Radiometric dating decay curves
An atom with six protons and eight neutrons is written as C.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.It is obvious from this example that the exactness of our knowledge of the half-life will depend on the exactness with which we can measure the initial size of the sample and the rate at which it decays.Each unstable isotope, then, has its own characteristic half-life.However, two atoms can have the same atomic number and different atomic weights.So, for example, C (carbon-14) has six protons and eight neutrons.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.
An example is Be ought to be particularly susceptible to such effects: it is a very small atom, and it decays by electron capture. For the same reasons, even smaller variations have been achieved by putting Re (rhenium-187).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!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.
Not all isotopes undergo decay: those that do are called unstable isotopes (or radioactive isotopes) and conversely those that don't are called stable. As we can see from this example, it is perfectly possible for different isotopes of the same element to differ in their stability.