Half-Life and Nuclear Reactions

ReviewO We learned that all radioactive

atoms eventually decay into stable isotopes.O We did not talk about how long this

takes.

Half-LifeOCannot predict when a radioisotope (RI)

will decay.O Can only give probability.

OHalf-life – Amount of time during which one-half of a radioactive substance will decay.O During a RI’s half-life, every atom has a 50%

chance of decaying.O Half-lives can be as short as a fraction of a

second or as long as billions of years.

Half-LifesO The half-life of 26Al is 710,000 years.

O If you start with 28.0 grams of 26Al, how much will remain after 1,420,000 years?O At start, 28.0 grams remain.O At 710,000 years, 14.0 grams remain.O At 1,420,000 years, 7.0 grams remain.

Half-LifesO The half-life of 61Fe is 6.0 minutes.

How much time must pass before a 600-mg sample decays to 75 mg?O At start, 600 mg remain.O At 6.0 minutes, 300 mg remain.O At 12.0 minutes, 150 mg remain.O At 18.0 minutes, 75 mg remain.

Half-LifesO In 48 minutes, 12 mg of 212Rn will decay to 3

mg. What is the half-life of 212Rn?O At start, 12 mg remain.O At (1st half-life), 6 mg remain.O At (2nd half-life), 3 mg remain.O It takes 2 half-lives for 12 mg to decay to 3 mg.

O 2 half-lives = 48 minutesO 1 half-life = 24 minutes

Nuclear FissionO A heavy nucleus is struck by a neutron.

O It becomes unstable and splits into smaller fragments, releasing energy.

O In some cases extra neutrons are also released.O The neutrons can strike other nuclei and cause

them to split.O Chain reaction.

Nuclear Fission

Energy

235Uno

no

no

no

90Rb

143Cs

Uses for Nuclear Fission

O Nuclear reactors use fission to produce energy.

Uses for Nuclear Fission

O Weapons: the bombs that destroyed Hiroshima and Nagasaki, Japan, were fission bombs.

Nuclear FusionO Two light nuclei are joined to

produce a larger nucleus.O Energy is released.

Nuclear Fusion

no

2H

3H

Energy

4He

Uses for Nuclear Fusion

O Nuclear fusion can be performed at low temperatures, but it does not produce much useable energy.

O Fusion only produces significant energy at extremely high temperature and pressure.O The Sun produces energy via nuclear

fusion.

Uses for Nuclear Fusion

O Weapons: Thermonuclear (hydrogen) bombs.O Use a conventional fission bomb to trigger

nuclear fusion in a lithium hydride shell.O Much more destructive than fission bombs.O Never used in war.