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The Atom Mrs. DeMedio Chemistry Unit 4 Objectives You will be able to: Evaluate the factors that influence the deflection of a charged particle. Compare and contrast the atomic models of Dalton and Thomson. Identify the characteristics of alpha, beta, and gamma radiation. Analyze how Rutherford’s atomic model explains the results of his gold foil experiment. Part I- The Atom Democritus Fifth century B.C. Greek philosopher proposed that all matter was composed of indivisible particles called “atomus” (Greek for indivisible). Everyone thought he was CRAZY!!!! Democritus is CRAZY!!!! Nothing happened with atoms until… John Dalton A. Experiment 1. Dalton experimented with gases and water evaporating into the atmosphere. 2. His question was why do gases not act as solids or liquids? B. Results Gases in the atmosphere were mixed together and not layered. 1. 2.Water vapor is absorbed into the atmosphere. C. Inferences from experiments. 1. Dalton’s Atomic Theory All matter consists of tiny particles called atoms. All atoms of the same element are chemically identical. Atoms of different elements are chemically different. Chemical reactions occur when atoms are joined, separated or rearranged. Compounds are formed from atoms of the constituent elements. D. Dalton’s Atomic Model 1. Dalton viewed the atom as a hollow sphere. Indivisible Neutral charge Smallest particle Dalton’s Atomic Model Part II: The Electron Thomson A.Experiment 1. J. J. Thomson wanted to test Dalton’s model of the atom. 2. Thomson used an evacuated Cathode Ray Tube. CATHODE RAY TUBE 3. An element was stimulated with electricity. 4. Raytraveled from the positive to the negative electrode. J.J Thomson 5. He placed a positive magnet on one side of the ray and a negative magnet on the other. 6. Ray deflected from the negative plate to the positive plate 7. Inferred ray must contain negative particles. B. Results 1. Joseph John Thomson (1897) a. Ray has constant charge to mass ratio-> all same particle b. Cathode rays are composed of identical negatively charged particles (electrons) C- Inferences from the properties of electrons 1. Atoms are neutral, so there must be positive charges to balance the negatives 2. Electrons have little mass, so atoms must contain other particles that account for most of the mass. D.Atomic Model 1. Thomson’s Plum Pudding Model a. His results led him to see the atom as a positively charged base with – particles interspersedmuch like plum pudding. Thomson’s Plum Pudding Model Part III: electron neutron proton The Nucleus Rutherford A. Experiment 1. Ernest Rutherford (1911) a. Alpha particles (helium nuclei) fired at a thin sheet of gold Rutherford’s Experiment B- Results Very few particles were greatly deflected back from the gold What does that sheet mean about the 1. atom? D-Rutherford’s Atomic Model 1. The atom has a dense center, called the nucleus, which is positively charged. 2. The nucleus is surrounded by mostly empty space with small electrons floating around in the space. C- Inferences Assumed that the positively charged particles were bounced back if they approached a positively charged atomic nucleus head-on (Like charges repel one another) 1. 2. The atom had a nucleus. a. nucleus is very small, dense and positively charged b. most of the atom is empty space