1. Unit 3 The History of the ATOM and Atomic Structures
Democritus & Dalton
Schrodinger
Rutherford
Thomson
Bohr

2. Democritus 460-370 B.C. Greek Philosopher
He called nature’s basic particle an atomos, based on the Greek word “indivisible.”
Had no evidence, so people didn’t take him seriously.
He considers education to be the noblest of pursuits, but cautioned that learning without sense leads to error. (THERE MUST BE A POINT!!!)
Aristotle said empty space did not exist & rejected Democritus; said matter was EARTH, WATER, AIR & FIRE

3. John Dalton 1803 Dalton’s Atomic Theory
All elements are composed of tiny indivisible particles called atoms.
Element A
2. Atoms of the same element are identical. The atoms of any one element are different from those of any other elements.

4. Dalton’s Atomic Theory
3. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds.

5. Dalton’s Atomic Theory
4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

6. J.J. Thomson 1897 (with Crookes & Millikan)
discovery of the electron (e-)
disproved Dalton’s theory; atoms DID have smaller parts after all
Used a cathode ray tube to prove that there were charged particles (stream of (-) particles) within the atom

7. Thomson Model Plum pudding model aka chocolate chip cookie model
Millikan found out that electrons were negatively charged and had a mass of 1/1840 of a hydrogen atom
Thomson Model
Plum pudding model aka chocolate chip cookie model

8. Video Links to Experiments
JJ Thomson’s Cathode Ray Tube
How does it work?

9. Ernest Rutherford 1911
Used alpha particles to shoot at gold foil. Got unexpected results. Something was deflecting particles back to the source.
Buried next to Sir Issac Newton and Lord Kelvin in Westminster Abbey.

10. Nucleus
Empty Space
Electrons

11. Rutherford said, “Atoms have a nucleus!”
Dense & positively charged
Contains most of the mass of the atom
Contains the protons (positive charge; heavy)
Electrons are around the outside of the nucleus flying around the empty space

12. Video Links to Experiments
Rutherford’s Gold Foil Experiment
What did he do?

13. James Chadwick Discovered the neutron in 1932 Has a neutral charge
Found in the nucleus
About ½ of the mass of the nucleus
Wanted to know where the extra mass was coming from in an atom; shot beryllium
Eventually led to experiments to bombard atoms with neutrons –> uranium  bombs!

14. Niels Bohr - 1913 Created the Bohr model
Electrons travel in definite orbits/ energy levels around the nucleus
aka “Planetary Model” – orbit the nucleus like planets around the sun
Electrons have fixed amounts of energy “quanta”
Low energy  closer to nucleus
High energy  further from nucleus

15. Bohr Model

16. Werner Heisenberg, 1927
It’s impossible to know both the location and the motion of an electron at the same time
Heisenberg Uncertainty Principle

17. Quantum Mechanical Model (aka wave mechanical model)
Erwin Schrodinger – 1926
Electrons DO NOT orbit the nucleus
Electrons DO have quanta of energy that determine placement in an atom

18. Electron Cloud
Area where there is a high probability that the electron will reside there 90% of the time

19. Schrodinger’s Cat

23. Subatomic Particles

24. Electron Cloud
- electrons o + - - + o
Nucleus
- protons
- neutrons

25. p+ n0 e- Found in the nucleus Mass = 1 (amu) Relative charge = +1
Same mass as a proton
Mass = 1 amu
Relative charge = 0
Found in the electron cloud around the nucleus
Mass 1836 times smaller than a proton
Therefore, mass doesn’t really count! ( amu)
Relative charge = -1

26. Atomic Number Magnesium = Z = 12 12 protons Atomic Number Z = p+
the number of protons in the nucleus
ALL atoms of the PROTONS
SAME element have the IDENTIFY
SAME atomic number and THE ELEMENT!
SAME protons
In a neutral atom, p+ = e-
Magnesium = Z = 12
12 protons
18.99

27. Mass Number (nucleus!) Protons? Electrons? Neutrons? 82 82 82 125
Mass number = p n0
(electrons don’t have much mass!!!)
Z = 82  LEAD, Pb Mass 207
Protons? Electrons? Neutrons? 82 82 82
125
#n0 = MN - # p+
= 207 – 82
= 125 neutrons

28. Mass – atomic = #n (M-A=N) = 14 – 6 = 8 neutrons
An element has an atomic number of 6 and a mass number of 14.
What is the element?
How many neutrons?
Z = 6 CARBON
Mass Number = 14
Mass – atomic = #n (M-A=N)
= 14 – 6
= 8 neutrons

29. 18.99 17 2 Average atomic mass (not the mass #)
Group number (column) the second digit tells us the number of valence (outer) electrons in a Bohr atom
Period number (row) the number of energy levels outside of the nucleus
18.99
Average atomic mass (not the mass #)

30. Bohr model of the atom (protons & neutrons)
Yep! Somebody lied when they told you 8! We’ll go into details in Unit 4. Hold your horses…

31. Identify this atom: (assuming it is neutral p+ = e-)
NUCLEAR NOTATION 16 O 8
HYPHEN NOTATION
Oxygen - 16

32. If we drew a Bohr model of sodium:
How many protons?
How many neutrons?
How many energy levels?
How many electrons? Na 23

33. Scanning Electron Microscopes