Key vocab:

What is a Radioactive Isotope?

What is Radioactive Decay?

What is Half Life?

Radioactive elements are unstable. They decay, and change into different elements over time.

Not all elements are radioactive. Those that are and are the most useful for geologic dating are:

U-238 (Uranium) Half-life = 4.5 Billion YearsK-40 (Potassium) Half-life = 1.25 Billion YearsC-14 (Carbon) Half-life = 5, 730 Years

Radioactive Isotopes

Radioactive Decay and Half Life

Here are some facts to remember:

1.The half-life of an element is the time it takes for half of the material you started with to decay.

2. Each element has its own half-life

3. Each element decays into a new element C14 decays into N14 (Nitrogen) while U238 decays into Pb206 (Lead), etc.

4. The half-life of each element is constant. It’s like a clock keeping perfect time.

Now let’s see how we can use half-life to determine the age of a rock or other artifact.

Radioactive Decay and Half Life

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and turns red. C14 – blue N14 - red

As we begin notice that no time has gone by and that 100% of the material is C14

Half

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

The grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red

Half

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

After 1 half-life (5730 years), 50% ofthe C14 has decayed into N14. The ratioof C14 to N14 is 1:1. There are equalamounts of the 2 elements.

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red .C14 – blue N14 - red

Half

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

Now 2 half-lives have gone by for a totalof 11,460 years. Half of the C14 that waspresent at the end of half-life #1 has nowdecayed to N14. Notice the C:N ratio. Itwill be useful later.

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red

Half

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

3 12.5% 87.5% 1:7

After 3 half-lives (17,190 years) only12.5% of the original C14 remains. Foreach half-life period half of the materialpresent decays. And again, notice the ratio, 1:7

What is the half life represented in this graph?