Radioactive Time Difference

Half-life is not a constant rate; it is a slowing rate.

Eh?

Tornado is right. It is exponential decay. The mean lifetime is longer than the half life.

Well aware what half-life is. It's the way #1 was phrased that provoked my comment.

The Big Bang was about 14 Billion years (1,000 million) ago. The Earth came into existence 'only' about 4.6 Billion years ago. The heavier elements of which the Earth is composed are the stuff left over after (numerous) supernova explosions, which occurred in the time frame of 1 Billion to 9 Billion years after the Big Bang.

Also - the decay rate of a radioactive material depends on which isotope it is; i.e., how many neutrons are present in the nucleus. U-238 has a fairly long half-life life, whereas U-235 has a short half-life.

And there really weren't dinosaurs when Raquel Welch stared in One Million Years B.C.

Just saying.

"...U-235 has a short half-life..."

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U-238's half life of almost 5 billion years is relatively long compared to the half live of many other isotopes. It doesn't really mean U-235's half life of a little over 700 million years is short though. It doesn't seem right to call any length of time that is within 2 magnitudes of order off the age of the universe as short. Perhaps 'less long' ?

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Good question.

http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Uranium-Resources/The-Cosmic-Origins-of-Uranium/

Current thinking is that this planet was created several billion years after the Big Bang....This Wiki link describes the Radiometric dating process....

..." uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample"...

http://en.wikipedia.org/wiki/Radiometric_dating

Is the question making the assumption that all lead started as uranium? Is that a correct assumption?

No...... "Lead (Pb) has four stable isotopes: 204Pb, 206Pb, 207Pb, 208Pb. Lead-204 is entirely a primordial nuclide and none of it is radiogenic. The isotopes lead-206, lead-207, and lead-208 represent the ends of decay chains called the uranium series (or radium series), the thorium series, and the actinium series respectively. These series represent the decay chain products of long-lived primordial U-238, Th-232, and U-235 respectively."Source(s):http://en.wikipedia.org/wiki/Isotopes_of...

All elements started as Hydrogen, the most abundant element in the Universe (maybe even Multiverse!). Fusion has created ALL of the more complex elements. Some of them are not stable on earth, and hence decay to lower states (entropy at work).

Thank you all, good answers.

Regards JD.