Notes 11.3: Radioactive Decay & Half-lives

Nuclear Reactions – Changes the nucleus of the atom. Can change one element into a different element Involve much more energy than a chemical reaction. Convert mass into energy (E=mc2)

Energy= (mass) (speed of light)2 Nuclear Fusion When 2 smaller atomic nuclei combine. The result releases a lot of energy. Happens at very high temperatures, making two nuclei slam together. FUSION in the sun 2 H2  1 He + ENERGY Small mass loss converted to large energy release. This relationship was discovered by Albert Einstein E = mc2 Energy= (mass) (speed of light)2

Nuclear Fission - When a large atomic nuclei splits into 2 smaller nuclei, releasing a lot of energy - Typically starts when nucleus is hit by a neutron - Uranium-235 splits into smaller atoms - Chain reactions continue, into smaller atoms

Fission Chain Reactions Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 620

Radioactive decay - occurs when a nucleus of an unstable element decays into a new, more stable element. Since it is a random process, the probability is predictable (think of flipping a coin).

Absorption of Radiation Timberlake, Chemistry 7th Edition, page 84

Alpha Decay alpha particle radioactive isotope neutron proton Timberlake, Chemistry 7th Edition, page 87

Beta Decay Timberlake, Chemistry 7th Edition, page 90

Half life - the time required for half the nuclei in a large sample to decay. This quantity can be predicted with great accuracy.

Half-life of Radiation Initial amount of radioisotope 0 1 2 3 4 Number of half-lives Radioisotope remaining (%) 100 50 25 12.5 After 1 half-life After 2 half-lives After 3 half-lives t1/2 t1/2 t1/2

Half-Life Plot Amount of Iodine-131 (g) Time (days) 20 Half-life of iodine-131 is 8 days 15 1 half-life Amount of Iodine-131 (g) 10 16 2 half-lives 5 24 3 half-lives 32 4 half-lives etc… 40 48 56 8 Time (days) Timberlake, Chemistry 7th Edition, page 104

Each radioactive element has its own unique half life. Half lives vary from less than 1 second to billions of years. Radon 51.5 seconds Iodine 133 21 hours Carbon 14 5,730 years Uranium 238 4.5 billion years Thorium 232 14 billion years

Carbon 14 - has a half life of 5,730 years. Since all living things contain carbon (and some carbon 14) finding out how much carbon 14 remains in a (formerly) living thing is useful in determining how old the organism is.

Years Amount Fraction 0 100g 1 5,730 50g 1/2 11,460 25g 1/4 17,190 12 Years Amount Fraction 0 100g 1 5,730 50g 1/2 11,460 25g 1/4 17,190 12.5g 1/8 22,920 6.25g 1/16 28,650 3.125g 1/32 34,380 1.5625g 1/64

Radiometric dating has been used to determine the age of rocks. Scientists use radioactive elements with extremely long half-lives to do this. The age of the Earth has been calculated this way.