1. A History of Atomic Theory Atomic Models

2. What is a model ? MODEL: detailed, 3-D representation of an object,
(typically on smaller scale than original)
model airplane
model car
clothing model

3. Let’s Take a Trip Through Time!

4. It Started with the Greeks
DEMOCRITUS
• Lived in Greece 2500 years ago ( BC)
• “Father of modern science”

5. Demacritus said: “All matter is made of atoms that are tiny, indestructible and indivisible.”

6. Democritus Where did Democritus get his ideas for
Greek banknote with Democritus and drawing of atom
Where did Democritus get his ideas for
small, indestructible, indivisible atoms?

7. Atoms are small…
new rings
old ring
EVIDENCE: Old gold rings wear away slowly, getting thinner and thinner, but you never see gold atoms on your finger, so gold atoms must be very small!

8. Atoms are indestructible…
EVIDENCE:
Earth has been around long time
Mountains wash away but rocks continue to exist
New plants grow where old plants die

9. Atoms are indivisible…
EVIDENCE: None
This was HYPOTHESIS made by Democritus to explain nature as he saw it.

10. What did Democritus think atoms looked like?
Appearance of atoms assumed based on behavior:
Liquids pour
Solids are hard and rigid

11. Liquids pour. Why? Liquids require a container
If liquid atoms were like little balls they would roll out when you tip the container
Greek idea of liquid atom

12. Solids are rigid. Why? cocklebur plant
cockleburs stick on clothing and each other
little hooks on cockleburs
Velcro® fastener
Velcro hooks and loops
Greek idea of solid atom

13. Aristotle had other ideas
Greek banknote and coin picturing Aristotle
ARISTOTLE: Famous Greek philosopher, born 384 BC.
student of Plato (another famous philosopher)
teacher of Alexander the Great (who later conquered the world)

14. Aristotle’s idea of matter
Aristotle did not believe Democritus’s idea of atoms
was correct
Aristotle believed all matter made from four elements:
Earth
Air
Fire
Water

15. Aristotle’s idea of matter
Aristotle was more famous than Democritus, so people believed him, even though he was wrong!
Democritus’s idea of atomos (atoms) was lost for nearly 2000 years until John Dalton brought it back in 1803

16. Atom idea lost for ~2000 years

17. John Dalton, New Atom,1803 Born in England, 1766
Studied chemistry, physics, and color blindness
Brought back Democritus’s idea of an indivisible atom

18. Color blindness Can you see a number in this box?
If not, you may be color blind.
(More males are color blind than females)

19. Start writing -
Foldable information

20. Dalton’s Theory of Atoms
Five parts (postulates):
# 1:
Elements are made of extremely
small particles called atoms

21. # 2:
Atoms of given element are identical in size, mass, & other properties;
Atoms of different elements are different in size, mass, & other properties
(later found not to be exactly correct)

22. #3: Atoms cannot be subdivided, created, or destroyed
(later proved wrong)

23. So according to Dalton’s theory:
atoms are considered to be like billiard balls

24. #4:
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
(H2O :1 ratio H:O)

25. #5: In chemical reactions, atoms are combined, separated and/or rearranged

26. Ex. of a chemical reaction:+ +  +
2H2O Na  2NaOH + H2

27. Dalton’s Chemical Symbols
We use different symbols today

28. Matter is electrical! Michael Faraday (English chemist, born 1791)
1834: learned matter interacts with electricity
Realized electricity had to be made up of particles that could be counted, but didn’t know what they were (electrons)

29. J.J. Thomson English physicist (1856-1940) won Nobel Prize in 1906
1897:
discovered ELECTRON
- studied cathode rays using
Crooke’s tube
- showed atoms were
divisible

30. Crooke’s tube
| cross-shaped anode
cathode |
– cross-shaped shadow
Cathode rays (stream of electrons) move from metal cathode (on left) to cross-shaped anode (on right) & cast cross-shaped shadow on glass (on right)
So … Electrons are particles with negative charge

31. Cathode Ray Tube Experiment
thought cathode rays were streams of particles smaller than atoms

32. electric &
magnetic fields
deflect beam of
charged particles

33. What do we mean by charge?
ELECTRICAL CHARGE:
property of matter:
either more or fewer electrons than protons
ELECTRONS are negatively charged (-1)
PROTONS are positively charged (+1)
MATTER IS NEUTRAL (no charge):
(+) charges equal (-) charges
charges cancel out each other
batteries have (+) and (-) ends

34. More About Charge…
opposite charges attract, like charges repel

35. JJ Thomson’s discovery of (-) charged electrons proved that atoms were divisible!

36. Thomson’s “Plum pudding” atom
– electron
ENGLISH PLUM PUDDING
raisin

37. RULE: Matter is NEUTRAL (no charge) so… for every one (-) charged electron there must be one some thing of (+) charge

38. Dalton’s Billard Ball Model
we move from
Dalton’s Billard Ball Model to
Thomson’s “Plum pudding” model

39. Thomson’s Plum Pudding Model:
(+) charge evenly spread
out while…
(-) charge found in bits –
like raisins in plum pudding
(or chocolate chips in cookies)

40. Ernest Rutherford 1871-1937 1920: discovered proton
1908: won Nobel Prize (Chemistry)
Discovered that most of
mass of atom is in the center
(nucleus)

41. Rutherford’s Experiment – 1911
Rutherford’s exp’t: animation

42. Rutherford tested plum pudding model of atom:
sent tiny radioactive alpha (a) particles, like bullets, towards thin sheet of gold foil

43. Rutherford gold foil experiment
expected a particles to pass straight through, like this…
most a particles went straight through or didn’t bump into anything showing most of atom is empty space

44. most a particles went straight through, BUT a few were deflected, meaning a particles must have hit something:
really heavy OR
(+) charged

45. gold foil experiment scattering results showed: Plum pudding model of atom was INCORRECT, so… Rutherford devised new model that fit his results… proposed Nuclear Model

46. NUCLEAR model Rutherford concluded:
all atoms have dense, positive (+) center
(center contains most of mass of atom)
atom is mostly empty space
(except very tiny (–) electrons; but didn’t say where are)
electrons (–) /
NUCLEAR model
nucleus
/ [protons (+)]

47. Rutherford model
did not speculate how
electrons arranged
around (+) center
(name nucleus came later)

48. Problem with Rutherford’s Model
To prevent (-) electrons from being attracted to (+) nucleus, electrons must orbit nucleus like Earth orbits sun
But…
e- need energy to orbit nucleus;
Rutherford didn’t address this

49. So how BIG is the nucleus compared to the entire atom?

50. If atom is as big as football stadium
nucleus smaller than flea on 50-yard line!

51. If atom as big as period at end of sentence in textbook,
it would have mass of 70 cars! .

52. So how big is an atom? most atoms are 1-2 angstroms across
1 Å = 1 X m
6 Billion Cu atoms in a line = less than 1 meter!
Can we “see” an atom?

53. James Chadwick 1891-1974 worked with Rutherford
1932: discovered neutron
1935: Nobel Prize (Physics)

54. Niels Bohr 1885 - 1962 created quantized atom model, 1915
worked on Manhattan Project
won Nobel Prize in 1922
Bohr’s model: e- only move from one energy level to another in atom:
absorb energy when jump from lower to higher levels
emit energy when jump from higher to lower levels

55. Bohr addressed issue of atom’s electrons:
nucleus surrounded by e- orbiting at different energy levels
e- have definite orbits

56. Quantum Mechanical Theory Electron in a Hydrogen atom

57. Bohr’s new atomic model had quantized energy levels
e- only move by jumping levels (n = 1, n = 2, etc.)

58. electrons:
absorb energy when they jump away from nucleus
emit energy when they jump towards nucleus

59. Bohr’s Planetary Model
electrons travel only in specific orbits
each orbit has definite energy
inner most orbit (n=1): least energy
outer most orbit (n=7): most energy
atoms emit radiation when e- jumps from outer orbit to inner orbit
e – in outermost orbits determine atom’s chemical properties

60. Erwin Schrödinger Wave Model
Austrian scientist ( )
1933: won Nobel Prize (Physics
calculated wave model of hydrogen atom (1926)
atomic model called:
modern model atom model
quantum mechanical model
wave model
cloud model

61. If we could see an electron it might look like this “cloud”
Schrödinger’s wave model of atom:
e- behaves as energy wave AND as matter particle (light also behaves as particle and wave)
Einstein had predicted that energy and matter were related in his equation E = mc2
If we could see an electron it might look like this “cloud”

62. Modern/Wave/Cloud Model
electron’s energy is quantized (specific values)
electrons in probability zones called “orbitals”, not orbits
- location cannot be pinpointed
electrons are particles & waves at same time
electrons move around nucleus at speed of light (3 x 108m/sec)

63. Orbitals

64. A Missing Particle – The Neutron
1932: James Chadwick
discovered last major piece of atom: neutron
(“neutral”, no charge)
1935: won Nobel Prize (Physics)
neutron& proton weigh about same while electron weighs almost nothing
( )

65. The Modern Model an atom is: mostly empty space
nucleus: most of atom’s mass
nucleus contains protons & neutrons
electrons in energy levels around nucleus
electrons jump between levels, emitting & absorbing energy as jump

67. The development of atomic theory represents the work of many scientists over approximately 100 years

68. Next Atomic Theory ?
Which one of you will discover something new to add to the atomic model of the atom and win a Nobel Prize in the future?