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Prescriptive Activities: Electrons: PF#60 Student Page Materials: • Student handout Directions: Read and complete the activities/questions below The model of the atom has changed over time as more experiments have been performed and more evidence has been collected. Consider the pictures below that show how the model has changed over time. The models have changed as we have gotten more information about the nature of the atom. An early model of the atom was the indivisible ball “atoms” . This idea was promoted Ancient Greek philosophers such as Democritus. The belief then was that matter was composed of small indivisible particles. Much later came the plum pudding model promoted by Thomson. In this model the negative charge of the electrons and the positive charge of the nucleus were spread out in the atoms. It was an improvement over the early atom motel because it could explain how atoms might be different from one another and explained the charges. The Rutherford model of the atom designed after the gold foil experiment came next. In this model, there is a nucleus with a positive charge and a large area outside of the nucleus that made up the rest of the atom and the electrons could be found in the large area. This model explained why most particles would go straight through a sheet of gold foil and only some would bounce back. The atom must be mostly empty space and the nucleus must be positively charged and in the center. 1 Prescriptive Activities: Electrons: PF#60 Student Page The Bohr model of the atom followed. In the Bohr model, nucleus was the center but now the electrons “lived” in energy levels. This addition to the Rutherford model helps explain why only certain energies of light where absorbed or released by atoms (e.g., the hydrogen spectrum or why certain materials are used to make particular colors of fireworks). However people started believing that these energy levels or shells were like real life things. Like there is an egg shell in the atom where the electron lives. This is not true, because we know now that the electrons do not rest on shells or other structures in the atom. The electron is there because it is attracted to the nucleus and is repelled by other electrons. There are no shells or rings structures in the atom only places where we might find the electron. The idea about shells is useful because when we organize electrons in an atom, they are grouped together by the amount of energy they have. These amounts of energy are we call shells. So they are not structures but an amount of energy an electron might have. We now have an idea about where the electrons can be found in the space around the nucleus and we can say that we are most likely to find then one place and not another, but we cannot know for sure where the electron is. This is called the cloud model of the electron or the quantum model. The cloud shows where you are likely to find the electron. The darker the cloud, the more likely you are to find the electron there. If the cloud is lighter, then you are less likely to find the electron there. A wrong idea is to think that the electrons are like rain in the clouds in the sky. This is wrong because the electron clouds do not exist, it is just a model showing you where you MIGHT find the electrons. This is where we are now in our model. Discussion Questions:
1. Draw each of the models above and label the nucleus and electrons. Note that in the cloud model you can only say that they might be found in a certain place. 2 Prescriptive Activities: Electrons: PF#60 Student Page 2. Find some examples of the Bohr model of the atom that are seen on internet. Collect some samples of the Bohr model of the atom you find and state where you found it. 3. The shell model of the atom looks convincing. How is the cloud model of the atom different than the shell model of the atom (Bohr model)? 4. When is the shell model useful and when is the shell model a problem? Resources You can check out some sample models of the atom at http://phet.colorado.edu/simulations/sims.php?sim=Models_of_the_Hydrogen_Atom While this activity is about light interacting with the atom, it shows you different ways to visualize the model of the atom. The last two models are quantum models. 3