1. Do Now
Which nuclear emission has the greatest mass and the least penetrating power?
an alpha particle 2. a beta particle
a neutron 4. a positron
When an atom emits a beta particle, the total number of nucleons
Decreases 2. increases 3. remains the same

2. Atomic Structure: The Nucleus, Part 3
Williamsburg High School for Architecture and Design
Mr. Quinn and Ms. Tom
9/24/13

3. Remember Our Types of Radiation!
Name
Mass
Charge
Speed
Identity
Alpha 4 +2
Slow He
Neutron 1
Beta - o -1
Fast
Electron
Beta + +1
Positron
Gamma
Light Speed
Photon
Now let’s get after it!

4. But Why Does it Happen? Types of decay:
Beta - turns a neutron into a proton
Beta + turns a proton into a neutron
Alpha makes nuclei smaller
Neutron also makes nuclei smaller

5. But Why Does it Happen?
Why are all Beta – emitters above the line of stability?
Why are all Beta + emitters below the line of stability?
Where would the neutron emitters be on this table?
Is radioactive decay more likely or less likely when the energy to escape is close to the energy holding the nucleus together? Why?

6. How Fast Does It Happen? Half-Life
The amount of time it takes for half the sample to decay, on average
Half-Lives
Fraction Left
% Left
1/1
100 1
1/2 50 2
1/4 25 3
1/8
12.5

7. Team Together #1
A sample exists for three half-lives. What fraction of the sample remains?

8. Team Together #2
A 100 gram sample has a half life of 25 years. How many grams remain 125 years later?

9. Team Together #3
Only 4 grams of a 256 gram sample remain after 1200 years. What is the half-life of the sample?

10. Practice Problem #1
What percentage of a sample remains after four half lives?

11. Practice Problem #2
An isotope has a half-life of 400 years. What fraction of atoms remain after 1200 years?

12. Practice Problem #3
An isotope has a half-life of 200 years. If we begin with 1000 atoms of sample, how many atoms are left after 400 years?

13. Practice Problem #4
A sample has decayed from 200 atoms down to 12 atoms. How many half-lives has it been through?

14. Practice Problem #5
A sample has only 6.25% of the atoms it once had. If the half-life of the sample is 50 years, how old is the sample?