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chap6 - Chapter 6 Electronic Structure of Atoms The last...

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Chapter 6 - Electronic Structure of Atoms The last two chapters discussed types of reactions and the energy changes associated with them. We now begin a look at atoms and molecules at a fundamental level. Earlier we saw that energy is consumed or released when a chemical reaction occurs. Why? Because during reactions some bonds brake and others form and all will have different strengths. For example, typically stronger bonds replace weak bonds and energy is released. Chemical bonds involve the interaction of electrons between atoms. For this reason, we must understand the interaction of electrons with nuclei and each other. That is what this chapter is all about. 6.1 The Wave Nature of Light When a fire burns you can feel the heat from the flames without touching them. The reaction between wood and oxygen releases this energy. Electromagnetic radiation carries energy through space and is sometimes called radiant energy . There are a number of different kinds of such energy including: visible light, X-rays, and radiowaves, as well as the infrared radiation coming from the fire. They all share the property of traveling in waves at 3.00 x 10 8 m/s in a vacuum ( you should remember this value ). wavelength ( λ ) amplitude Wavelength , λ , is the distance between successive peaks. Amplitude is the distance from the centerpoint to the top of the wavecrest. The distance covered by one wavelength is sometimes called a wavecycle or more simply, a cycle . The (wave)cycle is frequently used as a counting term. For example, 5 wavecycles passed this point in one second. It is important to remember that when waves move through a medium they don’t carry the
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2 medium with them. This is a demonstration you can do at home. First, fill a bathtub a few inches deep with water. (The deeper the better.) Place a cork (or small floating ball) at one end, but at least 6 inches from the nearest wall. Position yourself so you are looking straight down at the cork from above. Have someone else drop an object (e.g. a tennis ball) at the other end at roughly the same relative position. (It is important that there be 2-3 feet between the cork and the dropped object.) As the waves travel past the cork, you will see it bob up and down, but it should not move much towards or away from the dropped object. (It may move a little because the cork and dropped object will be near each other.) This experiment works better in a lake. Frequency, ν , is the number of wavecycles that pass a given point in one second. Thus, the velocity of the wave is λν . Since we know that all radiant energy travels at the speed of light, this becomes c = λν . Does this make sense? In high frequency light many wavecycles pass a given point in one second. If this is the case, the wavelengths must be short to get them through in the allotted time since the speed is fixed.
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