Forms of radioactive dating

For our purposes, the most important are: When decay takes place, the original atom is called the parent atom, and the new atom produced by decay is called the daughter atom.

Those isotopes which are produced by radioactive decay are said to be radiogenic.

An isotope is defined by its atomic number and its atomic weight.

In the example in the previous paragraph, we have shown the notation used for isotopes.

With the exception of gamma decay, which need not concern us here, this will involve changing the number of protons, or neutrons, or both, and so also changing the atomic number, the atomic weight, or both.

There are a number of mechanisms by which decay may take place.

Some small variations have been observed in certain isotopes depending on which other chemicals they form molecular bonds with.

The fact that it has six protons is revealed by the "C", which is the chemical symbol for carbon; by definition, all carbon atoms have six protons.According to physicists, radioactive decay occurs at random: an atom of (for example) Ne (neon-22) just because its number has come up. Consider, by analogy, a man playing Russian Roulette with a six-shooter.Every single time he plays, he has a one-in-six chance of dying, and this is true no matter how long he's been playing. If we have an atom of Na, then no matter how old it is, it has a 50% chance of decaying in the next 2.6 years; and if it survives that period of time, then it has a 50% chance of decaying in the next 2.6 years; and so on.However, as this will happen nowhere on Earth outside a physics laboratory, we can take its half-life to be 43 billion years for all geological purposes.So to really affect the half-life of an isotope one needs to resort to highly artificial methods — such as dropping it into the core of a nuclear reactor.The reader should recall from high school that the nucleus of an atom consists of protons (positively charged particles) and neutrons (uncharged particles which have almost exactly the same mass as protons).The nucleus is surrounded by a cloud of electrons, negatively charged particles having negligible weight.Because the energy emitted from the nucleus when it decays is so very small, it is possible to change its half-life by ionizing it.If it is completely stripped of all its electrons, its half-life falls from 43 billion years to 33 years!What is more, for each isotope, this half-life is constant: it is a property of the isotope, and virtually unaffected by external circumstances. In the first place, it should be true in principle: it can be deduced from the underlying laws of quantum mechanics.In the second place it is confirmed by actual observation: shortly after the discovery of radioactive decay, scientists began trying to change the decay rate by subjecting unstable isotopes to heat, pressure, magnetism, and so forth, with negative results.

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