1. The Atomic Theory Aim: What is the atom and how has it changed since its discovery?

2. Atomic Theory Do Now Name a murder mystery movie that you’ve seen? In that movie who did you originally think was the killer/thief was? As the movie when on, did you have to change your mind about who really was the guilty person?

3. Atomic Theory Atomic Theory is like a mystery, that scientist have been trying to solve for centuries. But instead of trying to find who is the guilty party, this mystery is an attempt to find what the atom really looks like. Each time new evidence is discovered the appearance of the atom changes to incorporate this new information.

4. Atomic Theory The Greek Philosopher Democritus (460 – 370 BC) thought that if you cut a substance in half a over and over again, eventually you would end up with a particle that could not be divided anymore. Democritus proposed that this particle that could not be cut any more, would be called the atomos. Which means unable to divide.

5. Atomic Theory According to Democritus all atoms were small hard particles made of the same material formed into different shapes and sizes

6. Atomic Theory Democritus thought: –Matter could only be cut up so many times, eventually there would be a piece that could not be cut any smaller. –All matter is made of these particles that he called Atomos. –Atomos were made of the same substance, but different shapes and sizes made different materials.

7. Copy into your notes Using a separate sheet of paper, draw a model of three atoms according to how Democritus thought atoms (atomos) looked like.

8. Copy into your notes Atom: The smallest unit of an element that maintains the properties of that element.

9. Atomic Theory During the years since Democritus first theorized the atom, scientist learned more about atoms through many experiments. The model of the atom has been altered and continues to be changed, to reflect each new piece of information we learn.

10. Atomic Theory John Dalton, a British Chemist in the late 1700’s, did many experiments with different substances in order to learn more about the atom. His experiments suggested that elements combine in certain proportions because they are made of single atoms

11. Homework Explain why the appearance of the atom has changed over time. How did Democritus come up with his model of the atom? What did Democritus’ atomos look like? Include a picture to help explain your description.

12. Atomic Theory Dalton published his Theory in 1803 and in it he said that –All substance are made of atoms, –All atoms are small particles that cannot be created, divided, or destroyed –Atoms of the same element are exactly alike and atoms of different elements are different –Atoms can join with other atoms to make new substances.

13. Atomic Theory Daltons Atom Small, Indivisible, and indestructible The Atom are tiny spheres that were hard and unbreakable

14. Atomic Theory In 1897 a British Scientist named J.J. Thomson showed that Dalton was incorrect about atoms being indivisible. There were things that were smaller than a atom and these things could be separated from the atom

15. 1) All gas is removed from the glass tube 2) An invisible beam was produced when the tube was connected to a source of electricity 3) Metal Plates could be charged to change the path of the beam. 5) When the plates were charged the beam made a glowing spot here 4) When the plates were not charged the beam made a glowing spot here

16. Atomic Theory Thomson concluded that: 1.the beam was made of particles that have negative electric charges 2.These negatively charged particles are present in every kind of atom 3.The atom was divisible.

17. Copy into your notes Electron: a subatomic (smaller than an atom) particle that has a negative charge. The negatively charged particles that Thomson discovered are now called electrons

18. Homework What did Daltons model of the atom look like. Include a picture to What did Dalton theory say about atoms? How did Thomson’s experiments change the way atoms look? How did it stay the same? What did Thomson’s model of the atom look like?

19. Atomic Theory Thomson’s Atom Plum Pudding Model of the atom Named after a popular desert of the time, my Plum Pudding Model had the electrons embedded in a soft atom like fruit in a yogurt

20. Atomic Theory In 1909 a student of Thomson’s named Ernest Rutherford designed an experiment to test Thomson’s theory. Rutherford aimed a beam of, charged particles at a thin sheet of gold foil. Rutherford expected for the heavy alpha particles to pass right through the thin sheet of foil. What he found was really surprising. Every once in a while one of these particles bounced right back or were deflected to the side.

21. An element such as radium produced the positively charged alpha particles The Lead focuses the particles in to a small stream Most of the particles pass straight through the gold foil Some Particles were slightly deflected from the straight path A few particles bounced straight back

22. Atomic Theory Of this surprising result Rutherford stated: –“It was quite the most incredible event that has ever happened … It was almost as if you fired a fifteen-inch shell into a piece of tissue paper and it came back and hit you” –Imaging throwing a brick at a piece of toilet paper and it bouncing back at you.

23. Atomic Theory Rutherford theorized that in order to get results in which “Bricks” thrown at paper fly back at you. The atom could not be like a pudding or yogurt.

24. In order to get alpha particles to bounce back or get deflected the atoms of the gold foil had to be mostly empty space with a small dense core. When the alpha particle hit the atoms core, the particle bounced back. Most of the time, however, the particles go straight through the foil encountering only empty

25. Rutherford’s Model of the Atom Small core with the electrons occupying the space around it My atom has a very small center with the electrons spinning around this nucleus

26. Copy into your notes Nucleus: an atoms central region, which is made up of protons and neutrons and is positively charged. Electron Cloud: the region around the nucleus of an atom where electrons are likely to be found.

27. Atomic Theory The next leap in the hunt for the atom was thanks to the work of Niels Bohr in 1913. Bohr was a Danish scientist who worked with Rutherford and studied the way that atoms reacted to light. Bohr wondered why atoms emitted different wavelengths of light when exposed to different amounts of energy.

28. When the electrons lose the energy, they would fall back down to a more stable energy level. And a specific color of light would be emitted. When exposed to energy, these electrons jump from one energy level to another. Bohr theorized that the electrons of an atom didn’t just orbit the nucleus randomly, but that they followed particular paths called energy levels.

29. Bohr’s Planetary Model I call this my planetary model because I theorized that the electrons orbit around the nucleus like planets around the sun

31. Modern Atomic Theory Aim: What is the Modern Atomic Theory

32. Have you ever taken a picture that came out blurry? What happened to make your picture that way? Take a look at the picture on the next slide and make an inference about what was happening when the picture was taken?

34. Modern Atomic Theory Many more scientists have added to our understanding of the atom since Bohr. We now “know”, thanks to physicists like Schrödinger and Heisenberg, that electrons don’t travel in definite paths as Bohr suggested. Instead electrons are found in regions around the nucleus called the orbitals

35. Modern Model of the Atom Electrons moving at nearly the speed of light around the nucleus could be found in at any place in one of these orbitals at any instant These orbitals helped to explain why, when bonded to other atoms the molecules take on certain shapes

36. Modern Atomic Theory States 1.Elements are composed of atoms, which are extremely small. 2.All atoms of a given element have the same chemical properties 3.All atoms of a given element contain the same number of protons. 4.Compounds are formed by the chemical combination of two or more different kinds of atoms. 5.Atoms are the units of chemical change

37. Atoms are composed of three types of particles :` Particles SymbolChargeLocation Protonsp+p+ Positive chargeFound in the nucleus NeutronsN0N0 Neutral (no) charge Found in the nucleus Electronse-e- Negative chargeFound orbiting the nucleus

38. Homework Using the notes and sketches that you took to day, make an timeline of the advances in the search for the atom –should be in chronological order –Should include the scientist involved –Should include a model of the atom –And evidence for the change from previous model