Date post: | 01-Jan-2016 |
Category: |
Documents |
Upload: | merritt-pickett |
View: | 19 times |
Download: | 2 times |
Nuclear Chemistry
Structure and Stability of Nuclei, Fission, Fusion,
and Radiation
Standards11. Nuclear processes are those in which an atomic nucleus changes, including radioactive decay of
naturally occurring and human-made isotopes, nuclear fission, and nuclear fusion. As a basis for understanding this concept:
a. Students know protons and neutrons in the nucleus are held together by nuclear forces that overcome the electromagnetic repulsion between the protons.
11. b. Students know the energy release per gram of material is much larger in nuclear fusion or fission reactions than in chemical reactions. The change in mass (calculated by E = mc2 ) is small but significant in nuclear reactions.
11. c. Students know some naturally occurring isotopes of elements are radioactive, as are isotopes formed in nuclear reactions.
11. d. Students know the three most common forms of radioactive decay (alpha, beta, and gamma) and know how the nucleus changes in each type of decay.
11. e. Students know alpha, beta, and gamma radiation produce different amounts and kinds of damage in matter and have different penetrations.
11. f.* Students know how to calculate the amount of a radioactive substance remaining after an integral number of half-lives have passed.
11. g.* Students know protons and neutrons have substructures and consist of particles called quarks.
Nuclear Chemistry
has to do with an atom’s nucleus
plural of nucleus = nuclei
What’s a nucleus? Let’s reviewAn atom’s nucleus contains almost all of an atom’s mass, but takes up very little of it’s volume.
Subatomic Particlesthe particles that make up an atom
• Protons – high mass, positive charge. Found in nucleus.
• Neutrons – high mass, no charge. Found in nucleus.
• Electrons – low mass, negative charge. Found orbiting around nucleus. (abbreviated e– )
Comparison of Masses
Proton Neutron
Electron
An AtomSize of atom
Size of nucleus
2 protons = He = helium
Basic Electrical Charge Laws
+ and – : Attract(pull
together)
– and – : Repel(push away)
+ and + : Repel(push
away) Like charges repel and Opposites attract
So why don’t the protons in the nucleus fly apart?
nuclear force
electrostatic force
is stronger over short distances
is stronger over long distances
Key11NaSodium22.99
Atomic Number•Number of Protons•Number of Electrons (when atom is neutrally charged)
•Property unique to each element
Key
Average atomic mass*•Weighted Average number of Protons and Neutrons (approximately)
NaSodium22.99
11
IsotopesWhen atoms have the same number of protons, but different numbers of neutrons, they are called isotopes.
Examples: Carbon-14 has 6 protons and 8 neutrons.Carbon-12 has 6 protons and 6 neutrons.
Carbon-14 is found more often in living organisms than in non-living matter. It also undergoes radioactive decay – which is why it is used for fossil dating.
Isotopes
More Examples: Uranium-238 has 92 protons and 146 neutrons.Uranium-235 has 92 protons and 143 neutrons.
Uranium-235 is more rare in nature than Uranium-238, but it also undergoes nuclear chain reactions more easily, which is why small amounts of Uranium-235 are used in nuclear weapons and nuclear power plants.
Some isotopes are more stable than others.
Which isotope is more abundant?Neon-20Neon-22
How many neutrons in each? Neon-20Neon-22 22 – 10 = 12 neutrons
20 – 10 = 10 neutrons
Which isotope is more abundant?Bromine-79Bromine-80
How many neutrons in each? Bromine-79Bromine-80
Lead has two isotopes with the following relative amounts: 80% Lead-207 20% Lead-208What would the average atomic mass calculate to be?
0.8(207) + 0.2(208) = 207.2
Chlorine has two stable isotopes:Chlorine-35Chlorine-37
What are the relative abundances (in %) of each isotope? x(35) + (1-x)(37) = 35.4535x – 37x + 37 = 35.45-2x + 37 = 35.45
-2x = -1.55
-2x = -1.55
x = 0.775
??%100 –??%
–37 –37
-2 -2
x1 – x
Chlorine has two stable isotopes:Chlorine-35Chlorine-37
What are the relative abundances (in %) of each isotope? x(35) + (1-x)(37) = 35.4535x – 37x + 37 = 35.45-2x + 37 = 35.45
-2x = -1.55
-2x = -1.55
x = 0.775
77.5%22.5%
–37 –37
-2 -2
0.7750.225
So why don’t the protons in the nucleus fly apart?
nuclear force
electrostatic force
is stronger over short distances
is stronger over long distances
Sometimes these forces are overcome. Fission – a nucleus breaks apart.(ex. atomic bomb, nuclear power plants, radioactive decay)
Fusion – a nucleus merges with another.(ex. the sun, hydrogen bomb, experimental fusion reactors)
Both fission and fusion release radiation.
It is called radiation because it radiates out.
Common Types of Radiation
α (alpha): helium nucleus at high speed. β (beta): electron at high speed.
γ (gamma): high energy photon.
He42
e0-1
γ00ha
rder
to b
lock
Symbol
Other Types of Radiation
neutron at high speed.
Proton (or hydrogen nucleus) at high speed.
β+ positron at high speed (the anti-matter version of an electron).
n10
p11
β+01
Symbol
Na2311 Na22
11
atomic mass
number of protons
We can use conservation of mass and charge to figure out nuclear reactions just like chemical reactions.
e0-1Th234
90 +
He42U238
92 + ???
???
We can use conservation of mass and charge to figure out nuclear reactions just like chemical reactions.
e0-1Th234
90 +
He42U238
92 +
???
Th23490
We can use conservation of mass and charge to figure out nuclear reactions just like chemical reactions.
e0-1Th234
90 +
He42U238
92 + Th23490
Pa23491
Half-Life• Half-life – how much time has passed
when half of the original amount remains.
• How much of the original remains after two half-lives?
• How much after three half-lives?
• How much after four half-lives?
or 50 %12
or 25 %14
or 12.5 %18
or 6.25 % 116
Table of Half-LivesIsotope Name Symbol Half-LifeUranium-238 238
92U 4.51 × 109 yearsUranium-235 235
92U 7.1 × 108 yearsCarbon-14 14
6C 5,730 yearsStrontium-90 90
38Sr 28 yearsCobalt-60 60
27Co 5.27 years
Table of Half-LivesIsotope Name Symbol Half-LifeThorium-234 234
90Th 24.1 daysProtactinium-234 234
91Pa 6.75 hoursPolonium-218 218
84Po 3.08 minutesRadon-219 219
86Rn 4.00 secondsPolonium-214 214
84Po 1.6 × 10-4 seconds
As a archaeologist you find a dead rat that contains 0.0009 grams of Carbon-14. A rat that died a year ago has 0.01 grams of Carbon-14. How long ago did the rat die? t½ = 5,730 years0.0009 g0.01 g
= 111.1
12
116
132
164
14
18
1 H.L. 2 H.L. 3 H.L. 4 H.L. 5 H.L. 6 H.L.
As a archaeologist you find a dead rat that contains 0.0009 grams of Carbon-14. A rat that died a year ago has 0.01 grams of Carbon-14. How long ago did the rat die? t½ = 5,730 years
111.1
116
18
3 H.L. 4 H.L.
3(5,730) = 4(5,730) =
17,190 years 22,920 years
17,190 – 22,920 years ago
Strontium-90 is one of the “fallout” products from atmospheric testing of nuclear weapons. If there were 50,000 grams of Strontium-90 in the U.S. southwest region when the 1963 testing ban began, how much Strontium-90 currently remains? t½ = 28 years2013 – 1963 =
12
116
132
164
14
18
1 H.L. 2 H.L. 3 H.L. 4 H.L. 5 H.L. 6 H.L.
50 years 50 yr28 yr = 1.8 half-
lives
Radioactive Decay Formulas
A = A0 e-( )tln (2)
t½1
2 t½ = A0
A
ln (2)
ln ( ) t
3 t =A0
Aln ( ) ln (2)
t½
A0 = original amountA = current amountt = current timet½ = half-life
Energy-Mass Equivalence
Sometimes in nuclear change a small amount of the mass disappears. It has been converted to a large amount of energy according to the formula: E = mc2
speed of light: c = 3 × 108 m/s
energy = (mass) × (speed of light)2
J = kg × (m/s)2
Fission Nuclear Reaction
4 e– in valence shell
Which isotope is more abundant?Lead-207Lead-208
How many neutrons in each? Lead-207Lead-208