1. Unit 3 - Atomic Structure
Chapter 5
Addison/Wesley Textbook

2. Earliest Model
450 BC – Democritis, a Greek philosopher, first uses the word “atomos” which means indivisible
Definition of atom today – Smallest particle of an element that still retains properties of that element

3. Late 1700’s Lavoisier – Law of Conservation of Matter
Proust – Law of Constant Composition
This says that the same compound from any source always contains the same elements in the same proportion by mass

4. First Atomic Theory John Dalton Proposed in 1803
Compilation of other people’s work and a little of his own
Still true except for one part
Good biography: um_sci/physics/whatis/biograp hy/dalton.html

5. Dalton’s Atomic Theory
Each element is composed of tiny, indivisible atoms
Each element’s atoms are the same and unique
Atoms are only rearranged in any chemical reaction
A compound has the same number and kind of atom.

6. The Atom Today
Since 1981 we have “seen” atoms with a scanning tunneling microscope.
Uses a fine tip and a stable environment to trace the electronic field and image it on a computer
Lots of galleries on the Web:
html

7. A Good Example

8. Discovery Atomic Structure
Early research comes from physicists’ work on electricity
“Electricity” is property of “electron”, which is amber
In ancient Greece, pieces of amber were rubbed and static electricity discharged
Ben Franklin did early research in late 1700’s

9. Great Experimenter

10. His work Discovered two kinds of charges, positive and negative
Opposite charges attract
Like charges repel
Objects pick up charges
They discharge when touched to ground

11. Lightning New research all the time Great photography – Check YouTube
/3214/02.html
hy03.sci.phys.mfw.lightning/

12. Electricity Research after Franklin
Physicists liked to zap things in the mid 1800’s
Cathode ray tube was device used by many (diagram)
Same device used as TV screen

13. Cathode Ray Tube Figure 02-03 Title: Cathode-ray tube. Caption:
(a) In a cathode-ray tube, electrons move from the negative electrode (cathode) to the positive electrode (anode). (b) A photo of a cathode-ray tube containing a fluorescent screen to show the path of the cathode rays. (c) The path of the cathode rays is deflected by the presence of a magnet.
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Cathode Ray Tube

14. Figure 02-04
Title:
Cathode-ray tube with perpendicular magnetic and electric fields.
Caption:
The cathode rays (electrons) originate from the negative plate on the left and are accelerated toward the positive plate on the right, which has a hole in its center. A beam of electrons passes through the hole and is then deflected by the magnetic and electric fields. The three paths result from different strengths of the magnetic and electric fields. The charge-to-mass ratio of the electron can be determined by measuring the effects that the magnetic and electric fields have on the direction of the beam.
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15. How it Works Metal is electrified in an evacuated tube
All metals gave a greenish ray going to the positive electrode
Ray could be attracted by a positive charge, repelled by a negative charge.
It could actually make a paddle wheel move - particle

16. Discovery of the Electron
JJ Thomson – Cavendish Lab
Used cathode ray tube to determine amount of deflection
Determined that particle has a negative charge
Determined the charge to mass ratio of the particle
Animation: hill.com/sites/ /student_view0/chapte r2/animations_center.html#

17. Finding the Charge of an Electron
American physicist – Robert Millikan
Famous Oil Drop Experiment (handout)‏
See animation

18. Figure 02-05
Title:
Millikan's oil-drop experiment.
Caption:
A representation of the apparatus Millikan used to measure the charge of the electron. Small drops of oil, which had picked up extra electrons, were allowed to fall between two electrically charged plates. Millikan monitored the drops, measuring how the voltage on the plates affected their rate of fall. From these data he calculated the charges on the drops. His experiment showed that the charges were always integral multiples of x C, which he deduced was the charge of a single electron.
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19. Explanation Drops of oil are sprayed into a chamber
X-rays cause electrons to be formed
they cling to oil (in varying numbers)‏
Drops pass through a set of electric plates which have a charge
Millikan adjusted charge to balance the charge on each drop
Found the greatest common factor

20. Conclusion Charge on an electron is 1.60 X 10-19 Coulombs
Mass of an electron is 9.11 X grams
Virtually without mass

21. Discovery of Radiation
Henri Becquerel accidentally discovered radiation in 1896
Photographic plate wrapped and put in drawer for weekend gets exposed
Rock was “radiating” something
Rock was pitchblende which contains radium

22. Characteristics of radiation
Spontaneously emitted by some elements
Studied by Marie and Pierre Curie
They discovered several elements, including uranium and polonium
Atom emits radiation and then changes
This gave clues to what atom is actually made of

23. Marie and Pierre Curie
Good site
ory/curie/

24. Figure 02-06
Title:
Marie Sklodowska Curie ( ).
Caption:
When M. Curie presented her doctoral thesis, it was described as the greatest single contribution of any doctoral thesis in the history of science. Among other things, Curie discovered two new elements, polonium and radium. In Henri Becquerel, M. Curie, and her husband, Pierre, were jointly awarded the Nobel Prize in physics. In 1911 M. Curie won a second Nobel Prize, this time in chemistry.
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25. Further Research on Radiation
Ernst Rutherford is brought to Cavendish Lab in early 1900’s
Studied radioactivity
Analyzed nature of radiation
Handout

26. Figure 02-08
Title:
Behavior of alpha (α), beta (β), and gamma (γ) rays in an electric field.
Caption:
The α rays consist of positively charged particles and are therefore attracted to the negatively charged plate. The β rays consist of negatively charged particles and are attracted to the positively charged plate. The γ rays, which carry no charge, are unaffected by the electric field.
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27. Magic Bullet Alpha Particle chosen Right size
Could be detected afterwards
Helium nucleus – 2 protons and 2 neutrons
+2 charge

28. Gold Foil Experiment Rutherford got grad students to design set up
Geiger and Marsden
Wanted to confirm Thomson’s “Plum Pudding” model of the atom – electrons stuck in positive pudding
Handout

29. Figure 02-09
Title:
J. J. Thomson's "plum-pudding" model of the atom.
Caption:
Thomson pictured the small electrons to be embedded in the atom much like raisins in a pudding or seeds in a watermelon. Ernest Rutherford proved this model wrong.
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30. Figure 02-10
Title:
Rutherford's experiment on the scattering of α particles.
Caption:
The red lines represent the paths of the α particles. When the incoming beam strikes the gold foil, most particles pass straight through the foil, but some are scattered.
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31. Explanation Find a source of alpha particles
Aim them at a piece of gold foil
Check to see where they come out by counting fluorescent spots

32. Results Most went through
Very small number were deflected almost straight back
Only explanation was that all matter was concentrated into a dense nucleus
Nucleus had a positive charge
Electrons traveled in empty space around the nucleus
Movie: Empty Space

33. Results Most went through
Very small number were deflected almost straight back
Only explanation was that all matter was concentrated into a dense nucleus
Nucleus had a positive charge
Electrons traveled in empty space around the nucleus
Movie: Empty Space – next slide

34. Atom is Empty Space
From NOVA

35. Figure 02-11
Title:
Rutherford's model explaining the scattering of α particles.
Caption:
The gold foil is several thousand atoms thick. Because most of the volume of each atom is empty space, most α particles pass through the foil without deflection. When an α particle passes very close to a gold nucleus, however, it is repelled, causing its path to be altered.
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36. Modern Atomic Theory
There are 3 major subatomic particles (protons, neutrons, and electrons).
There are basic particles that make these up but we will not discuss them
The proton also came from the cathode ray tube
The neutron was discovered by Chadwick, a student of Rutherford in 1935.

37. Summary of Particles OUTSIDE NUCLEUS VERY SMALL LARGE -1 NONE +1
ELECTRON
NEUTRON
PROTON

38. Planetary Model Proposed by Rutherford
Electrons orbit nucleus like planets around sun
Atoms are neutral so
#protons = #electrons
Charge on electron: X C or “1”
Mass of proton: 1.67 X g or 1 amu
(atomic mass unit)‏

39. Atomic Number Defined by Henry Mosely (1887-1915)‏
Student of Rutherford
Unique for each element
Number of protons in the nucleus
What is atomic number of
nitrogen? Uranium?

40. Isotopes Means “type or form”
All atoms of the same element have the same number of protons
There may be different types of the same elements, called isotopes
Vary in number of neutrons, mass
Try Carbon-12 and Carbon-14

41. Characteristics of Isotopes
Varying masses
Same chemical and physical properties
Some may be unstable, and therefore radioactive

42. Symbol Carbon-12 12 is mass number, # protons + # neutrons
Also written
126C
Mass # - Atomic # = # of neutrons

43. Atomic Mass Mass of an isotope in amu’s is simply the Mass number
Most elements have several common
isotopes
Mass on periodic table must reflect this,
that is why there are decimals
Weighted average calculation (like grades)‏

44. Calculation
Multiply the mass of each isotope by its abundance as a decimal
Add each of these to get weighted average
Try one

45. Mass Spectrometer Inject gaseous form of element
Strip electrons (positive charge)‏
Sort by size with a magnetic field
Computer counts the isotope and gives
a readout

46. Figure 02-13
Title:
A mass spectrometer.
Caption:
Cl atoms are introduced on the left side of the spectrometer and are ionized to form Cl+ ions, which are then directed through a magnetic field. The paths of the ions of the two isotopes of Cl diverge as they pass through the magnetic field. As drawn, the spectrometer is tuned to detect 35Cl+ ions. The heavier 37Cl+ ions are not deflected enough for them to reach the detector.
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47. Animation
EMOS/MassSpec.html

48. Figure 02-14
Title:
Mass spectrum of atomic chlorine.
Caption:
The fractional abundances of the 35Cl and 37Cl isotopes of chlorine are indicated by the relative signal intensities of the beams reaching the detector of the mass spectrometer.
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