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History of Atomic Theory Atomic Models Models are often used for things that are too small or too large to be observed or that are too difficult to be understood easily In the case of atoms, scientists use large models to explain something that is very small Models of the atom are used to explain data or facts that were gathered experimentally. Our knowledge of the structure of matter has been developed over time. “Atomos” “Planetary” Heisenberg “Cloud” Model 1932 Chadwick “neutron” 1932 “Orbitals” “Solid Sphere” “Plum Pudding” Werner Heisenberg The “Electron Cloud” Model Electrons travel around the nucleus in orbits. Cannot know where electrons will be at any given moment. Electrons travel so fast, they appear to form a “cloud” around the nucleus. Early Models of the Atom Democritus and the Greeks Universe was made of empty space and tiny bits of stuff Called these tiny bits of stuff atomos Atoms could not be divided John Dalton The “Solid Sphere” Model All elements are composed of “uncuttable” particles. Atoms of the same element are the same Atoms of different elements are different. “Solid Sphere” Model J. Joseph Thomson “Plum pudding” model Atom made of a positively charged material with the negatively charged electrons scattered through it. Ernest Rutherford The “Planetary” Model “Planetary” Model Mostly empty space Small, positive nucleus Contained protons Negative electrons scattered around the outside Niels Bohr Bohr or “Orbital” Model Electrons move in definite orbits around the nucleus like the planets of the solar system. James Chadwick Discovered the neutron Werner Heisenberg The “Electron Cloud” Model Electrons travel around the nucleus in orbits. Cannot know where electrons will be at any given moment. Electrons travel so fast, they appear to form a “cloud” around the nucleus. What do we know NOW? What do you need to know? You don’t have to – memorize every single scientist and every single model. You need to understand – that the model has changed over time and how it has changed – That scientists build upon previous scientists’ ideas – That the model could change in the future with new technology Atomic Models p.15-16 Use your notes Draw a picture of the atomic model Write a short description of the idea the model shows List the Pros (what is correct about it) and Cons (what is incorrect about it or missing) Atoms under a microscope We cannot see an entire atom under a microscope, so atoms are still too small to see with a microscope completely. So we say they are too small to see with a microscope! The “Solid Sphere” Model Picture Description Pros Cons •Solid sphere • Round •Cannot cut elements • uncuttable •No empty space • Same atom = same element •No protons, neutrons or electrons •Same atom = same element Solid Sphere Scientist:_________________ John Dalton •Different atom = different element • Different atom = different element •No orbitals/ energy levels •No nucleus The “Plum Pudding” Model Picture Description •Positively charged sphere with negative electrons stuck in it; like a raisin muffin. Positively charged sphere electrons Scientist:_________________ J. Thomson Pros •Discovered the electron •Positive and negative charges Cons •Positive and electrons mixed together (no orbitals) •No protons, neutrons or a nucleus •No empty space The “Planetary” Model Picture Description •Mostly empty space + •Electrons orbit randomly around a positively charged nucleus. Scientist:_________________ E. Rutherford Pros Cons •Empty space •Electrons orbit •Electrons orbit around randomly nucleus • Protons (positive •Protons discovered charge in the center nucleus) •No neutrons The “Orbital” Model Picture Description •Electrons move in specific orbits around the positive nucleus Scientist:_________________ Niels Bohr Pros •Electrons travel in orbitals (or energy levels) Cons •No neutrons The “Electron Cloud” Model Picture Description •Electrons form a negatively charged cloud around the nucleus •Can’t tell where electrons are (move so fast!) Scientist:_________________ W. Heisenberg Pros •Described electrons more accurately (fast) Cons •Not completely accurate (we have never seen an atom) Analysis Questions 1. No, they used previous ideas and built upon them (added to them) and changed them a little 2. Plum pudding added positive and negative charges to the solid sphere before, planetary model kept + and – charges and added new locations for parts of atom…etc 3. Yes, if we get better technology Magic Square The magic number is… • 39! • If you did it correctly, all totals should be 39…if all totals aren’t, you did it wrong. Don’t just change it to 39, fix it…I will be grading it for correctness. • If you lie, zero. Study Guide Answers 1. Atom 2. Atom 3. No; too small 4. No 5. B 6. It will change in the future as we get new technology and new information 7. No, we have never seen it so we cannot know for sure if it is completely correct 8. Build upon previous knowledge…they don’t just throw away old info, they add to it and make changes to make it more correct (look at atom models). Study Guide (cont.) 9. It is too small to see 10. Protons = Atomic Number • Electrons = Atomic number • Neutrons = Mass – atomic number 11. Electrons 12. 6; on top of the symbol 13. 12; below the name 14. Location • Protons: nucleus • Neutrons: nucleus • Electrons: around the nucleus (in energy levels) 15. An element 16. Se; Selenium Study Guide (cont.) 17. 6 (have to be equal to balance out the positive and negative charge) 18. Protons=3, neutrons=7-3=4 19. Boron Drawing (on white board) 20. Molecule H 21. Water molecule (or any other) H • It has 2 or more atoms O 22. 2 hydrogen atoms and one oxygen atom 23. Nucleus (protons and neutrons) 24. Atoms 25. Picture C 26. A 27. A (Picture A and B) 28. B (Picture B, C, and D) Brain Drain 1. An atom is the smallest unit of matter. It has protons and neutrons in the nucleus. It has electrons in energy levels orbiting the nucleus. The protons are positive, the neutrons are neutral, and the electrons are negative. (It’s negative to be electrocuted!). 2 electrons can fit in the first energy level, 8 e- can fit in 2nd level, and 18 ecan fit in 3rd energy level. P+ and N are big, e- are small. 2. Nobody has seen the atoms because it is so small and we don’t have technology that is advanced enough. We have done experiments to infer what an atom looks like. 3. Atoms are too small to be seen, electrons move so fast we don’t know where they are, too much empty space. Brain Drain (cont) 4. Atoms are just one atom like one hydrogen. Molecules are 2 or more atoms bonded together like hydrogen gas (2 hydrogen atoms) or water (2 hydrogen atoms + an oxygen atom) 5. Started as a solid sphere, added positive/negative charges, added electrons, added empty space….etc. New information and technology led to these discoveries. 6. Aluminum • Mass number=27, Atomic number=13 • 13P+ • 13e• 14N (27 minus 13) Page 15 Answers 1.Dalton’s model showed only solid spheres for atoms-charges missing, no orbitals. 2. Thompson added charges to the atoms (positive space with electrons in it) 3. Added the nucleus and protons, with electrons on the outside 4. We still Bohr’s model to learn about atoms because it is so easy to explain and use to show the nucleus and electrons. 5. What did Chadwick add to Bohr’s model? He added the neutron to atoms 6. Heisenberg used and electron cloud because it describes how you cannot tell where the electrons are at any given time and they move so fast they appear as a cloud 7. They did not start from scratch, they used previous knowledge and built on it.