1. Notes 11.3: Radioactive Decay & Half-lives

2. 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)

3. 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 H  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

4. 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

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

6. 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).

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

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

9. Beta Decay
Timberlake, Chemistry 7th Edition, page 90

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

11. Half-life of Radiation
Initial amount
of radioisotope
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

12. 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

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

14. 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.

15. 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, g 1/8 22, g 1/16 28, g 1/32 34, g 1/64

16. 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.