radioactive dating of rocks

some natural elements "automatically change" into other elements thru a process of radioactive decay.

these changes do not occur because of changes in electron structure as in chemical reactions, but take place due to changes in the nucleus of the atom.

These are nuclear reactions.

half-life -- the time it takes half of the a substance to change into another material.

Here are some half-lives of various materials

Polonium-2l5 --- 0.00l8 seconds
Polonium-2l6 --- 0.l6 seconds
Bismuth-2l2 --- 60.6 minutes
Sodium-24 --- l5.0 hours
Iodine-l3l --- 8.l4 days
Phosphorus-32 --- l4.3 days
Iron-59 --- 46.3 days
Cobalt-60 --- 5.26 years
Strontium-90 --- l9.9 years
Radium-226 --- l,620 years
Carbon-l4 --- 5,930 years
Chlorine-36 --- 400,000 years
Uranium-235 --- 7l0 m. years
Uranium-238 --- 4.5 b. years

Critical assumptions that must be made to validate the ages obtained:

the radioactive decay rate has not changed since Day #l
when dating U--Pb changes - no lead was there to being with

Every 4.5 billion years, half of the U-238 will change to Pb-206. This is a Half Life of U-238.

When the melted rock freezes the radioactive clock starts "ticking."

As rocks melt inside the earth they either stay there ( magma ) or flow onto the surface ( lava ) and freeze ( turn to a solid ). When rocks are "dated" you are measuring the time elapsed since it was last a liquid.

if we had 80 grams of U-238

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then in 4.5 billion years the sample looks like this:

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It has gone thru one half-life: half of the sample is U-238, and half of the sample is Pb-206.

If it went thru only half of a half-life, only l/4th of the atoms would be changed to lead ( 20 of them )and the other 3/4ths ( 60 of them ) would still be U-238.

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The rock would be l / 2 of the age of a half-life: l / 2 of 4.5 b. years -- 2.25 b.y.

If it went thru only l0% of a half-life, only l / 20th of the atoms would be changed to lead ( 4 of them ) ( l / l0th x l / 2 = 1 / 20th ), and the other 95% would still be U-238 ( 76 of them ).

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The rock would be l / 20th the age of a half-life: l / 20th of 4.5 billion years -- 900 m.y. 12 / 31n / 95 - 147am