• The nuclei of some atoms (e.g. Uranium) are unstable and will decay to become either some nearby element in the periodic table or some isotope of the same element.

• 235U 207Pb -- (704 million years)
• 87Rb 87Sr -- (48.8 billion years)
• 14C 14N - (5730 years)

• The time in parenthesis is the ``half-life'' of the specified process. The half-life represents the time required for 1/2 of the atoms in a sample to decay to the other state.

• Like popcorn, some atoms decay right away, while others survive much longer than the average.

• There is no way to predict when a particular atom will decay. We know very precisely the probability that an atom will decay in a given time interval, but whether it really does depends on the roll of the cosmic dice.

• If you start with a pure sample of one of the above unstable elements, after the time in parenthesis has elapsed, half of that sample will have decayed.

• After one more half life, 3/4 of the original sample will have decayed (i.e. half of the remaining half).

• If you are using Uranium/Lead dating, how do you determine how much lead was in the rock originally??? A rock which is half uranium and half lead, may be very young and just had a large amount of lead to start.

• Lead comes several stable forms -- one has 207 protons and neutrons (the decay product of 235Uranium), and one with 204 in particular.

• Uranium 235 exculsively decays to the form with 207 protons and neutrons.

• The ratio of 207Pb to 204Pb in naturally occurring lead in any rock (i.e. no contamination from the decay of 235U) is a constant.

• Measure the amount of 204Pb in the sample under study and that measurement will tell you how much of 207Pb was there originally.

• Any excess 207Pb will have been produced by the decay of Uranium.

• In practice, the radioactive dating techniques used today measure the time since a rock was last molten.

• The age of the Earth is 4.6 billion years. Nevertheless, according to radioactive dating techniques, the ``ages" of rocks found on its surface are far younger.

• The material making up these rocks has been part of the Earth for all 4.6 billion years

• however all of these rock were formed when molten material on or beneath Earth's surface solidified only recently.

• The oldest rocks in the Solar System -- the ones which have not been melted since they were first formed -- are the meteorites which fall to Earth from interplanetary space.

• The meteorites are pieces of the asteroids -- leftover fragments of planetesimals from the era of the formation of the planets.

• They solidified early in the history of the Solar System and have not melted since.

• Jupiter stirred up the motions of the nearby planetesimals and prevented them from accumulating into a planet.

• These stirred planetesimals collided at high speed and fragmented. They were unable to stick together and accumulate to form a large planet.

• These leftover planetesimal pieces now form the asteroid belt.

• Asteroids collide and fragment. Pieces (and sometimes whole asteroids) can be gradually directed to the Earth via gravitation nudges from the planets.

• When radioactive dating techniques are applied to meteorites they are uniformly found to be 4.56 billion years old - which establishes the time of the formation of the planets.

Revised October 29, 2004