Discovering the Atom

Discovering the Atom by Stephanie Praamsma

1a. According to Democritus, what is the composition of matter? According to Democritus all matter is composed of atoms: bits of matter too small to be seen. 1b. What is the root of our word "atom" and how is this root misleading? The word “atom” comes from the Greek word atomos, ‘a’ means ‘not’ and ‘tomos’ means cut, so the greek words definition is ‘uncuttable’. This is misleading because atoms can actually be split. 2. The Greeks believed that fire was a state of matter. How did atomists describe fire? Atomists before Leucippus and Democritus described fire as being one of the four origins of matter, but Democritus and Leucippus described fire as being just like everything else and being composed of atoms. 3. What are the five main ideas of Dalton's theory? What idea compromised the acceptance of his work? All matter consists of tiny particles called atoms, atoms are indestructible and unchangeable, all atoms of an element are identical, atoms of different elements have different masses, and when elements react, their atoms combine in simple, whole number ratios. These are the Dalton’s five main postulates. The idea that when elements react, their atoms sometimes combine in more than one simple, whole-number ratio was the idea that compromised the acceptance of his work because everyone was impressed by the power and simplicity of this idea. 4. Which of the basic ideas of Dalton's Atomic Theory is an extension of what the Greeks believed about atoms? Dalton’s first idea, all matter consists of tiny particles called atoms, is an extension of what the Greeks believed: that after a certain point matter cannot be cut into smaller pieces that retain the properties of the whole. 5. How did William Crookes determine the nature of the beam in cathode ray tube? He determined the nature of the beam by placing barriers in the cathode ray’s path, and using magnets to change its path. 6. What did he conclude from his experiment? He concluded that cathode rays acted like light, seemed to travel in straight lines, cathode rays were NOT light (because light is not affected by magnets), and have some sort of charge. 7. How did Thomson use the cathode ray tube in his research? Thomson used the cathode ray in his research by deflecting the ray with a negatively and positively charged plates. He did this by removing more of the gas in his tube than Hertz had, creating a better vacuum. He also positioned a coil of wire, producing a magnetic field which also deflected the cathode rays, so that the deflection was opposite of the one produced by the electrical field. Then he adjusted them so that the forces were balanced. 8. What did Thomson conclude based on his research?

Since the cathode rays moved toward the positively charged plates, he concluded that the rays must be negatively charged. He also came up with these formulas: F=Eq, F=Bqv, Eq=Bqv, v=E/B. 9. How did Thomson’s findings revise Dalton’s Atomic Theory? His findings revised Dalton’s Atomic Theory by proving that atoms were not the smallest parts of all matter, but are made of smaller particles, like the electon. 10. Press the bulb of the atomizer and observe. Describe what happens to the oil drops.

11. Now turn on the circuit, by pressing the button on the bottom right so that it goes from reading OFF, to reading ON. Spray the atomizer again. Describe the behavior of the drops.

12. Now play with the voltage by changing the potential difference. Spray the atomizer and observe. Move the voltage up and down and observe the drops. Record the voltage that makes some of the drops stay suspended in the middle of the apparatus.

13. Based on the interaction between the oil drops and the electric field, what did Millikan conclude?

14. Describe and sketch Thomson’s model of the atom. What is wrong with the picture presented on the www.iun.edu page?

15. Go to the micro.magnet.fsu site. What do you observe when the slit width is adjusted to 4.0 nm?

16. How did Rutherford’s experiment disprove Thomson’s model of the atom?

17. What new model did Rutherford propose? Sketch it.

18. Which subatomic particle did James Chadwick discover? Compare its mass to that of a proton. 19. Who was credited with the discovery of X-rays?

20. What important discovery did Becquerel make upon examining a piece of pitchblende wrapped up in paper? (Pitchblende is a source of uranium)

21. What two elements did the Curies discover?

22. Becquerel was credited with discovering radioactivity, the spontaneous emission of radiation by a material. He also noticed that both X-rays and the radiation emitted by uranium had strong effects on photographic film, yet they are not the same. What important difference between radiation emitted by uranium and X-rays led him to this conclusion?