Models

1. Models • A model is a drawing, mathematical equation, physical structure or other representation • uses familiar ideas to explain unfamiliar hypotheses or theories. • A model can be changed as new information is collected. • Examples of models DNA – Biology Bohr Model – Chemistry Law of Electrostatics Physics Astronomy – Earth Science

2. Investigating Atoms and Atomic Theory • Students should be able to: • Describe the particle theory of matter. PS.2a • Use the Bohr model to differentiate among the three basic particles in the atom (proton, neutron, and electron) and their charges, relative masses, and locations. PS.3 • Compare the Bohr atomic model to the electron cloud model with respect to their ability to represent accurately the structure of the atom.PS.3

3. Atomos: Not to Be Cut The History of Atomic Theory

4. Atomic Models • This model of the atom may look familiar to you. This is the Bohr model. In this model, the nucleus is orbited by electrons, which are in different energy levels.

5. Democritus’ Model – 400 BC • The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball →

6. Who are these men? In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

7. Democritus 400 BC • This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago. • He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

8. Atomos • His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. • This piece would be indivisible. • He named the smallest piece of matter “atomos,” meaning “not to be cut.”

9. Atomos • To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. • Atoms were infinite in number, always moving and capable of joining together.

10. Did people believe in Democritus Model of Matter? This theory was ignored and forgotten for more than 2000 years!

11. Why? • Aristotle and Plato had a more respected, (and ultimately less “correct”) theory • The 4-Element Model Aristotle and Plato favored the earth,fire, air and waterapproach to the nature of matter. Their ideas were more accepted because of they were both very well-known philosophers. Democritus’ atomos idea was buried for approximately 2000 years.

13. Dalton’s Model • In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

14. Dalton’s Theory • He deduced that all elements are composed of atoms. Atoms are • indivisible • indestructible • Atoms of the same element are exactly alike. • Atoms of different elements are different. • Compounds are formed by the joining of atoms of two or more elements.

15. . • This theory became one of the foundations of modern chemistry.

16. Inferences Made By Indirect Evidence • Inferences:logical explanations or conclusions based on observations and/or facts; often made from INDIRECT EVIDENCE • Indirect Evidence: evidence gathered or observations made when we cannot observe the phenomenon or the object directly

17. Scientists started to use Indirect Observation to Make Inferences • Indirect Observation: Used when we cannot directly observe the object under study. We subject the object to various tools or methods or treatments and we observe the results. We then make inferences. AKA we shake our gifts!! • Object #1 • Object #2 • Object #3

18. Thomson’s Plum Pudding Model • In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

19. Thomson Model • He proposed a model of the atom that is sometimes called the “Raisin-Bun or Plum-Pudding” model. • Atoms were made from a positively chargedsubstance with negatively charged electrons scattered about, like raisins in a pudding.

20. Thomson Model • Thomson studied the passage of an electric current through a gas. • As the current passed through the gas, it gave off rays of negatively charged particles.

21. Thomson Model Where did they come from? • This surprised Thomson, because the atoms of the gas were uncharged. Where had the negative charges come from?

22. Thomson concluded that the negative charges came from within the atom. A particle smaller than an atom had to exist. The atom was divisible! Thomson called the negatively charged “corpuscles,” today known as electrons. Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom (to balance out the negative particles) But he could never find them.

23. Dalton, Crookes, Thomson • Dalton • Crookes • Thomson • http://vimeo.com/77101317

24. Rutherford’s Gold Foil Experiment • In 1908, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure.

25. Rutherford’s experiment involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) • Watch it here http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

26. Rutherford’s Observations • Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. • Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

28. Rutherford Movie Clip • http://chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHERFD.html

29. Rutherford’s 3 Conclusions • This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. • Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged “bullets.” • He called the center of the atom the “nucleus” • The nucleus is tiny compared to the atom as a whole.

30. Rutherford • Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. • The negatively charged particles were scattered outside the nucleus around the atom’s edge.

31. Bohr Model • In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.

32. Bohr Model • According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certaindistances from the nucleus.

33. Wave Model

34. The Wave Model • Today’s atomic model is based on the principles of wavemechanics. • According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.

35. The Wave Model • In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has. • According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

36. Electron Cloud: • A space in which electrons are likely to be found. • Electrons whirl about the nucleus billions of times in one second • not moving around in random patterns • Location of electrons depends upon how much energy the electron has.

37. Electron Cloud: • Depending on their energy they are locked into a certain area in the cloud. • Electrons with the lowest energy are found in the energy level closest to the nucleus • Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

39. DO THIS NOW!!!! • FIRST • Make sure all of the scientists pages of the booklet are completed! • Sign up to present your answers! • Make sure your lab reports are handed in! • Complete the timeline • Yes, there are scissors on the side counter • Yes, there is glue • PLEASE RETURN ALL ITEMS! • Rafael- clean up your area before you leave!

40. Present Your Answer!! Booklet pages “Developing Models of Matter” (p 8-13) • 5 pages of reading • 5 questions (complete sentences) #1 Nathan #2 Brayden #3 Kobi #4 KALEB #5 Dayna

41. “Progression of the Atomic Model” (2 pages) William 9-4 “Science in Ancient Times (14 pages) Key Words – element Jillian atom Joanna pure substance ____________ hypothesis Calon___________ property EMORY alchemist ____________ #1 Element definition Matt McMyn #2 Priestley preparing oxygen Matthew L. #3, 4 Property and Examples Imesha #5 same substance ____ Questions #1 Dalton __________________ #2 Thomson’s Plum Pudding Model _____________

42. Present Your Answer!! “Developing Models of Matter” (p 8-13) 9-5 • 5 pages of reading & 5 questions (complete sentences) #1 KEVIN #2 Angeli #3 Rajeevan # 4 Maxwell #5 Christian

43. Complete “Progression of the Atomic Model” (2 pages) 9-5 Complete “Science in Ancient Times (14 pages) Key Words – element Louis atom __________ pure substance ____________ hypothesis _____________ property EMORY alchemist ____________ #1 Element definition Aiah_ #2 Priestley preparing oxygen Imoudu #3, 4 Property and Examples Kushpreet #5 same substance Julia Questions #1 Dalton _____________ #2 Thomson’s Plum Pudding Model _____________