Defining the Atom

1. Defining the Atom • Atoms are TINY! • One penny contains 2.4 x 1022 atoms • Earth’s population = 6 x 109 people • Radius of one atom is about 1 x 10-10 m • What is inside the atom?

2. Reviewing the Atom

3. What is our current model of the atom?

4. How did all this information come about? What is the evidence?

5. Early Ideas About Matter: Democritus • Ancient scholars thought matter was made of earth, air, fire, and water • Democritus (~400 BC) was the first person to propose the idea that matter was not infinitely divisible, but made up of individual particles called atomos

6. John Dalton John Dalton was an English chemist and physicist best known for his work in the development of the modern Atomic Theory. http://www.chemheritage.org

7. Dalton’s Atomic Theory (1803) 1. All matter is composed of tiny undividable particle called atoms Originally proposed by Democritus

8. Dalton’s Atomic Theory (1803) 2. Atoms of the same element are identical in their properties. Atoms from different elements are different from those of any other. Contributed by Dalton himself

9. Dalton’s Atomic Theory 3. Atoms combine in simple whole number ratios to form molecules. (Because atoms are unbreakable they must combine in whole number ratios). Incorporates the law of constant composition from Proust

10. Dalton’s Atomic Theory 4. In chemical reactions, atoms are not broken or changed into another type. Incorporates Lavoisier’s theory.

11. Dalton’s Model of the Atom

12. Comparison of Democritus and Dalton http://abyss.uoregon.edu/~js/ast123/lectures/lec04.html

13. Daltons Theory

14. How does Dalton’s theory hold up today?

15. 1. All matter is composed of tiny undividable particle called atoms • FALSE Atoms Exist: Proof = Scanning Tunneling Microscope (STM) However, atom can be divided into sub atomic particles. http://www.nisenet.org/publicbeta/articles/seeing_atoms/index.html http://www.nisenet.org/publicbeta/articles/seeing_atoms/images/STM-med.jpg

17. 2. Atoms of the same element are identical in their properties. Atoms from different elements are different from those of any other. • FALSE. Isotopes of an element have nuclei with the same number of protons but different numbers of neutrons. http://faculty.weber.edu/bdattilo/shknbk/notes/time.htm

18. 3. Atoms combine in simple whole number ratios to form molecules. • TRUE http://www.goalfinder.com/images/SPAPRO6/Dalton-multiple-proportions.jpg

19. 4. In chemical reactions, atoms are not broken or changed into another type. • FALSE. Nuclear Decay converts one type of atom to another http://uncyclopedia.org/wiki/Radioactive

20. Joseph John (JJ) ThomsonDiscovery of Electron, 1897 http://www.manep.ch/img/photo/challenges/nanotubes/thompson.jpg

21. Thomson used a cathode ray to tube to find that NEGATIVELY charged particles existed in the atom. • How did he discover this? • He knew they were negative because they moved to the POSITIVE pole of the ray tube. • He called the NEGATIVE particles, ELECTRONS

22. Cathode Ray Tube

23. Thomson's Cathode Ray Tube

24. The Atom is Divisible! • Work done by J.J. Thomson and others proved that the atom had pieces called electrons • Thomson found that electrons are much smaller than atoms and carry a negative charge

25. Thomson’s Model of the Atom:The Plum Pudding Atom

26. Thomson’s Interpretation - the Plum Pudding Model • The atom is breakable • must have positive charge to balance negative charge of electrons • because there was no experimental evidence of positive matter, Thomson assumed there must be positive energy

27. Consequences of thePlum-Pudding Model • there must be a lot of empty space in the atom • since the electrons are negative, it is assumed you must to keep them apart so they will not repel each other

28. Robert Millikan and the Oil Drop Experiment 1916

29. Millikan’s Discoveries • Determined charge of an electron • Charge = -1 • Also calculated the mass of electron (1916) • Similar values as today

30. Goldstein, 1886

31. Structure of the Nuclear Atom • Protons • Eugene Goldstein observed a positive charged particle in cathode ray tubes in 1896 • He found particles traveling toward the NEGATIVE pole of cathode ray tube (the opposite of Thomson) • Positive particle = PROTON

32. Ernest Rutherford 1911 http://nobelprize.org/nobel_prizes/chemistry/laureates/1908/rutherford-bio.html

33. Rutherford’s Experiment: Trying to prove the plum pudding model. • To prove something is empty…. • Try to shoot something at it and see if it goes through • Target: use large atoms • use very thin sheets of target so do not absorb “bullet” • Bullet: use particles that are smaller than the atom, that you can control and shoot • Alpha a particles are small parts of the nucleus that are EMITTED form a source of radioactive material

34. Alpha Particles Striking Screen Rutherford’s Experiment Radioactive Sample Lead Box Fluorescent Screen Gold Foil

35. Rutherfords Experiment

36. Results of Rutherford’s Experiment 1. Over 98% of particles passed straight through gold foil. 2. About 2% of particles went through but were deflected by large angles. 3. Less than 0.01% of particles bounced off gold foil came straight back. • “…as is you fired a 15” canon shell at a piece of tissue paper and it came back and hit you.” -Rutherford http://upload.wikimedia.org/wikipedia/commons/thumb/c/c3/Rutherford_gold_foil_experiment_results.svg/220px-Rutherford_gold_foil_experiment_results.svg.png

37. Rutherford’s Interpretation of the Results • Over 98% of particles passed straight through gold foil • Most of the atom must be made of empty space • About 2% of (+)particles went through but were deflected by large angles • There must be something with a (+) charge in the atom that repels the alpha particles

38. Rutherford’s Interpretation of the Results • Less than 0.01% of particles bounced off gold foil came straight back. • There must be something VERY DENSE in the atom that deflected the alpha particles

39. Rutherford’s Atomic Model The Nucleus: Contains the positive particles and the mass of the atom Electrons can move around anywhere in atom. Rutherford discovered the existence of the nucleus http://www.faqs.org/docs/qp/chap04.html

40. Problems with Rutherford’s Model 1. Atomicmass • From the results of his experiments, Rutherford was able to calculate the mass of the atom. • However, he was unable to get an accurate mass, he could only get about half the actual mass. For example, he calculated the mass of Boron to be 4 amu. • What was he missing??? • Hint: See Chadwick’s Discovery

41. Problems with Rutherford’s Model 2. Inherent Instability of the Atom. When the electrons circle round the nucleus, they are continuously emitting radiation.  While doing so, they should lose energy, and thus spiral into the nucleus.  But they don’t. http://www.faqs.org/docs/qp/chap04.html

42. Niels Bohr (1915) • Solved the energy problems in Rutherford’s model http://nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-bio.html

43. Quantum Theory • Bohr proposed that while circling the nucleus of the atom, electrons could only occupy certain discrete orbits, that is to say energy levels. As long as electrons stay in these energy levels, they are stable. • A quantum of energy: a discrete amount of energy

44. Bohr Model • Energy exists in steps, it is not continuous • Energy in: Electrons jump up (Absorption) • Energy out: Electrons jump down (Emission)

45. Bohr Model of the Atom • An electron moves around the nucleus only in certain allowed circular orbits • Planetary model

46. James Chadwick (1932) Discovered the neutron http://www.tamu-commerce.edu/physics/links/chadwick.jpg

47. Discovery of Neutron http://library.thinkquest.org/27954/Image24.gif http://www.allaboutspace.com/agifs/Atomstructure.GIF

48. A summary of the History of the Atom Image ref: mrschmidchemistry.com

49. Summary of Scientist • Lego Atomic History

50. Atom is mostly empty space