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AP Atomic Structure Models What Is a Model?

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AP Atomic Structure Models What Is a Model? AP Atomic Structure Models What is a Model? On a scrap piece of paper, write down your definition of a model with at least two examples. A model is a representation of an object, idea, action, or concept. Models A model in science is usually an approximation of the “real thing.” This means that all models have simplifications that may lead to omitted information. Models When a model is selected for a specific purpose, it should be able to help predict a “behavior.” The Four Historical Models of the Atom 1. Particle model 2. Plum Pudding model 3. Rutherford model 4. Bohr model Modern Model Quantum Mechanical Model Why do we have so many historical models? Are some of them still useful? When we initially began the process of determining the structure of the atom, we first had to overcome the prevailing scientific concept that matter was continuous. This idea of continuous matter was from Aristotle. (a Greek philosopher 384-322 B.C.E.) Particle Model John Dalton’s four ideas about the particle nature of matter helped push forward the acceptance of the first model. 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. John Dalton 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction. Other scientists that helped move forward an atom centered world 1. Robert Boyle and Isaac Newton (position papers) 2. Lavoisier (conservation of mass) 3. Proust (law of definite proportion) 4. Amedeo Avogadro (constant number of molecules) 5. Gay-Lussac (gases) Particle Model Why nothing inside? There was no evidence at the time that there was an internal structure. It was such an unimaginably small thing that something to fit inside it was considered impossible. Cathode Ray Tube J.J. Thomson was the individual credited with the discovery of two subatomic particles: electron 1st and the proton 2nd. http://www.youtube.com/watch?v=7YHwMWcx eX8 The positive electrode is the anode and the negative electrode is the cathode. The rays travel to the anode. Therefore, the charge must be negative. Robert Millikan Obtained the 1st accurate measurement of an electron’s charge. He used oil drops and charged plates Using the charge to mass ratio that J.J. Thomson found and the actual charge of the electron from Millikan, the mass of the electron was discovered. 9.101*10-31 kg Or it is 1/1836 of a proton Plum Pudding Model Gold Foil Experiment Lord Rutherford provided evidence that the plum pudding model might not work based upon faulty predictions. It predicts that if you shoot small particles at the atom, many of these particles will reflect back. Very few will pass through. http://www.learnerstv.com/animation/animation.p hp?ani=%20121&cat=chemistry Observations: When the alpha particles (He+2) “hit” the gold foil, they passed through. Need to modify the Plum Pudding Model Move the positive protons into the center and call that the nucleus Rutherford Model of the Atom 1920 Lord Rutherford predicted the existence of a third particle This was due to the lack of mass accounted by the total mass of the protons and the electrons 1930 1st evidence of the 3rd particle 1932 Chadwick had concrete evidence published which established the existence of the neutron This particle had no charge and it approximately had the same mass as a proton. Bohr Model (Planetary Atomic Model) Of these models, which are still used? Particle Model – drawing molecules, phases of matter Bohr Model – showing number of protons, neutrons, and electrons .
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