half-life and nuclear reactions review we learned that all radioactive atoms eventually decay into...
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Half-Life and Nuclear Reactions
Review
We learned that all radioactive atoms eventually decay into stable isotopes.– We did not talk about how long this takes.
Half-Life
Cannot predict when a radioisotope (RI) will decay.– Can only give probability.
Half-life – Amount of time during which one-half of a radioactive substance will decay.– During a RI’s half-life, every atom has a
50% chance of decaying.– Half-lives can be as short as a fraction of a
second or as long as billions of years.
Half-Lifes N-13 is radioactive.
– It decays into C-13 via a process called electron capture. The half-life of 13N is 10 minutes.
– If you start with 1000 atoms of N-13, approx. how many will remain after 10 minutes?
• 10 minutes = 1 half-life, so there would be about 500 atoms left.
– Approx. how many will remain after 20 minutes?• 20 minutes = 2 half-lives, so there would be about 250
atoms left.– Approx. how many will remain after 30 minutes?
• 30 minutes = 3 half-lives, so there would be about 125 atoms left.
Half-LifesAmount of N-13 Remaining
0
200
400
600
800
1000
1200
0 10 20 30 40 50
Time (minutes)
Ato
ms
of
N-1
3
Half-Lifes
The half-life of 26Al is 710,000 years.– If you start with 28.0 grams of 26Al, how
much will remain after 1,420,000 years?• At start, 28.0 grams remain.• At 710,000 years, 14.0 grams remain.• At 1,420,000 years, 7.0 grams remain.
Half-Lifes
The half-life of 61Fe is 6.0 minutes. How much time must pass before a 600-mg sample decays to 75 mg?– At start, 600 mg remain.– At 6.0 minutes, 300 mg remain.– At 12.0 minutes, 150 mg remain.– At 18.0 minutes, 75 mg remain.
Half-Lifes
In 48 minutes, 12 mg of 212Rn will decay to 3 mg. What is the half-life of 212Rn?– At start, 12 mg remain.– At (1st half-life), 6 mg remain.– At (2nd half-life), 3 mg remain.– It takes 2 half-lives for 12 mg to decay to 3
mg.• 2 half-lives = 48 minutes• 1 half-life = 24 minutes
Nuclear Fission
A heavy nucleus is struck by a neutron.– It becomes unstable and splits into smaller
fragments, releasing energy.– In some cases extra neutrons are also
released.– The neutrons can strike other nuclei and
cause them to split.• Chain reaction.
Nuclear Fission
Energy
235Uno
no
no
no
90Rb
143Cs
Uses for Nuclear Fission
Nuclear reactors use fission to produce energy.
Uses for Nuclear Fission
Weapons: the bombs that destroyed Hiroshima and Nagasaki, Japan, were fission bombs.
Nuclear Fusion
Two light nuclei are joined to produce a larger nucleus.– Energy is released.
Nuclear Fusion
no
2H
3H
Energy
4He
Uses for Nuclear Fusion
Nuclear fusion can be performed at low temperatures, but it does not produce much useable energy.
Fusion only produces significant energy at extremely high temperature and pressure.– The Sun produces energy via nuclear
fusion.
Fusion in the Sun
The Sun is mostly made of hydrogen. Intense heat and pressure at its center
causes nuclear fusion to occur. This releases energy that supports the
mass of the Sun and prevents it from collapsing.
The Sun
Uses for Nuclear Fusion
Weapons: Thermonuclear (hydrogen) bombs.– Use a conventional fission bomb to trigger
nuclear fusion in a lithium hydride shell.– Much more destructive than fission bombs.– Never used in war.