1. Atoms and their structure
Chapter 2
Atoms and their structure

2. History of the atom Original idea of the atom:
Ancient Greece (400 B.C.)
Greek philosophers Democritus and Leucippus
Empty space and small particles

3. The Beginning of Atomic Theory
Atomos - not to be cut, indivisible
Atoms - smallest particles of matter, different types of atoms exist for every type of matter

4. Lavoisier French chemist (1743-1794)
Observed chemical changes in sealed containers
Mass of reactants = Mass of products
Conservation of Matter

5. Who’s Next? John Dalton: A teacher in Late 1700’s England
Summarized results of his experiments and those of other’s in:
Dalton’s Atomic Theory
Combined ideas of elements with that of atoms

6. Dalton’s Atomic Theory
1. All matter is made of tiny particles called atoms.
2. Atoms are indestructible and cannot be divided into smaller parts.
3. Atoms of the same element are identical, those of different atoms are different.

7. Theory or Law? A scientific law is a fact of nature
Ex: the sun rises, conservation of matter
A theory explains the law
Dalton’s theory explains why matter in conserved

8. Parts of Atoms J. J. Thomson - English physicist, 1897
Cathode ray tube experiment.
Vacuum tube - all air has been pumped out.

9. Thomson’s Experiment
Voltage source - +
Vacuum tube
Metal Disks

10. Thomson’s Experiment
Voltage source - +

11. Thomson’s Experiment
Voltage source - +

12. Thomson’s Experiment
Voltage source - +

13. - + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative to the positive end

14. - + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative to the positive end

15. - + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative to the positive end

16. - + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative to the positive end

17. Thomson’s Experiment
Voltage source
By adding an electric field

18. Thomson’s Experiment
Voltage source + -
By adding an electric field

19. Thomson’s Experiment
Voltage source + -
By adding an electric field

20. Thomson’s Experiment
Voltage source + -
By adding an electric field

21. Thomson’s Experiment
Voltage source + -
By adding an electric field

22. Thomson’s Experiment
Voltage source + -
By adding an electric field

23. Thomson’s Experiment Voltage source + -
By adding an electric field he found that the moving pieces were negative

24. Thomson’s Model Found the electron
Couldn’t find positive charge(for a while)
Said the atom was like plum pudding
A bunch of positive stuff, with the electrons able to be removed

25. Other Subatomic Particles
Proton - positively charged particles
1,840 times heavier than the electron
Neutron - no charge, but the same mass as a proton.
Where are the particles?

26. Rutherford’s Experiment
Ernest Rutherford - English physicist, (1910)
Used radioactivity
Alpha particles - positively charged He nuclei given off by polonium
Shot them at gold foil
which can be made a
few atoms thick

27. Radioactivity

28. Rutherford’s experiment
When the alpha particles hit a fluorescent screen, it glows.
Americium 241, just over 1cm from the screen.

29. Florescent
Screen
Lead block
Uranium
Gold Foil

30. He Expected…
…The alpha particles to pass through without changing direction very much
Because…
The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

31. What he expected

32. Because…

33. Because he thought the mass was evenly distributed in the atom

34. Because he thought the mass was evenly distributed in the atom

35. What he got

36. How Rutherford explained it
Atom is mostly empty
Small, dense, positive particles at center
Alpha particles are deflected by it if they get close enough (like repels like) +

37. +

38. Subatomic Particles Relative mass Actual mass (g) Name Symbol Charge
Electron e- -1
1/1840
9.11 x 10-28g
Proton p+ +1 1
1.67 x 10-24g
Neutron n0 1
1.67 x 10-24g

39. Structure of the Atom There are two regions: 1. The nucleus
- protons and neutrons
- positive charge
- almost all the mass
2. Electron cloud
- Most of the volume of an atom
- The region where the electron can be found

40. Counting the Pieces Atomic Number = number of protons
# of protons determines kind of atom
the same as the number of electrons in the neutral atom
Mass Number = the number of protons PLUS the number of neutrons
Includes all the things with mass

41. Isotopes Dalton was incorrect.
Atoms of the same element can have different numbers of neutrons!
different mass numbers
called isotopes

42. Periodic Table Info
Contains the symbol of the element, the atomic mass and the atomic number
Symbol: letters in the middle
Atomic mass: decimal number (usually at bottom)
Atomic number: whole number (usually at top)

43. Periodic Table Info
Label the following diagram:

44. Atomic Mass How heavy is an atom of oxygen?
There are different kinds of oxygen atoms.
Look at Average atomic mass.
Based on abundance of each element in nature.

45. Atomic Mass
Calculate the atomic mass of copper if copper has two isotopes.
69.1% of Cu atoms have a mass of amu and the rest have a mass of amu.

46. Bohr’s Model Why don’t electrons fall into the nucleus?
Move like planets around the sun.
Circular orbits at different levels.
Amounts of energy separate one level from another.

47. Bohr’s Model
Nucleus
Electron
Orbit
Energy Levels

48. } Bohr’s Model Fifth Fourth Increasing energy Third Second First
Further away from the nucleus means higher energy.
There is no “in between” energy
Energy Levels
Fifth
Fourth
Third
Increasing energy
Second
First
Nucleus