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The Story Behind Atomic Theory Summary Sheet Model Diagram of the Atom Main Ideas and Explanation All matter is made up of indivisible units called atomos (uncuttable in Greek). Greek Democritus i) All elements are made of atoms that are indivisible. ii) All atoms of one element are identical. Atoms of different elements are different. iii) Atoms cannot be created, destroyed, or subdivided. iv) Atoms of one element combine with atoms of another element in simple, whole number ratios to form compounds. (Law of Definite Proportions) Dalton The atom has a positively charged electron cloud with negatively charged electrons flowing freely throughout. Thomson Plum Pudding Model Rutherford This model introduced the nucleus. The center of the atom is where the majority of the mass and charge is located. The nucleus contains protons and neutrons and has an overall positive charge. The remaining area is an electron cloud which is negatively charged making up the majority of the atom’s volume. Based on the activity today, and the associated reading, complete the following questions. 1. What particles are found in the nucleus of the atom? Describe these particles. Protons and neutrons. Protons are positively charged particles and neutrons are neutral particles and they contribute to the mass of the atom. 2. What is the third particle found in the atom? Where is it located? The electron is the third particle. It is located outside the nucleus. 3. How was it determined that the charge of an electron is negative? By using magnetic or electric fields placed near the electron beam in the cathode-ray tube, the beams bend. Depending on the direction the beam bends dictated the charges. 4. What observation led to the discovery of protons? How did we know that they were positively charged? The gold foil experiment showed that not all alpha particles passed through the foil in a straight line path. Some were deflected which lead to the belief that there was a center in the atom where the mass was concentrated and it had an overall positive charge. 5. Describe the experiment which caused the Thomson model to be replaced by the Rutherford model of the atom. Using a very thin foil of gold, a few atoms thick, bombard it with positively charged alpha particles. According to Thomson’s model, the particles should pass straight through. 6. Contrast the Thomson model with the Rutherford model of the atom. What did Rutherford’s experiment indicate about the structure of an atom? Thomson’s model has the positive and negative charges balanced throughout the whole atom. Rutherford’s model indicates there is a central mass that is positively charged with the negatives around it. 7. Suppose that in a hypothetical scattering experiment all of the alpha particles went through the gold foil. Propose a model of an atom that would account for this observation. If all the alpha particles went through the foil, this would indicate that there were no positive charges in the atom. 8. What were the four main postulates of Dalton’s atomic theory? The four main postulates are: i)All elements are made of tiny particles, called atoms (indivisible). ii) All atoms of one element are identical, though atoms of different elements are different, specifically in terms of mass. iii) Atoms can’t be created, destroyed, or subdivided. iv) Atoms of one element combine with atoms of another element to form compounds. The molecules of these compounds always have the same proportions of elemental atoms. 9. What parts of Dalton’s atomic theory no longer apply to our view of the atom? Atoms can be subdivided. All atoms of one element can differ in atomic mass (the number of neutrons in the nucleus – isotopes).