Brief Timeline of Atomic Theory

2. Brief Timeline of Atomic Theory

3. Democritus • 400BC • Greek philosopher

4. Hard Particle (Cannonball)Theory • Proposed that they world was made up of tiny, indivisible particles moving through a void of empty space • “atom” comes from the Greek word “atomos”, meaning indivisible (cannot be divided)

5. John Dalton • 1808 AD • First modern atomic theory

6. Daltons Atomic Theory • All matter is composed of tiny, indivisible particles called atoms • All atoms of an element are identical • Atoms of different elements are all different • Atoms combine in simple ratios to form compounds

7. J.J. Thomson • 1897-1904 • “Plum Pudding Model” • Cathode Ray tube experiment • demo

8. demo

9. Cathode Ray Tube • Thompson showed that cathode rays (electrons) were composed of negatively charged particles that separated from the gas atoms inside the tube • Significant because: this meant that atoms are not hard, indivisible particles. Atoms are composed of smaller “subatomic” particles

10. Thomson’s Plum Pudding Model • The atom was a hard sphere that was positively charged with negatively charged electrons that “dotted” the atom like raisins in plum pudding

11. The discovery of radioactivity • Henri Becquerel • 1896 • Discovered that uranium ore released rays that could expose photographic film

12. The discovery of radioactivity • Marie & Pierre Curie • Extracted 2 new elements from uranium (U)ore: radium (Ra) and polonium (Po) Marie Curie

13. Ernest Rutherford Magnetic Field Experiment Gold Foil Experiment (1911) Lead to discovery of the nucleus, as a positively charged center of atom, containing the mass Most of the atom is negatively charged empty space, electrons are outside the nucleus • Was able to separate radioactive rays into 2 types: alpha (a) & beta (B) • Determined that a rays were composed of helium nuclei (He +2 charge)

14. Magnetic Field Experiment

16. Gold Foil Experiment

17. Gold Foil Experiment

18. Gold Foil Experiment

19. Gold Foil Experiment

20. Rutherford’s Atomic Model

21. Rutherford’s “Nuclear Model” • Most of the atom is negatively charged empty space, surrounding a small, positively charged nucleus, containing most of the mass of the atom

22. Modern Theory of Atomic Structure • Developed by Niels Bohr, based on the science of nuclear physics • Bohr determined that an element's position on the periodic table was related to its electron configuration.

23. Electron configuration Electron configuration – shows how many electrons are in each energy level or “ring” • Ex: Carbon 2-4

24. Bohr’s Planetary Atomic Model • NielsBohr (1922) • Determined that electrons rotate around the nucleus in discrete paths or rings

25. Planetary Model of Atomic Structure

26. Wave-Mechanical Model • Current (modern) theory of atomic structure • Moseley used x-ray analysis to calculate an integer for each element: these integers are the atomic numbers

27. Wave-Mechanical Model • There is a tiny, dense positively charged nucleus at the center of a huge negatively charged electron cloud

28. Wave-Mechanical Model

29. Orbital • Region of probability of finding an electron

30. “The whole point:” • The modern model of the atom is the result of many investigations that have been revised over a long period of time by many scientists • Atomic theory song

31. Place the models of atomic structure in order from earliest to the modern theory:

32. Basic Atomic Structure • The nucleus occupies less than 0.01% of the total volume of an atom but accounts for 99.97% of its mass. Thus most of an atom isEMPTY SPACE wherethe ELECTRONS are found, this is called an ELECTRON CLOUD. • One atomic mass unit is 1/12TH THE MASS OF A CARBON-12 ATOM. This is the standard by which the masses of all other elements are determined. It is abbreviated “u”.

33. Subatomic Particles

35. Use your Periodic Table to complete the following:

36. The only number that never changes for an element is ATOMIC NUMBER !!

37. Atomic Structure 1

38. Atomic Structure 1

39. **Shade the columns representing the nucleons light blue

40. ISOTOPE • Forms of the same element having different mass due to different number of neutrons. • Indicated by “element name-mass”

41. 158O 168O Name: _______________ Mass: ________________ Protons: ______________ Neutrons: _____________ Name: _______________ Mass: ________________ Protons: ______________ Neutrons: _____________

42. Practice:

43. The mass number on the periodic table indicates the weighted average of all the naturally occurring isotopes of an element To calculate a weighted average: % X mass + % X mass + ….. 100 100

44. Neon is naturally found in nature having 90.51% mass of 20.00u, 0.24% mass of 21.00u and 9.22% mass of 22.00u. Calculate the weighted atomic mass of neon.

46. Calculate the weighted average atomic mass of the elements below. Show all work, round to the nearest hundredth. a.)99.63%14N & 0.37%10N b.)69.1%63Cu (actual mass of 63.93g) & 30.9%65Cu (actual mass of 64.93g) c.)78.9%24Mg, 10.00%25Mg & 11.01%26Mg

47. You can estimate which isotope is found in the highest abundance as the one with a mass closest to the mass listed on the periodic table Example: Chlorine-35 mass 34.969g Chlorine-37 mass 36.966g Look on the periodic table for the mass of chlorine ____________________________ The more abundant isotope has a mass closer to the mass given on the periodic table:_____________

48. Practice: Which isotope of silicon would be found in the highest percentage? 2814Si, mass 27.977 2914Si, mass 28.976 3014Si, mass 29.974 Why?

49. Atomic Structure 2

50. 2.) Calculate the weighted average of the following naturally occurring isotopes. SHOW ALL WORK!