Radioactive Isotopes and Half Life

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What is a Radioactive Isotope?

What is Radioactive Decay?

What is Half Life?

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

Not all elements are radioactive. Those that are listed below are the most useful for geologic dating of fossils are:

U-238 Half-life = 4.5 Billion YearsK-40 Half-life = 1.25 Billion YearsC-14 Half-life = 5, 730 Years

Radioactive Isotopes

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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 it’s own half-life.

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3. Each element decays into a new element - C14 decays into N14

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, fossil or other artifact.

Radioactive Decay and Half Life

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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 Time elapsed

0 100% 0% 0 years

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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 Time Elapsed

0 100% 0% 0 years

1 50% 50% 5,730 years

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.

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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 Time Elapsed

0 100% 0% 0 years

1 50% 50% 5,730 years

2 25% 75% 11,460 years

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.

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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 Time Elapsed

0 100% 0% 0 years

1 50% 50% 5,730 years

2 25% 75% 11,460 years

3 12.5% 87.5% 17,190 years

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

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Band of StabilityFor low atomic numbers the stable nuclei are those with a ratio of protons to neutrons approx. 1:1

As the atomic # increases, the stable ratio increases to about 1.5: 1.

Stable, naturally occurring isotopes

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Band of StabilityIf an isotope falls above the band, then it needs to lose neutrons and gain more protons to become stable. How would it achieve that?

If an isotope had too many protons and too many neutrons, how would it become stable?

Stable, naturally occurring isotopes

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Beta Deca

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AlphaDecay

Positron Emission