1. ATOMIC STRUCTURE, NUCLEAR CHEMISTRY & UNIT 6
What is an Atom?
ATOMIC STRUCTURE,
NUCLEAR CHEMISTRY &
UNIT 6

2. Democritus
He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further?
Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

3. History of the Atom Aristotle – famous philosopher
Believed all substances made of 4 elements
Blending these in different proportions to get all substances
Fire – hot
air – light
Earth – cool, heavy
Water – wet

4. History of the Atom Late 1700s – John Dalton
Teacher who summarized results of his experiments and those of others
Combined idea of elements with that of atoms

5. Dalton’s Atomic Theory
All matter is made of tiny, indivisible particles call atoms
Atoms of the same element are identical
Atoms of different elements combine in whole number ratios to form compounds
Chemical reactions involve the rearrangement of atoms. No atoms are created or destroyed.

6. Atoms
Smallest particle of an element that still retains the properties of that element

7. JJ Thompson
In 1897, Thompson discovered the first component part of the atom: the electron, a particle with a negative electric charge. 
In 1904, he proposed an initial model of an atom, since nicknamed "Thompsons pudding". 
He imagined the atom as a sphere full of an electrically positive substance mixed with negative electrons "like the raisins in a cake".

8. Thomson’s Experiment
Voltage source
Vacuum tube
Metal Disks

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 end to the positive end

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

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

16. - + Thomson’s Experiment Voltage source
Passing an electric current makes a beam appear to move from the negative end 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 he found that the moving pieces were negative

21. Millikan’s Experiment
Atomizer
Oil droplets - +
Oil
Telescope

22. Millikan’s Experiment
X-rays
X-rays give some electrons a charge.

23. Millikan’s Experiment
From the mass of the drop and the charge on
the plates, he calculated the mass of an electron

24. Other Particles
Proton – positively charged pieces 1840 times heavier than electrons, discovered by E. Goldstein
Neutron – no charge but the same mass as a proton, by J. Chadwick
Where are these pieces?

25. Rutherford’s Experiment
Ernest Rutherford – English physicist
Believed in the plum pudding model of the atom

26. Rutherford’s Experiment
Wanted to see how large the positive particles are
Used radioactivity
Shot alpha particles at gold foil
Alpha particles – positively charged pieces,

27. Rutherford’s Experiment
When an alpha particle hits a fluorescent screen it glows

28. Fluorescent
Screen
Lead block
Uranium
Gold Foil

29. Rutherford’s Experiment
He expected the alpha particles to pass through without changing direction very much
Because…
The positive charges were thought to be spread out evenly
Not enough to stop the alpha particles

30. What he expected

31. Because

32. He thought the mass was evenly distributed in the atom

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

34. What he got

35. How he explained it Atom is mostly empty. Small dense, positive
piece at center.
Alpha particles
are deflected by it
if they get close enough. +

36. +

37. Density and the Atom
Since most of the particles went through, it was mostly empty space
Because the alpha particles turned so much, the nucleus was heavy
Small volume, big mass, big density
The small dense positive area is the nucleus

38. The Nucleus

39. Structure of the Nuclear Atom
Unit 6 – Part 2
Structure of the Nuclear Atom

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

41. Counting the Pieces
Atomic Number – the number of protons in the nucleus
All protons are alike so…
# of protons determines kind of atom
The same as the number of electrons in a neutral atom

42. Counting the Pieces Mass number = # of protons + # of neutrons
Protons and neutrons account for most of the mass of an atom

43. X Symbols Mass number Atomic number
Contain the element symbol, mass number, and atomic number
Mass
number X
Atomic
number

44. F Symbols 19 9 Find: # of protons # of neutrons # of electrons
Atomic #
Mass # F 19 9

45. Br Symbols 80 35 Find: # of protons # of neutrons # of electrons
Atomic #
Mass # Br 80 35

46. Symbols If an element has 91 protons and 140 neutrons, what is the
Atomic #
Mass #
# of electrons
Complete symbol

47. Isotopes Dalton was wrong
Atoms of the same element can have different numbers of neutrons
This means different mass numbers
Called isotopes

48. Naming Isotopes Can also put mass # after name of the element
Carbon-12
Carbon-14
Uranium-235

49. Atomic Mass There are different kinds of oxygen atoms.
More concerned with the average atomic mass

50. Atomic Mass Based on the abundance of each in nature
Do not use grams because numbers would be too small

51. Measuring Atomic Mass Use the atomic mass unit (amu)
One twelfth of a carbon-12 atom
Each isotope has its own atomic mass
Determine average from % abundance

52. Calculating Averages
Multiply atomic mass by % abundance then add results

53. Atomic Mass - Example
Calculate the atomic mass of copper if copper has two isotopes % has a mass of amu and the rest has a mass of amu.

54. Atomic Mass – Example 2
Magnesium has 3 isotopes % has a mass of % has a mass of The rest has a mass of amu. What is the atomic mass?

55. Atomic Mass Is not a whole number because it is an average
The number on the periodic table