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Parts of an Atom Atom – are the fundamental particles of matter. Means “uncuttable”, are about 1/10 of a billionth of a meter across. Are composed of protons (p+), neutrons (nº) and electrons (e-) Neutrons and protons are located in the center (nucleus) Atoms are mostly empty space - if an electron were basketball sized, the nucleus would be a car 20 miles away. Proton – means “first” Relatively large, have a positive charge. The # of protons in the nucleus determines what element the atom is (a.k.a. the atomic Neutron – means neutral Approx. same mass as proton, no charge. Determines whether an element is radioactive Determines how much an element weighs - neutron + proton = atomic mass Forms of an element that have different numbers of neutrons are isotopes. Electron Small ( ~ 1/2000 mass of proton), negative charge. Are attracted to protons Move around in patterns known as orbitals. - The orbital (or shell) is the distance from the nucleus. - The oribitals have names and are limited to the number of electrons they can hold. The electrons in the outermost orbital are known as Nucleus Contains protons and neutrons Makes up most of the mass of an atom. Small part of atoms volume. valance electrons. Electron Cloud • Space in which electrons orbit the nucleus at high speed. • Large part of atoms volume. • Subdivided into Energy Levels or Shells. • Electrons occupy the lowest energy levels. number) Station 2: Atomic Models Station 3: Plum Pudding Atoms In 1897 J.J. Thomson discovered the electron, a negatively charged particle more than two thousand times lighter than a hydrogen atom. An atom made of thousands of electrons would have a very high, negative electric charge. This was not observed, as atoms are usually uncharged. Because matter is electrically neutral, there must be a positively charged particle that balances the negative charge on the electrons in an atom. If electrons are very light, these positively charged particles must carry the mass of the atom. Thomson therefore suggested that atoms are spheres of positive charge in which light, negatively charged electrons are embedded, much as raisins might be embedded in the surface of a pudding. This plum pudding model was generally accepted. Even Thomson's student Rutherford, who would later prove the model incorrect, believed in it at the time. Station 4 The Rutherford Model In 1909 Ernest Rutherford conducted what is now a famous experiment where he bombarded gold foil with alpha particles (Helium nuclei). A source which undergoes alpha decay is placed in a lead box with a small hole in it. Any of the alpha particles which hit the inside of the box are simply stopped by the box. Only those which pass through the opening are allowed to escape, and they follow a straight line to the gold foil. The figure below shows the experiment. Observations Most of the alpha particles pass straight through Some of the alpha particles get deflected A very few get deflected greatly. Even fewer get bounced back Conclusions The atom is 99.99% empty space. The nucleus contains a positive charge and most of the mass The nucleus is 100,000 times smaller than the atom. Station 5: Rutherford Scattering simulation [PHET, Chemistry, Rutherford Scattering]. https://phet.colorado.edu/en/simulation/rutherfordscattering This simulation gives a microscopic picture of Rutherford’s famous experiment in which he shot alpha particles at a thin foil of gold. Begin with the “Plum Pudding Model” Turn the gun on for 10 seconds and record how the alpha particles move in relation to the atomic model: Switch to “Rutherford Atom” Change the number of protons to “20” at the right side of the screen. Turn the gun on again and record your observations: Slide the “number of protons” bar to the right little by little and observe the “particles” as they approach the central mass. Record how the deflection of the particles changes as one adds more protons: Station 6: “Models of the Hydrogen Atom” simulation. Open the “Models of the Hydrogen Atom” simulation. http://phet.colorado.edu/new/simulations/sims.php?sim=Models_of_the _Hydrogen_Atom Select “Experiment” and “White Light” Click the button on the gun to start the experiment. Describe the movement or color changes that you see: Switch to “Prediction” in the upper left corner of the simulation. For each of the models, run the simulation for 10 seconds and describe the movement/color changes you observe. Billiard Ball Plum Pudding Solar System Bohr deBroglie Schrödinger Which one matches what actually happens in the “Experiment” from the first part?