1. Particles, Nuclei, and Isotopes
The Nuts and Bolts of
Chemistry

2. Atomic History: An Overview
Early Greeks - Democritus vs Aristotle
Dalton’s Atomic Theory
Thomson - discovery of electrons
Rutherford - discovery of nucleus
Chadwick - discovery of neutron
Bohr - planetary model of atom

3. Atomic History: An Overview
Early Greeks - Democritus vs. Aristotle
Dalton’s Atomic Theory
Thomson - discovery of electrons
Rutherford - discovery of nucleus
Chadwick - discovery of neutron
Bohr - planetary model of atom

4. Early Greeks What is the nature of matter?
-Infinitely divisible pieces of “stuff”
-Earth, Air, Fire ,and Water
No experiments

5. Early Greeks
Aristotle
Democritus vs

6. Democritus vs. Aristotle
New idea
“atomos” - indivisible
Eventually, can’t divide matter any more
First “atomic theory”
Aristotle
Didn’t buy it…
All things are infinitely divisible
Guess who won?

7. CHAMPION
(Aristotle)
For 2000 years, scientists thought all matter was infinitely divisible…

8. Atomic History: An Overview
Early Greeks - Democritus vs Aristotle
Dalton’s Atomic Theory
Thomson - discovery of electrons
Rutherford - discovery of nucleus
Chadwick - discovery of neutron
Bohr - planetary model of atom

9. Dalton’s Atomic Theory
John Dalton
1807
Idea of “atom”
Solid spheres that were indestructible
Unique to each element
Combine evenly
Reactions are rearrangements

10. Dalton’s Atomic Theory
Atoms combine evenly in compounds
Small, whole number proportions
(e.g.) water - H20 (not H1O1/2 ) H O H O

11. Atomic History: An Overview
Dalton’s Atomic Theory
Thomson - discovery of electrons
Rutherford - discovery of nucleus
Chadwick - discovery of neutron
Bohr - planetary model of atom

12. Thomson’s Discovery Studied electricity not atomic structure
His 1st observation was that a magnet could deflect the current and realized that it was made of particles
So to study current he pumped all the air out of a glass tube and applied voltage to two metal plates + (anode)/- (cathode)
J.J. Thomson 1897

13. Thomson’s Discovery (cont…)
He then sent a ray of particles through the tube and saw that they came out of the cathode side and were attracted to the anode side and the tube glowed
This told him that the particles were negative… he called his discovery a Cathode Ray Tube
Sends a “ray” of - particles
Used in TVs and Computers
J.J. Thomson 1897

14. Thomson’s Discovery The beam was attracted to the positive plate.
So, cathode rays are negative
He realized that these negative particle must be ELECTRONS

15. Thomson’s Discovery
Most books give Thomson credit for discovering the proton
He and Millikan found the mass of an electron to be much smaller than an atom
So, electrons are VERY VERY small
Protons must be large in comparison
Plum Pudding Model

16. Plum Pudding Model Preface: “Plum Pudding” atomic model
atoms are solid
made of positively-charged material
with negative “bits” scattered throughout
(like raisins in plum pudding)
(or raisin bread)

17. Atomic History: An Overview
Dalton’s Atomic Theory
Thomson - discovery of electrons
Rutherford - discovery of nucleus
Chadwick - discovery of neutron
Bohr - planetary model of atom

18. Rutherford’s Discovery
Ernest Rutherford
Image courtesty of

19. Gold Foil Experiment To test the Plum Pudding Model (1907):
Shot alpha rays at thin gold foil
(about 2000 atoms thick)
EXPECTED to see the ray scatter as it hit all the solid positive atoms
Like spray from a nozzle

20. Gold Foil Experiment
Here is what they saw:

21. Click here to see an animation.
Gold Foil Experiment
In Detail:
Click here to see an animation.

22. Gold Foil Experiment RESULTS Most of the particles were not deflected
Some were minimally deflected
VERY few (1 in 20,000) bounced back
“as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Rutherford

23. Gold Foil Experiment CONCLUSIONS Plum pudding model wrong
A “nucleus” exists
It is tiny
It is densely-packed and positively-charged (this is the only way a strong positive particle could be deflected)
Empty spaces exist in atoms
LOTS of it!!!!

24. Gold Foil Experiment How much empty space?
Use a billiard ball to represent a nucleus
The electrons occupy a volume one kilometer in ALL DIRECTIONS
Most of that space is EMPTY.

25. Atomic History: An Overview
Dalton’s Atomic Theory
Thomson - discovery of electrons
Rutherford - discovery of nucleus
Chadwick - discovery of neutron
Bohr - planetary model of atom

26. Chadwick’s Discovery (1932)
PROBLEM
There was more mass in nucleus than explained by protons alone
Where did it come from?
NEUTRONS
Chadwick

27. Atomic History: An Overview
Dalton’s Atomic Theory
Thomson - discovery of electrons
Rutherford - discovery of nucleus
Chadwick - discovery of neutron
Bohr - planetary model of atom

28. Bohr’s Atomic Model Nucleus has + charge Electrons have - charge
Why don’t electrons simply “fall into” the nucleus?
Bohr

29. Bohr’s Atomic Model Bohr pictured atoms as little solar systems
Nucleus in center
Electrons “orbiting” in circles
We now know this isn’t correct
But it helps to explain many
things about atoms

30. What Theory Do We Currently Support?
Electron Cloud Model
Nucleus is composed of Protons and Neutrons
Electrons spin very quickly around the nucleus forming a “giant cloud” of negative energy
These electrons can move further away or closer to the nucleus at anytime
Their distance and space is infinite!

31. HOMEWORK
Notes 4.2

32. Protons, Neutrons, and Electrons
Three Main Subatomic Particles
Protons
Located in nucleus
Positively charged (+1)
Mass = “1” mass unit (or 1 gram/mole)
Number of protons = atomic number (Z)

33. Protons, Neutrons, and Electrons
Three Main Subatomic Particles
Neutrons
Located in nucleus
no charge (0)
Mass = “1” mass unit (or 1 gram/mole)

34. Protons, Neutrons, and Electrons
Three Main Subatomic Particles
Electrons
Located outside nucleus
negative charge (-1)
So small we assume mass = 0
Actual mass of g/mol

35. Protons, Neutrons, and Electrons
Atomic number (Z)
shown in lower left
ALWAYS equals the number of protons
Equals the number of electrons in a neutral atom
Isotope Notation C 6

36. Protons, Neutrons, and Electrons
Mass Number (A)
shown in upper left
Is the sum:
neutrons + protons
In this example, carbon has 6 protons and 7 neutrons
Isotope Notation C 13 6

37. Protons, Neutrons, and Electrons
We call this atom
“carbon-13”
Isotope Notation C 13 6

38. Protons, Neutrons, and Electrons
Atoms with the same number of protons can have different numbers of neutrons.
We call such atoms isotopes

39. Protons, Neutrons, and Electrons
Atoms with the same number of protons can have different numbers of neutrons.
We call such atoms isotopes C C C 12 13 14 6 6 6
carbon -12
carbon -13
carbon -14

40. Protons, Neutrons, and Electrons12 13 14 6 6 6
6 protons
6 electrons
6 neutrons
6 protons
6 electrons
7 neutrons
6 protons
6 electrons
8 neutrons
All three kinds of carbon atoms have the same chemistry!

41. Protons, Neutrons, and ElectronsH Pb F 1
208 +2 19 1 82 9
1 proton
1 electron
0 neutrons
Neutrons = A - Z
1 - 1 = 0
82 protons
80 electrons
126 neutrons
Neutrons = A - Z
= 126
9 protons
9 electrons
10 neutrons
Neutrons = A - Z
= 10

42. Protons, Neutrons, and Electrons? 7 3
What element is this?
How do you know?

43. Protons, Neutrons, and ElectronsLi 7 3
What element is this?
How do you know?
Lithium
the atomic number is 3
look on the periodic table

44. Protons, Neutrons, and ElectronsPb ? K
207
235 ? ? 92 19
# of protons (Z)
# of neutrons
# of electrons
mass number (A)
name 82
125
207
lead-207 92
143
235
uranium-235 19 21 40
potassium-40

45. Homework… Complete the Atomic Structure Worksheets
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