Atomic Structure http://en.wikibooks.org/wiki/High_School_Chemistry/Further_Understanding_of_the_Atom http://en.wikibooks.org/wiki/High_School_Chemistry/Further_Understanding_of_the_Atom

Dalton's Atomic Theory Held up well to a lot of the different chemical experiments that scientists performed to test it. In fact, for almost 100 years, it seemed as if Dalton's Atomic Theory was the whole truth. As it turns out, Dalton had a lot right. He was right in saying matter is made up of atoms He was right in saying there are different kinds of atoms with different mass and other properties He was "almost" right in saying atoms of a given element are identical He was right in saying during a chemical reaction, atoms are merely rearranged He was right in saying a given compound always has atoms present in the same relative numbers. But he was wrong in saying atoms were indivisible or indestructible.

J.J. Thompson In 1897, a scientist named J. J. Thompson conducted some research which suggested that Atomic Theory wasn’t the entire story. As it turns out, atoms are divisible. In fact, atoms are composed of smaller subatomic particles.

Discovering Electrons In the mid-1800s, scientists were beginning to realize that the study of chemistry and the study of electricity were actually related. First, a man named Michael Faraday showed how passing electricity through mixtures of different chemicals could cause chemical reactions. Shortly after that, scientists found that by forcing electricity through a tube filled with gas, the electricity made the gas glow! Scientists didn't, however, understand the relationship between chemicals and electricity until a British physicist named J. J. Thomson began experimenting with what is known as a cathode ray tube. Cathode Ray Tube Experiment

Cathode Ray Tube Experiment

Cathode Ray Tube Experiment

J. J. Thomson made a radical proposal: Maybe atoms are divisible! J. J. Thomson suggested that the small, negatively charged particles making up the cathode ray were actually pieces of atoms. He called these pieces "corpuscles", although today we know them as "electrons". Thanks to his clever experiments and careful reasoning J. J. Thomson is credited with the discovery of the electron.

Thomson's Model of the Atom Thomson's "plum-pudding" model for the atom. Notice how the "plums" are the negatively charged electrons, while the positive charge is spread throughout the entire pudding batter.

Thomson's Model of the Atom Negative electrons = Pieces of Fruit-Plums (chocolate chips) Positive material = Pudding (cookie dough batter) How this fits Thompson’s Observations: Isolated electrons using a cathode ray tube Never managed to isolate positive particles Result: Thomson theorized that the positive material in the atom must form something like the "batter" in a plum pudding, while the negative electrons must be scattered through this "batter".

Disproving the "plum pudding" model A man named Ernest Rutherford used alpha particles (Helium atoms that lost their electrons and had a positive charge)as tiny bullets to fire at all kinds of different materials. One experiment in particular, however, surprised Rutherford, and everyone else. Fired alpha particles at a very thin piece of gold foil Almost all of the alpha particles went straight through the foil as if they'd hit nothing at all Every so often, though, one of the alpha particles would be deflected slightly as if it had bounced off of something hard Structure of the Atom: The Rutherford Model

Discovering the Nucleus Rutherford Observation Most alpha particles passed through the gold foil without hitting anything at all Once in a while, though, the alpha particles would actually collide with a gold nucleus, causing the alpha particles to be deflected, or even to bounce right back in the direction they came from. Rutherford Explanation The positive matter forming the gold atoms was not distributed like the batter in plum pudding The positive matter was concentrated in one spot forming a small positively charged particle somewhere in the center of the gold atom. We now call this clump of positively charged mass the nucleus.

What about our “Batter” of electrons? Rutherford Suggestion Electrons might be circling or "orbiting" the positively charged nucleus as some type of negatively charged cloud instead Not much evidence at this time to suggest exactly how the electrons were held in the atom Modern Reconstruction of Rutherford's Experiment