up, while gravity pulled it downwards. By changing the plate's charge to a force that would counteract gravity, Millikan was able to know precisely how much force was needed to hold an electron up, and, through the use of a few equations, he was able to find the charge on the oil drop. By performing this experiment thousands of times (rather, his assistants did), he was able to find out the average increments that the oil drop's charge is, and can find out the charge on a single electron through more equations that he utilized.
If you would like more information on any of these people, I suggest either clicking on their name, and if that provides no information, searching for them in Google.
Significant Figures | The Chemistry Tutorial | Atomic Structure