Investigating Atoms and Atomic Theory

1. Investigating Atoms and Atomic Theory HISTORY OF THE ATOM AND ATOMIC THEORY

2. Atomos: Not to Be Cut

3. 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. • A model uses familiar ideas to explain unfamiliar facts observed in nature. • A model can be changed as new information is collected.

4. The atomic model has changed throughout the centuries, starting in 400 BC, when it looked like a billiard ball →

5. 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.

6. 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?

7. 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.”

8. 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.

9. This theory was ignored and forgotten for more than 2000 years!

10. Why? • The eminent philosophers of the time, Aristotle and Plato, had a more respected, (and ultimately wrong) theory. Aristotle and Plato favored the earth,fire, air and waterapproach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

11. 500BC-1720Alchemists Now, they wanted to live forever, so they started out with trying to make a potion that could make them live forever. They didn’t succeed, but they did make many experiments, and the scientific method. They also kept careful records.

12. 1777Antoine Lavoisier He had the first version of the Law of Conservation of Matter. He also named oxygen and hydrogen. He invented the first periodic table, which had 33 elements. http://www.seilnacht.com/Lexikon/lavoisi.JPG

13. 1780Charles Augustin de Coulomb He had the theory of Simple Machines. He also had Coulomb’s law which stated the interaction between electric charges. He also published the laws of friction. He used windmills and the elasticity of fibers and metals to come up with this law. http://www.bhak-bludenz.ac.at/physik/geschichte/physiker/bilder/coulomb.jpg

15. 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.

16. Dalton’s Theory • He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles. • 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.

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

18. 1879Sir William Crookes He came up with thallium and helium. He investigated canal rays, cathode rays, and plasmas. He used electricity in gases, and found as the pressure went down, the electrodes began to emit rays. http://www.vanderkrogt.net/elements/images/portret/Crookes.jpg

19. 1896Becquerel He came up with radioactivity. He found this out from unknown x-rays waves, which were produced by uranium. Penetrating radiation was also discovered by Becquerel. He also discovered that uranium was able to expose a photographic plate on black paper, and didn’t depend on the chemical state. http://dbhs.wvusd.k12.ca.us/webdocs/Gallery/Becquerel.GIF

20. 1895W.K. Roentgen He discovered x-rays when he was working with Sir William Crooke’s tube. He also found that x-rays pass through different materials at different temperatures. He was doing an experiment on cathode rays, when he messed up and discovered x-rays. http://hi.fi.tripod.com/timeline/images/wilhelm_rontgen.jpg

21. 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.

22. 1897Plum Pudding Model Discovered by J.J. Thomson. He said the atom was a sphere of positive electricity, with negative particles throughout. This came around right after he discovered the electron.

23. Thomson Model • He proposed a model of the atom that is sometimes called the “PlumPudding” model. • Atoms were made from a positively chargedsubstance with negatively charged electrons scattered about, like raisins in a pudding.

24. 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.

25. 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?

26. 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. But he could never find them.

27. 1898Marie & Pierre Curie They both picked up on Becquerel’s work on uranium, which lead them to find the elements radium and polonium. Though Marie went further with the radioactivity of elements, she didn’t get very far. All she said was, “they were somehow disintegrating over time and emitting radiation that exposed the plate.” ~ Marie Curie http://www.neoam.cc.ok.us/~rjones/Pages/online1014/chemistry/chapter_8/pages/history_of_atom.html#becquerel-curie%20anchor Pierre went on to say that atoms might be composed of even smaller things. http://www.aip.org/history/curie/images/photos/0203-matte.jpg

28. 1900Max Planck He was the person who came up with the original quantum theory. To come up with this, all he did was research from pervious scientists and a little bit on his own, and then wrote a book on the quantum theory (the theory: the behavior or matter and energy in an atom). He also was looking at color changes from energy. He also made a math equation to help with this theory. http://www.spaceandmotion.com/Images/planck-max-quantum-1.jpg

29. 1905Albert Einstein First, he found that nature and matter intertwine somehow, the theory of measurement came about from this. Second, he publish 5 papers on electrons. These contributed to the quantum mechanics. He said, “light consists of quanta, bundles of energy which behave somewhat like particles.” ~Albert Einstein http://www.crystalinks.com/einstein.html http://www.lucidcafe.com/library/96mar/96margifs/einstein.gif

30. 1908Robert Millikan He was by far the most famous American scientist. He wanted to find the electrical charge of electrons. He measured water droplets, and that wasn’t successful, so he measured oil droplets, where all this proved electrons were negatively charged. He also was a professor for many years, and wrote many textbooks on chemistry. http://www.chemistryexplained.com/images/chfa_03_img0536.jpg

31. Robert Millican • In 1907 performed the “oil Drop” experiment which determined the charge of an electron to be 1.6 x 10-19 coulombs. • Using the mass to charge ratio for electrons he determined the mass of an electron to be 9.10 x 10-28 g

32. 1908Hans Geiger He helped invent the Geiger Counter, and the Geiger-Marsden experiment led him to the discovery of the atomic nucleus. The counter was able to prove the Compton effect, because is detected the neutrons, electrons, and quanta. http://www.origin-life.gr.jp/2904/2904174/42.jpg

33. 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.

34. Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

35. 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.

37. 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.

38. 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.

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

40. 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.

41. 1913The Bohr Model Niels Bohr discovered this atom. It shows a positively charged nucleus surrounded by electrons. This was mostly to explain the Rydberg formula. The Bohr model is a hydrogen model. It helps with the learning of the quantum theory. http://en.wikipedia.org/wiki/Bohr_model

42. 1913Niels Bohr Niels figured out the structure of the atom, and their radiations. He also discovered the principle of complementary. He introduced electrons from the book he wrote on the structure of the atom. He also started the basis of the quantum theory. http://www.nrc-cnrc.gc.ca/education/elements/images/elements/Bohrium.jpg

43. 1922Planetary or Solar System Model Both Rutherford and Bohr contributed into this model. Rutherford said the atom is mostly empty space with a nucleus that has a positive charge surrounded by negative electrons. Then Bohr said they circulate the atom.

44. 1920’sElectron Cloud Model Discovered by Erwin Schrödinger and Werner Heisenberg. They said an atom consists of a dense nucleus and many proton and neutrons and is surrounded by electrons, but they all have different energy levels, and different charges.

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

46. 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.

47. Wave Model

48. 1926Erwin Schrödinger Erwin’s contributions were the wave mechanics and quantum mechanics. He wrote papers on wave mechanics, which lead him to the quantum mechanics. To find these theories out, he worked out math equations and used statistics. http://academic.brooklyn.cuny.edu/history/virtual/portrait/schroedinger.jpg

49. 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.

50. 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.