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Atoms, Molecules and Ions Chapter 2 Daltons Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms . All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. 1. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same. 1. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions. Daltons Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms . All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. 1. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same (Law of Constant Composition). 1. Chemical reactions only involve the rearrange- ment of atoms. Atoms are not created or destroyed in chemical reactions. Daltons Atomic Theory (1808) 2 8 X 2 Y 16 X 8 Y + Cathode Rays and Electrons A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes. The voltage causes negative particles to move from the negative electrode to the positive electrode. The path of the electrons can be altered by the presence of a magnetic field. Consider cathode rays leaving the positive electrode through a small hole. If they interact with a magnetic field perpendicular to an applied electric field, the cathode rays can be deflected by different amounts. Cathode Rays and Electrons The amount of deflection of the cathode rays depends on the applied magnetic and electric fields. In turn, the amount of deflection also depends on the charge to mass ratio of the electron. In 1897, Thomson determined the charge to mass ratio of an electron to be 1.76 10 8 C/g. Goal: find the charge on the electron to determine its mass. J.J. Thomson, measured mass/charge of e- Cathode Rays and Electrons Consider the following experiment: Oil drops are sprayed above a positively charged plate containing a small hole. As the oil drops fall through the hole, they are given a negative charge. Gravity forces the drops downward. The applied electric field forces the drops upward. When a drop is perfectly balanced, the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate. Cathode Rays and Electrons Using this experiment, Millikan determined the charge on the electron to be 1.60 10-19 C.... View Full Document

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