1. Chapter 4
Atomic Structure

2. History of the Atom 1. Democritus vs. Aristotle pg. 102-103
2. John Dalton and conservation of mass pg
3. Cathode ray tube and Sir William Crookes pg
4. Mass and charge of electron (J.J. Thompson) and oil drop experiment pg
5. Plum pudding model vs. Rutherford’s experiment pg

3. Democritus Greek philosopher who asked questions about matter.
Can you divide matter infinitely?
Democritus says no!
Tiny particles called atoms, indivisible!
Matter is composed of atoms,
which move through empty space.
Atoms are solid homogeneous, indestructible, and indivisible.
Atoms have different sizes and shapes. These properties, and movement determine properties of matter

4. John Dalton
Matter is composed of small particles called atoms that are indivisible and indestructible.
Atoms of a given element are identical in size, mass, and chemical properties, and are different from those of another element.
Different atoms combine in simple whole number ratios to form compounds.
In a chemical reaction, atoms are separated, combined or rearranged.

5. Dalton’s Inaccuracies
Atoms are not indivisible!
Subatomic particles – electrons, protons, and neutrons
Atoms of the same element can have slightly different masses! - isotopes

6. The Atom
EXTREMELY small particle of an element that retains the properties of that element is an atom.
If the atom is the size of an orange, an orange would be the size of the EARTH

7. Subatomic Particles - Electron
Cathode Ray Tube
Thin beam of electrons
travels from cathode to
anode!
Cathode rays are a
stream of charged particles. Particles carry a
negative charge…now called electrons!
Cathode
Vacuum
Anode

8. J.J. Thompson
Determined that the mass of the charged particle (electron) was much less than that of the hydrogen atom.
Dalton was WRONG about the atom being the smallest particle!

9. Millikan Oil-Drop Experiment
Determined the charge of an electron.
Charge up the oil particles
with electrons.
Change the electric field
changes the rate of oil
droplets!
Charge of electron x coulombs
Mass of electron = 9.1 x grams

10. Plum Pudding Model
Matter isn’t all negatively charged, so how do we have negatively charged subatomic particles without positively charged ones??
J.J. Thompson thought an atom was a positively charged sphere with electrons hanging out within.

11. Rutherford and the Nucleus
Experiment proved that plum pudding model was incorrect!
Atom is mostly empty
space through which e-
can move. Almost all of
the positive charge and
atomic mass resides in
the center – NUCLEUS!
Nucleus is positively charged to deflect alpha particles and to balance electron charge.

12. Subatomic Particles Electron – VERY tiny, negatively charged
Proton – located in the nucleus, charge opposite of an electron (positive!)
Neutron – located in the nucleus, same mass as a proton, neutral!

13. Warm – Up!
What experiment determined the mass and charge of an electron?
Dalton concluded that the atom was the smallest particle of matter. Was he correct?
What did the gold foil experiment prove?

14. Atomic Theory Today Quantum Mechanical Model
All atoms are made up of electrons, protons, and neutrons. Electrons are located outside of the nucleus, protons and neutrons are located inside the nucleus.
Electrons exist in a cloud surrounding the nucleus. Attracted to the nucleus so they hang around!
Nucleus accounts for 99.97% of the atomic mass, and occupies a VERY small volume.
A neutral atom has the same number of electrons and protons!

15. Current Atomic Model Neutral atom: # Protons = # Electrons
Simulation!!

16. Make sure you can answer…
What are John Dalton’s 4 theorems
How does John Dalton’s theory relate to conservation of mass?
How was the electron discovered?
Who discovered the mass of an e-? HOW?
What was Rutherford’s contribution?
Describe the structure of the atom.

18. Properties of Atoms Atomic # # of Protons = # of Electrons Atomic mass
(in neutral atom)
Atomic mass
a weighted average

19. Practice
What is the isotopic symbol for each?

20. Isotopes and Ions Isotope – Things to remember – Isotopic symbol : Ge
The # of protons of an element NEVER changes, and is ALWAYS the same as the Atomic #.
If the # neutrons is different = ISOTOPE
If the # electrons is different = ION
+ = cation Less electrons
- = anion More electrons
Atoms with the same number of protons but
different number of neutrons.
Isotopic symbol : 70 Ge 32

21. Why aren’t the masses of elements in whole numbers?
Mass of Atoms
Mass of electron = 1/1840th of a proton
Mass of proton ≈ mass of neutron
1 atomic mass unit (amu) ≈ mass of proton
Carbon 12 atom = 12 amu
Why aren’t the masses of elements in whole numbers?

22. Atomic Mass = Average of Isotopes
Weighted average mass – mass of each isotope contributes to total mass according to how much of that isotope exists. K
Three isotopes = 39K 40K 41K
Potassium 19 19 19
Percent Composition:
93.26%
0.01%
6.73%

23. Calculate the Atomic Mass of K
Use % composition and convert to relative abundance (divide by 100)
93.26% composition = relative abundance
2. Amu = ((Mass of Isotope1)x(Relative Abundance1)) + ((Mass of Isotope2)x(Relative Abundance2))…
((0.9326)x(39)) + ((0.0001)x(40)) + ((0.0673)x(41)) =
amu

24. Warm Up!
Element Atomic # Mass # Calcium Oxygen 8 17 Mercury What is the number of protons, electrons, and neutrons for each? What is the isotope symbol (shorthand notation) for each?

25. Agenda
Question for today: What does radioactive mean and what makes certain atoms radioactive?
Isotope calcs
Radioactive particles
Decay practice

26. Amu = (R.A.)x(Mass) + ((R.A.)x(Mass))…
What element is this?
Isotope Mass of Isotope Percent abundance
6X amu %
7X amu %
Find the atomic mass
What element is this? (Use the Periodic Table)
Boron has two isotopes: Boron-10 (% abundance – 19.8%, mass = amu) and Boron-11 (% abundance – 80.2%, mass – amu). Calculate the atomic mass of Boron.

27. Bromine has two isotopes with the first having a mass of 78
Bromine has two isotopes with the first having a mass of amu and occupying 50.69% and the second isotope having a mass of amu and occupying 49.31%. What is the average atomic mass of bromine?
Verify the atomic mass of Magnesium:
24Mg = amu and percent abundance of 78.99% , 25Mg = amu and percent abundance of 10.00%, 26Mg = amu and percent abundance of 11.01%.

28. One more…
Copper has two naturally occurring isotopes, Cu-63 and Cu-65. The atomic mass of Cu is amu. Calculate the percent abundances of the two isotopes.

29. Radioactivity – emit radiation
Nuclear reactions – change an element into a new element!! Lots of energy involved!
Unlike a chemical reaction because we are doing more than rearranging – we CHANGE the identity.
Change in the atom’s nucleus.
UNSTABLE nuclei are unhappy and lose energy by emitting radiation – radioactive decay.
They form STABLE atoms of a different element.

30. Radioisotopes Isotopes of atoms with unstable nuclei.
Undergo radioactive decay to attain stability.
Emit 3 types of radiation
alpha, a
beta, b
gamma, g

31. What are the charges on radioactive particles?

32. Types of Radiation
Alpha radiation – (remember the gold foil experiment?!?!) made up of POSITIVE “alpha particles”.
2 protons and two neutrons (no electrons!)
4He2+ or a 2

33. Alpha decay U He Th Ra He Rn Cm He Pu + + + 238 4 234 92 2 90 226 4
222 Rn + 88 2 86
247 Cm 4 He
243 Pu + 96 2 94

34. Types of Radiation Beta radiation – negatively charged beta particles
Unstable neutron turns into a proton and ejects 1 electron
e- or b

35. Usually omitted from nuclear equations.
Types of Radiation
Gamma radiation – emits gamma rays, high energy photon that has no mass nor charge.
Gamma rays almost always accompany alpha and beta radiation and account for the energy lost in the nucleus. g
Usually omitted from nuclear equations.
238 U 4 He
234 Th
2 g + + 92 2 90

36. Penetrating Power of Radiation

37. Penetrating Power
Least
Alpha particles most mass and charge. 4He2+ Beta particles less mass (only the mass of an electron) and a neg charge. Gamma rays have no mass and no charge.
Isotopic mass 2
Most

40. In the Nucleus Radioactive decay – transmutation
Atomic # is altered = identity of element changed
Nucleons + +
Strong nuclear force between all nucleons.
Repulsive force between 2 protons (electrostatic).
Neutron attraction have to overcome the repulsive forces –
as atomic # increases we need more neutrons to
stabilize the nucleus!!! +

41. Low atomic #’s have a 1:1 neutron to proton ratio 4He High atomic #’s are stabilized by a 1.5:1 ratio 200Hg If atom is not in band (belt) of stability it undergoes radioactive decay to get there! 2 80

42. Decay Practice b 238Pu 4He 234U 229Th 4He 225Ra 212Bi 4He 208Tl 208Tl+
a decay 94 2 92
Thorium-229 is used to increase the lifetime of fluorescent bulbs.
What type of decay occurs when thorium-229 decays to form radium-225?
Write out the nuclear equation.
229Th
4He
225Ra + 90 2 88 B A
Write a balanced nuclear equation for the decay shown on the right.
Identify A and B b
Bismuth -212
212Bi
4He
208Tl + A 83 2 81 a
208Tl
208Pb b + B 81 82

43. Warm – Up!!
What is the band of stability and how does it relate to the proton to neutron ratio?
How does the neutron to proton ratio change when polonium-210 decays into lead-206? What type of decay does polonium-210 undergo?
(Low atomic # elements are happy with a 1:1 ratio of neutrons to protons. Heavier elements need a 1.5:1 ratio and all elements above 82 are radioactive.)

44. Half Life
Time required for one half of the nuclei to decay into its products.
Strontium-90 half life is 29 years.
If you had 10 g now, in 29 years you would have 5g.

46. Half Life Calculations
N = N0 (½)n
N – remaining amount of element
N0 – initial amount of element
n – number of half lives that have passed
Kr-85 has a half life of 11 years. Kr is used in indicator lights of appliances. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left?
N = ?
N0 = 2 mg
n = 33 years/11 years (years that have passed/half life)

47. Kr-85 has a half life of 11 years
Kr-85 has a half life of 11 years. Kr is used in indicator lights of appliances. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left? N = 2.0 mg (½)(33/11) N = 2.0 mg (½)3 N = 2.0 mg (⅛) N = 0.25 mg left after 33 years

48. Half Life Practice
The half life of Ra-222 is 3.8 days. How much is left of a 10 mg sample after 15.2 days?
N = N0 (½)n
N = 10mg (½)(15.2/3.8)
N = 10mg (½)4
N = 10mg (1/16)
N = 0.625mg
0.25, 0.625, 800

49. Half Life Practice
Bandages can be sterilized by exposure to gamma radiation from cobalt-60, which has a half life of 5.27 years. How much of a 10 mg sample of cobalt-60 is left over after years? After four half lives? N = N0 (½)n N = 10 mg (½)10.54/5.27 N = 10 mg (½)4

50. Half – Life Calculations
Do the problem intuitively…
Think about how many half lives have passed and just do the division
Two half lives (10 mg/2)/2 = 2.5 mg
Four half lives 10 mg/2/2/2/2 = mg

51. Warm - Up!
Americium-241 has a half life of 430 years. How much of a 15 mg sample is left after 2150 years?
A radioisotope has a half life of 197 years. How much remains of a 2.0 g sample after 10 years?
Strontium has a half life of 29 years. How long will it take for a 56 g sample to decay to 1.75 g?
.469 mg, g

52. Nuclear Reactions
Induced Transmutation – FORCE an element to change its identity by bombarding it with radioactive particles!
Particle accelerators move particles at extremely high speeds to overcome repulsive forces.
4He
14N
17O + 1H + 2 7 8 1

53. Nuclear Fission
Splitting of a nucleus into fragments - LARGE release of energy!
Large atoms want to be smaller for stability (Atomic # larger than 60 = large atoms)
Kr - 92
neutron
U-235  U-236
Ba - 141

54. Neutrons Perpetuate Fission

55. Critical Mass Fissionable material must have sufficient mass to split
Not massive enough – subcritical – no chain reaction
Extremely massive – violent nuclear reaction

57. Violent Nuclear Reactions Subcritical masses that get
together to form supercritical
mass.
Equal to 20,000 tons of TNT

58. Nuclear Reactors Reactor core controlled by Cd or B to absorb neutrons
Assignment : Look up Chernobyl Nuclear Power Plant!

59. Nuclear Fusion
Bind low atomic mass (less than 60 because 60 is ideal) to form more stable atom.
Combination of nuclei called fusion.
How the sun works…
Need very high energy to initiate and sustain.
41H 2b
energy +
4He + 1 2

60. Nuclear waste
What happens to all the radioactive waste accumulated at a power plant?
Treated with advanced technologies so the material doesn’t deteriorate
Sealed and buried underground

61. Describe the process that occurs during a nuclear chain reaction and explain how to monitor a chain reaction in a nuclear reactor.
True/False
Great amounts of energy can be liberated from small amounts of matter in a nuclear reaction.
The amount of U-235 in a nuclear reactor should always be kept subcritical.
Nuclear power plants do not contribute to air pollution.
Nuclear power use is dangerous because plants are commonly are out of control.

62. Warm Up Isotope Percent Abundance Mass (amu) X-50 4.35 49.946 X-52
83.79
51.941
X-53
9.50
52.941
X-54
2.36
53.939
Find the atomic mass and identify the element.
How many electrons, and protons does this element have?
How many electrons protons and neutrons does 52X2+ have? Is it in cationic
or anionic form?

63. Warm – Up! What is an isotope? What is a radioactive isotope?
What does it mean for something to be radioactive?

64. Warm Up! What are the three types of radioactive particles?
What are the charges on those particles?
Complete the following nuclear reaction (Remember that b = e-)
137 Cs b
137 Ba + 55 56

65. Warm – Up!!
What happens to the atomic mass number and the atomic number of a radioisotope when it undergoes alpha emission?
High speed electrons emitted by an unstable nucleus are ________ particles.
What isotope of what element is produced if krypton-81 undergoes beta decay?
Write out the nuclear reaction.

66. Agenda: Nuclear Fission Nuclear Power Plants Homework: # 34 – 79 odds pg 894 due Thursday Test Thursday

67. Today’s Question
What are the main components of a nuclear power plant? How does a power plant control the chain reaction?

68. Nuclear Fission The splitting of an atom into parts
Uranium-235 when hit with a neutron at high velocity breaks apart into 2 new elements as well as additional neutrons.
Those neutrons go on to strike other U-235 atoms and cause a chain reaction

70. Fission reactions produce a ton of energy
Fission reactions produce a ton of energy. If a chain reaction is uncontrolled the consequences can be extreme

71. Nuclear Reactors
Nuclear fission produces the energy generated in a nuclear power plant
Uses fuel rods made out of enriched uranium
Control rods made out of other metals are used to control the chain reaction by absorbing extra neutrons
Heat generated from fission boils water and the steam spins a turbine which produces electricity

73. Nuclear waste
What happens to all the radioactive waste accumulated at a power plant?
Treated with advanced technologies so the material doesn’t deteriorate
Sealed and buried underground

74. What happens when we run out of U-235?
Breeder Reactors
-cost inefficient
-technically challenging

75. Nuclear Fusion
Process by which 2 or more atomic nuclei join together to form a single heavier nucleus
Generally produces large quantities of energy
Stars
Hydrogen bomb
Possible answer to energy source with the least amount of waste products

76. Nuclear Weapons 2 types Fission based and Fission/Fusion Based
Limited development by a few developed countries globally

77. Fission Weapons
Chemical explosives
start a chain reaction

78. Fission/Fusion Weapons
Fission reaction acts
as a trigger for
the fusion reaction