Do Now

Which nuclear emission has the greatest mass and the least penetrating power?

1. an alpha particle 2. a beta particle3. a neutron 4. a positron

When an atom emits a beta particle, the total number of nucleons

1. Decreases 2. increases 3. remains the same

Atomic Structure: The Nucleus, Part 3

Williamsburg High School for Architecture and Design

Mr. Quinn and Ms. Tom9/24/13

Remember Our Types of Radiation!

Now let’s get after it!

Name Mass Charge Speed Identity

Alpha 4 +2 Slow He

Neutron

1 0 Slow Neutron

Beta - o -1 Fast Electron

Beta + o +1 Fast Positron

Gamma o 0 Light Speed

Photon

● 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

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?

But Why Does it Happen?

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

0 1/1 100

1 1/2 50

2 1/4 25

3 1/8 12.5

Team Together #1

A sample exists for three half-lives. What fraction of the sample remains?

Team Together #2

A 100 gram sample has a half life of 25 years. How many grams remain 125 years later?

Team Together #3

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

Practice Problem #1

What percentage of a sample remains after four half lives?

Practice Problem #2

An isotope has a half-life of 400 years. What fraction of atoms remain after 1200 years?

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?

Practice Problem #4

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

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?