1. Chapter 7: The Theory of the Atom

2. The Present Day Model of the Atom: The Quantum Mechanical Model

3. SUBATOMIC PARTICLES Atoms are composed of three subatomic particles:
1. PROTON
2. NEUTRON
3. ELECTRON

4. The Proton Positively Charged Located in nucleus
Mass equal to the mass of a neutron
Discovered by Goldstein

5. The Neutron No Charge Located in nucleus
Mass equal to the mass of a proton
Discovered by Chadwick

6. The Electron Negatively Charged Located outside the nucleus
Mass approximately 1/2000th the mass of a proton.
Discovered by Thomson

7. Atomic Structure
Atomic number: the # of protons = the # of electrons in an atom
Atomic mass: the average mass of all isotopes (not rounded to a whole number)
Mass number: the # of protons & neutrons in an atom (atomic mass rounded to the whole number) 12 6

8. Calculating # of P, N & E # Protons = atomic number = # Electrons
(Atoms are neutral therefore the # of positive protons must equal the # of negative electrons)
Mass Number = # protons + # neutrons
= # of nucleons
# Neutrons = mass number - atomic number

9. Atomic Symbols Show the mass number and atomic number
Give the symbol of the element
mass number
23 Na sodium-23
atomic number 11

10. Atomic Symbols Al # P ____ # E ____ # N ____27
Al # P ____ # E ____ # N ____
Fe # P ____ # E ____ # N ____ 13 56 26

11. Isotopes
Atoms with the same number of protons, but different numbers of neutrons.
Atoms of the same element (same atomic number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl
chlorine chlorine - 37
Each one of these is called a nuclide of chlorine.

12. Isotopes

13. Atomic Mass = (%)(mass)+ (%) (mass) + (%)(mass)
Average Atomic Mass
Average atomic mass is an average of the atomic masses of each isotope of that element. Since there are different percentages of each isotope in existence, the relative abundance is also considered.
Atomic Mass = (%)(mass)+ (%) (mass) + (%)(mass)
100

14. Sample Problem
Neon has three naturally occurring isotopes. Neon-20 has a mass of grams and an abundance of 90.92%, Neon-21 has a mass of grams and an abundance of 0.257%. Neon-22 has a mass of grams and an abundance of 8.82%. Determine the average atomic mass of neon. ( )(0.9092) + ( )( ) + ( )(0.0882) grams 20.2 grams

15. Development of the Atomic Model
Democritus – 400 BC
Greek philosopher
“Thought” about the idea of the world composed of small particles called “atomos”
The rest of the universe was empty space

16. Rejection of atomos
Aristotle rejected Democritus’ ideas claiming that empty space did not exist.
Aristotle claimed the elements consisted of earth, fire, air and water.
As one of the most influential philosophers of his time, he lead people to reject the idea of atomos from Democritus. (held science back for 2000 years)

17. Development of the Atomic Model
J Dalton
“Atoms” Model
Matter is composed of indivisible particles called atoms.

18. Dalton’s Atomic Theory
All matter is made of atoms.
Atoms are indivisible and indestructible.
All atoms of one element are identical.
Atoms of a specific element are different from atoms of other elements.
Atoms combined in small whole number ratios.
In a chemical change, atoms are neither created nor destroyed, they are just rearranged.

19. 2 Parts of Dalton Incorrect
#2 – atoms are composed of smaller particles – electrons, protons and neutrons
#3 – isotopes exist – atoms of the same element with different amounts of neutrons – not identical

20. Crooke’s tube
Found that there was a ray traveling from the cathode to the anode in the tube.
The ray produced green flashes on a zinc sulfide coating at the other end of the tube.
A cathode ray was a stream of charged particles.
TVs have cathode rays striking light-producing chemicals coated on the screen.

21. Go to animation of a crooke’s tube
crooke's tube animation

22. Development of the Atomic Model
JJ Thomson
“Plum Pudding” Model
The atom is a positive sphere with negative electrons dispersed throughout.

23. J.J Thomson

24. Thomson’s Cathode Ray Tube Experiments:
Found that cathode rays were composed of charged particles that were attracted to the positive pole of a magnet
Concluded that the particles must be negative and called them electrons

25. Also: Demonstration of cathode ray tube cathode ray tube

26. Millikan’s Oil Drop Experiment
Determined the charge on an electron to a great degree of precision (within 1% of the actual value).
By varying the voltage in this apparatus, he could vary the rate at which the oil drops would fall.

27. Go to animation of Millikan’s Oil Drop Experiment

28. Development of the Atomic Model
E. Rutherford
“Nuclear” Model
The atom has a positive center with negative electrons around it.

29. Go to learners tv.com animation of Rutherford’s experiment
rutherford's gold foil experiment

30. Rutherford’s Gold Foil Experiment First Conclusion
Since alpha particles are positive and they are being deflected, they must be hitting something positive.
NUCLEUS IS POSITIVE!!!!!

31. Rutherford’s Gold Foil Experiment Second Conclusion
The object that was deflecting the alpha particles must be very small because only 1 out of every 10,000 particles was deflected.
NUCLEUS IS SMALL!!!!!!

32. Rutherford’s Nuclear Model
The atom had a “nucleus” that was small, positive and dense.

33. Rejection of Rutherford’s Model
The scientific community rejected the nuclear model. Scientists questioned the following:
“What prevents the negatively charged electrons from falling into the positively charged nucleus, resulting in the collapse of the atom?”

34. Development of the Atomic Model
N. Bohr
“Planetary” Model
Bohr proposed this model to help solve Rutherford’s problem.
It looks like the planets around the sun.

35. Bohr stated that:
Electrons are constantly moving in a fixed path around the nucleus.
Since they are moving, they do not collapse upon the nucleus.

36. The current model Quantum Mechanical Model
The electron cloud is more complex than “orbits” around the sun.
It is thought to have various energy levels and sublevels, each of which has a different size, shape and orientation around the nucleus.

37. Quantum Mechanical Model
The current model is based on the probability of finding an electron in a particular spot around the nucleus.
Complex mathematical calculations led to graphs such as these:

38. 226 Ra  222 Rn + alpha particle (α)
Reactions of Elements
In an ordinary chemical reaction, atoms are rearranged to form new materials, but the original identity of the atoms remains the same. (no new element) The electron cloud is involved.
2 Na Cl2  2 NaCl
In a nuclear reaction, atoms change into new types of atoms, releasing radiation in the form of particles or rays. (new element) The nucleus is involved.
226 Ra  Rn alpha particle (α)

39. Unstable Nuclei
Unstable nuclei emit radiation through a process called radioactive decay.
They will continue to decay until a stable nucleus is formed.
Stability is determined by the neutron to proton ratio in the nucleus.

40. Alpha Radiation Alpha Particle: [ α ] ( He) He Nuclei +2 Charge
Mass = 6.64 x grams
Can be stopped by Cardboard
ALPHA DECAY – release of He
U  Th He
Notice that the sum of the mass numbers and the atomic numbers is equal on each side. 4 2 4 2
238 92
234 90 4 2

41. Beta Radiation Beta Particle: [ β ] ( e) Electron -1 Charge
Notice that the sum of the mass numbers and the atomic numbers are equal on each side.
Beta Particle: [ β ]
( e) Electron
-1 Charge
Mass = 9.11 x grams
Can be stopped by Aluminum
Beta Decay -- Neutron => Proton + Electron
Overall loss of a neutron, but the proton goes to the nucleus of a new element and an electron is radiated
C  N e -1 14 6 14 7 -1

42. Gamma Radiation Gamma Ray: [ γ ] High-Energy Photons 0 Charge
Mass = 0 grams
Can be stopped by thick blocks of Lead
Gamma Decay – Only Energy is Given Off
Co  Co Energy (γ)
Notice that the mass numbers and the atomic numbers are equal on each side. 60 27 60 27

43. RADIATION ANIMATION
alpha beta gamma rays

44. Strength of the Three Types of Radiation

45. Patterns in Nuclear Stability
Like Charges REPEL, therefore lots of protons together doesn’t make sense.
However, at close distances a “Strong Nuclear Force” exists between protons and neutrons.
The “Neutron to Proton” ratios of Stable Nuclei INCREASE with Increasing Atomic Number.
More protons require more neutrons to overcome the repulsive forces among like charges.

46. Band of Stability Graph

47. Band of Stability
A band of area within which all stable nuclei are found.
The Band of Stability ends with Bismuth.
All nuclei w/ 84 or more protons are radioactive.

48. Complete the equations:
1)The alpha decay of iridium-174
2) The beta decay of platinum-199

49. Half-life Half-life (t½)
Time required for half the atoms of a radioactive nuclide to decay.
Shorter half-life = less stable.

50. RADIOACTIVE DECAY
DECAY

51. Other Nuclear Reactions
Fission – the splitting of atoms with the release of energy
Fusion – the combining of smaller atoms to form larger atoms with the release of energy

52. FISSION the splitting of atoms with the release of energy
1 g of 235U = 3 tons of coal

53. FISSION chain reaction - self-propagating reaction
critical mass - mass required to sustain a chain reaction

54. FISSION ANIMATION
fission reaction

55. FUSION combining of two nuclei to form one nucleus of larger mass
thermonuclear reaction – requires temp of 40,000,000 K to sustain
1 g of fusion fuel = 20 tons of coal
occurs naturally in stars

56. FUSION ANIMATION
fusion reaction

57. FISSION VS FUSION 235U is limited danger of meltdown toxic waste
thermal pollution
fuel is abundant
no danger of meltdown
no toxic waste
not yet sustainable

58. THE END