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Ally Denton's Atomic Theory Timeline Project
Transcript of Ally Denton's Atomic Theory Timeline Project
By: Ally Denton
"Development of the Atomic Theory." Holt Science & Technology: Physical Science. N.p.: n.p., n.d. 312-17. Holt McDougal Online. Web. 10 Oct. 2013.
"Science Clarified." Web log post. Atomic Theory. N.p., n.d. Web. 10 Oct. 2013.
Good, Maggie. 2011Period6Group2 - John Dalton. 2011Period6Group2 - John Dalton. N.p., n.d. Web. 10 Oct. 2013.
In 1897, a British scientist named J.J. Thomson discovered that an atom contained smaller parts called electrons. Thomson experimented with a cathode-ray tube, which is shown on the right. He found that positively charged plates attracted the beam that he decided was negatively charged. Thomson also decided that the negative charges, now called electrons, are in every atom. Using this newfound knowledge, Thomson created a new model of the atom, shown on the left, called the Plum Pudding Model, in which the electrons are scattered throughout the atom. The name comes from a dessert where the plums are scattered throught the pudding, thus the similarities between the two.
Ernest Rutherford, who had once been one of Thomson's students, tested Thomson's theory using his "Shooting Gallery" model, pictured on the right. He pointed a positively charged beam at specially coated gold foil to find that while most of the particles went in a straight line, some of them bounced back at him. Rutherford realized that Thomson's model contradicted what he had seen, so Rutherford created a new model for the atom in 1911 in which the diameter of the nucleus is 100,000 times smaller than the diameter of the whole atom. The nucleus to atom-size ratio would be like a pin in a baseball field, pictured on the bottom.
Around 440 BC, a philospher named Democritus hypothesized that at some point after cutting something in half, thta you would end up with a particle that could not be cut. Democritus called this particle the atom, which comes from the Greek word "atomos", which means "not able to be divided. He believed these atoms made up every object in our world, and they could be different shapes and sizes. One example of this is the picture of the cans on the left, these cans are all made up of aluminum atoms, which cannot be cut in half to create a smaller particle.
In the lates 1700s, A British chemist and schoolteacher named John Dalton wanted to know why elements combine in certain amounts to create specific compounds. He experimented using the three wooden balls on the left to determine that elements combined they way that they did because they were made up of single atoms. He published his atomic theory in 1803, which said:
•All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed.
• Atoms of the same element are exactly alike, and atoms of different elements are different.
• Atoms join with other atoms to make new substances.
In 1913, a Danish scientist named Niels Bohr, who worked with Thomas Rutherford, used his studies in the way atoms react to light to predict that electrons move around the nucleus in a set way on different energy levels. He compared this to rungs on a ladder; electrons could jump down to different energy levels, but they could not stay in between the levels, or rungs. Bohr's new model for the atom is pictured on the left.
Two more 20th century scientists who helped improve the atomic model were Werner Heisenberg and Erwin Shrodinger. These two discovered that the path an electron moves in cannot be predicted, but electrons can be found in certain areas called electron clouds. The modern model of the atom, which both Heisenberg and Schrodinger greatly helped in developing, is pictured both left and right.
Heisenberg / Schrodinger
Thomson used a cathode-ray tube similar to this one while conducting his experiments .
Rutherford's "shooting gallery" model he used to test Thomson's theory.
This pin represents the nucleus, and the baseball field represents the whole atom. This is a size accurate portrayal of what Rutherford hypothesized what the nucleus size compoared to the atom size would be.
This is the modern atomic model.