Isotopes
Unlike the number of protons, the number of neutrons in an atom is not diagnostic of an element. Atoms of any given element can come in different versions called isotopes, which have differing number of neutrons, but always the same number of protons.
- in other words, an element can have different numbers of neutrons. These different instances are known as isotopes.
- because different isotopes of an element are differentiated by their number of neutrons, therefore the atomic mass varies between these isotopes
- ex. Carbon-12 is an isotope of 6 protons and 6 neutrons. Carbon-14 is an isotope of 6 protons and 8 neutrons
Some isotopes are stable and some are unstable.
- ex. Lead-202 is an unstable isotope, while lead-204, lead-206, lead-207 and lead-208 are stable
Unstable means that the atoms spontaneously decay into something else at a predictable rate (though the decay occurs at unpredictable moments)
Another word for unstable is radioactive
In one kind of isotope instability, a neutron turns into a proton. This means that the mass number stays the same, but the atomic number goes up, so the atom becomes a different element.
- in another type of radioactive decay, the opposite happens
- Every unstable isotope decays at its own characteristic rate which is precisely known.
- decay is exponential
- ex. Starting with 100 grams of a radioactive isotope, a fixed proportion of the remaining amount will decay. For example, if that proportion was 10%, then it would start at 100g, then 90g, then 81g, and so on.
Half-life
The favoured measure of decay rate is the half-life, which is the time taken for half of the atoms to decay
- the half-life is the same, no matter how many atoms have already decayed (that is what exponential decay means)
The half-life of carbon-14 is between 5000 and 6000 years. Therefore, for specimens older than about 50,000 to 60,000 years, carbon dating is useless.
- the half-life of rubidium-87 is 49 billion years; the half-life of fermium-244 is 3.3 milliseconds. Such startling extremes show us the stupendous range of clocks available.
- potassium-40 has a half life of 1.26 billion years, and is therefore an isotope much used on the evolutionary timescale.
An isotope decaying means it turns into another element
- ex. Potassium-40 has a half life of 1.26B years. One element below potassium is argon. If we start with some quantity of potassium, after 1.26 billion years, half of the potassium-40 will have decayed to argon-40. After another 1.26 billion years, half of what remains (ie a quarter of the original) will have decayed, and so on.
- Therefore, we now understand how isotopes can be used as our clock. If we took a pound of potassium-40 and put it in a sealed box and waited for a few hundred million years, we would find that some proportion of those original potassium-40 atoms are now argon-40. By measuring the proportions, we can conclude how much time as elapsed.
Link rot
A 2003 study found that on the Web, about one link out of every 200 broke each week, suggesting a half-life of 138 weeks.
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