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 13 N 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-Lifes

The half-life of 26 Al is 710,000 years. –If you start with 28.0 grams of 26 Al, 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 61 Fe 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 212 Rn will decay to 3 mg. What is the half-life of 212 Rn? –At start, 12 mg remain. –At (1 st half-life), 6 mg remain. –At (2 nd 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 235 U nono nono nono nono 90 Rb 143 Cs

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 nono 2H2H 3H3H Energy 4 He

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.