1. The Atom
Chapter 4

2. History of the Atomic Theory
A. Democritus
B. Aristotle
C. Lavoisier
D. Proust
E. Dalton
F. Modern Atomic Theory
History of Atomic Structure
A. Thomson
B. Milikan
C. Rutherford
D. Bohr
E. Chadwick
F. Quantum Atom
Subatomic Particles
A. Atomic Number
B. Mass Number and Isotopes
C. Electrons and Ions
D. Nuclear and Hyphenation Notation
E. Average Atomic Mass
Weighing and Counting Atoms
A. Mole Atoms
B. Mole Mass
C. Mass Atoms

3. I. History of the Atomic Theory
Remember: a scientific theory explains behaviors and the ‘nature’ of things
Theories can be revised when new discoveries are made
The theory describing the composition of matter has been revised many times

4. I. History of the Atomic Theory
Democritus ( BC)
1.Matter is made up of “atoms” that are solid, indivisible and indestructible
2.Atoms constantly move in space
3.Different atoms have different size and shape
4.Changes in matter result from changes in the grouping of atoms
5. Properties of matter result from size, shape and movement

5. I. History of the Atomic Theory
B. Aristotle ( BC ) & Others
1. Four kinds of matter
a. Fire – Earth – Water – Air
2. One kind of matter can transform
into another
3. Rejected idea of the “atom” (idea then
ignored for almost 2000 years
4. This theory was more popular and
it was easier to accept

6. Aristotle’s Theory of Matter

7. I. History of the Atomic Theory
Antoine Lavoisier (1770s)
1. Experiment:
2 Sn + O  2 SnO
tin oxygen tin (II) oxide
mass before reaction = mass after reaction
2. Law of Conservation of Mass
a. Matter cannot be created or destroyed (in a chemical or physical change)

9. I. History of the Atomic Theory
Joseph Proust (1779)
1. Develops Law of Definite Composition- all samples of a specific substance contain the same mass ratio of the same elements
a. ex: all samples of CO2 contains 27.3%
carbon and 72.7% oxygen
b. therefore ‘elements’ are combining
in a whole number ratio – WHY????

10. I. History of the Atomic Theory - Dalton
John Dalton (1803)
1. Develops Law of Multiple Proportions
a. describes the ratio of elements by mass in two different compounds composed of the same elements
2. Example: carbon monoxide carbon dioxide
1 part oxygen : 2 parts oxygen
*when compared to the same amount of
carbon in each compound

11. I. History of the Atomic Theory- Dalton
3. Dalton collects data and develops his
atomic theory in 1803

12. 4. Dalton’s Background
Dalton became a school teacher at the age of 12 (he left school at age 11)
loved meteorology - pioneer in this field
studied works of Democritus, Boyle and Proust
Wrote New System of Chemical Philosophy in 1808

13. 5. Dalton’s Atomic Theory
1. Matter is made of small particles-atoms
2. Atoms of a given element are identical in size, mass, but differ from those of other elements*.
3. Atoms cannot be subdivided or destroyed*.
( supports law of conservation of mass)
4.Atoms combine in small whole number ratios to form compounds. (def comp,Mult prop)
5. Atoms combine, separate, or rearrange in chemical reactions.
* Modified in Modern Atomic Theory

14. JUST A THEORY…….
But it lead to the
Modern Atomic theory

15. F. Modern Atomic Theory
All matter is made up of small particles called atoms.
Atoms of the same element have the same chemical properties while atoms of different elements have different properties
Not all atoms of an element have the same mass, but they all have a definite average mass which is characteristic. (isotopes)

16. F. Modern Atomic Theory
Atoms of different elements combine to form compounds and each element in the compound loses its characteristic properties.
Atoms cannot be subdivided by chemical or physical changes – only by nuclear
changes

17. I. History of the Atomic Theory
1803
1897
1909
1913
1935
Today
solid
particle
electron
proton
e- orbit nucleus
neutron
Quantum Atom theory
Dalton
Thomson
Rutherford
Bohr
Chadwick
Schrodinger and others

18. History of the Atomic Structure A. J.J. Thomson (1856-1940)
Experiments with cathode ray tubes
a. atoms have (-) charged particles
which are smaller than atoms
b. determined charge/mass ratio of the
“electron”

19. Voltage source - +
Vacuum tube
Metal Disks

20. Voltage source - +

21. Voltage source - +

22. Voltage source - +

23. Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

24. Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

25. Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

26. Voltage source - +
Passing an electric current makes a beam appear to move from the negative to the positive end

27. Voltage source
By adding an electric field

28. Voltage source + -
By adding an electric field

29. Voltage source + -
By adding an electric field

30. Voltage source + -
By adding an electric field

31. Voltage source + -
By adding an electric field

32. Voltage source + -
By adding an electric field

33. Voltage source + -
By adding an electric field he found that the moving pieces were negative

34. Demonstration of the cathode ray experiment.

35. Thomson’s Model The Pudding Model
a. electrons present
b. atom is like plum pudding - bunch of positive stuff (pudding), with the electrons suspended (plums)

36. History of the Atomic Structure B. Robert Milikan (1868-1953)
1. Oil Drop Experiment (1909)
a. Discovered mass and actual charge of
electron (-1)
b. Mass is 1/1840 the mass of a hydrogen
atom
1) e – has a mass of 9.11 x g

37. Oil Drop

38. History of the Atomic Structure – Summary thus far
So, at this point we know:
- Atoms are divisible into smaller particles
Electrons are negatively charged
The mass of an electron is very small
HOWEVER
Atoms should have a (+) portion to balance
the negative part
- Electrons are so small that some other particles must account for mass

39. II. History of the Atomic Structure Ernest Rutherford (1871-1937)
C. Ernest Rutherford (1909)
1. Discovered the proton p+
2. Received Nobel Prize in Chemistry
3. Gold Foil Experiment (Expectations)
a. Shot alpha particles at atoms of gold
b. expected them to pass straight
through

40. Florescent
Screen
Lead block
Uranium
Gold Foil

41. He thought this would happen:

42. According to Thomson Model

43. He thought the mass of the positive charge was evenly distributed in the atom

45. Here is what he observed:

46. 4. Gold Foil Experiment Results
a. Most positive alpha particles pass right
through
b. However, a few were deflected
c. Rutherford reasoned that the positive
alpha particle was deflected or repelled
by a concentration of positive charge

47. The positive region accounts for deflection

48. 5. Gold Foil Experiment Conclusions
a. the atom is mostly empty space
b. the atom has a small, dense positive center
surrounded by electrons
Rutherford Model of the Atom

49. II. History of the Atomic Structure
At this point in 1909, we know:
p+ = 1.67 x g
e- = 9.11 x g
The charges are balance!
But,
How are the electrons arranged?
There is still mass that is unaccounted for

50. II. History of the Atomic Structure
Niels Bohr (1913)
1. Electrons orbit nucleus in predictable paths

51. History of the Atomic Structure E. Chadwick (1891 – 1974)
1. Discovers neutron in nucleus
2. Neutron is neutral - does
not have a charge n0
3. Mass is 1.67 x g
a. slightly greater than the mass of a proton

52. II. History of the Atomic Structure
F. The Quantum Atom Theory
1. The atom is mostly empty
space
2. Two regions:
a. Nucleus- protons and neutrons
b. Electron cloud- region where you have a 90% chance of finding an electron

53. II. History of the Atomic Structure
Charges balanced
Mass accounted for
However –
what about the
behavior of the
electrons?