1. TOPIC: Atomic Theory – how the model of the atom has evolved and the scientists responsible

2. The Complete Modern Atom
An atom is:
mostly empty space
nucleus has most of mass of an atom
Atom is divisible
Protons (+1 charge)
Neutrons (no charge)
Electrons (-1 charge)
nucleus contains protons & neutrons
electrons are in energy levels around nucleus

3. Here’s how we got there…

4. It Started with the Greeks
DEMOCRITUS
• Lived in Greece 2500 years ago ( BC)
• “Father of modern science”
said: “All matter is made of atoms that are tiny, indestructible and indivisible.”

5. 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!

6. 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

7. 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

8. Democritus
(Greek)
All matter is made of atoms that are tiny, indestructible, and indivisible (can’t be divided)
Atomos

9. Aristotle’s idea of matter
Aristotle did not believe Democritus’s idea of atoms
was correct
Aristotle was more famous than Democritus, so people believed him, even though he was wrong!
Aristotle believed all matter made from four elements:
Earth Water
Air (wind) Fire

10. Aristotle Thought Democritus was wrong
Matter made of 4 elements – Earth
Sun
Air
Water

11. Atom idea lost for ~2000 years

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

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

14. Dalton’s Theory of Atoms
Five parts to Dalton’s modern atomic theory:
# 1:
Elements are made of extremely
small particles called atoms

15. # 2:
Atoms of given element are identical in size, mass, and other properties;
atoms of different elements are different in size, mass, and other properties
(found out not exactly correct)

16. #3: Atoms cannot be subdivided, created, or destroyed (later proved wrong)

17. So Dalton’s atoms are kind of like billiard balls
#4:
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
(H2O Hydrogen :1 Oxygen)

18. #4:
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
(H2O :1 ratio H:O)

19. #5: In chemical reactions, atoms are combined, separated or rearranged 2H2O + 2Na  2NaOH + H2

20. #5: In chemical reactions, atoms are combined, separated or rearranged 2H2O  2H2 + O2

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

22. Dalton Billard Ball Atoms (tiny) Atoms of same element are identical *
Indestructible *
Combine in whole # ratio (H2O)
In chem rxn. are combined, separated, rearranged
*turned out to not be entirely be true
Billard Ball

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

24. Crooke’s tube
JJ Thomson’s discovery of the negatively charged electrons proved that atoms were divisible!

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

26. JJ Thomson Plum Pudding Model
Discovered the Electron (first subatomic particle to be discovered) using cathode ray tubes
Proved Dalton #3 to be wrong
the atom is divisible!
Plum Pudding Model

27. Ernest Rutherford 1871-1937 Discovered the proton in 1920
Won Nobel Prize in
chemistry in 1908
Discovered that most of
mass of atom is in nucleus
And the rest of the atom is empty space – GOLD FOIL EXPERIMENT

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

29. Rutherford gold foil experiment
Rutherford expected a particles to pass straight through, like this…
What he found is that most did go straight through (so atom is mostly empty space), BUT a few a particles bounced back (Concluded there is a nucleus)

30. Rutherford shot a particles, which have a positive charge, since they bounced back he concluded that the nucleus must have a _____________ charge
Positive
electrons (–) /
The NUCLEAR atom model
nucleus
/ [protons (+)]

31. Nuclear Model Rutherford + -discovered the proton (+ charge)
Gold Foil Experiment
Atoms have a positive nucleus
Nucleus contain most of the mass
Atom is mostly empty space
Nuclear Model +

32. Chadwick
- worked with Rutherford
- Discovered the neutron

33. Problem with Rutherford’s Model
OPPOSITES ATTRACT: What prevents (-) electrons from being attracted to (+) nucleus,

34. Niels Bohr

35. Bohr addressed the issue of electrons in the atom
Nucleus surrounded by electrons orbiting at different energy levels
Electrons have definite orbits

36. Bohr’s new atomic model had quantized energy levels, meaning the electrons could only move by jumping between levels (numbered n = 1, n = 2, n = 3, etc.)

37. Planetary Model Electrons travel around the nucleus in orbits (shells)
Bohr
Electrons travel around the nucleus in orbits (shells)
Gain/Emit energy when moving from level to level
Planetary Model e- e- e- + e-

38. Erwin Schrödinger Wave Model
Austrian scientist ( )
won the Nobel Prize in physics in 1933
calculated wave model of hydrogen atom in 1926
also called cloud model, quantum mechanical model, modern model of the atom
atom model we still use today

39. 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

40. If we could see an electron it might look like this “cloud”
In Schrödinger’s wave model of atom, electron behaves as energy wave as well as a 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”

41. Wave Mechanical (Cloud) Model = Modern Model
Schrodinger
Wave Mechanical (Cloud) Model = Modern Model
Electrons behave like waves
Not located in an exact orbit but rather a zone called an orbital
Orbital = probable location of electron

42. The Complete Modern Atom
An atom is:
mostly empty space
nucleus has most of mass of an atom
nucleus contains protons & neutrons
electrons are in energy levels around nucleus
electrons jump between levels, emitting and absorbing energy as jump