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Another radioactive element useful for dating is carbon-14, which decays into nitrogen-14. With a half-life of 5,730 years, carbon-14 decays much more rapidly than radium 87 and so is useful for measuring the age of objects from the recent historical and geologic past, such as fossils, bones, wood, and other organic materials. Whereas rubidium 87 is incorporated into rocks during their formation, carbon-14, which is an essential element of the cells of organisms, becomes incorporated into living tissues as organisms grow. The ratio of carbon-14 to stable carbon isotopes in the organism is the same as it is in the atmosphere. When a living organism dies, mo more carbon dioxide is absorbed and so new carbon isotopes are added. The daughter nitrogen-14 isotope, existing in gaseous form, leaks out of the dead organism, and thus, we cannot use it to compare the ratio of original to daughter as is done with rubidium-87 and its daughter. However, as the amount of carbon-14 in the dead organism becomes less over time, we can compare the proportion of this isotope remaining with the proportion that is in the atmosphere and from this calculate the approximate number of years since the organism has died. Dating dead organic material by this method is moderately reliable in samples up to about 50,000 years old, but beyond that the accuracy becomes unreliable.

9, According to paragraph 5, why can’t scientists compare the ratio of carbon-14 to nitrogen-14?

A, the amount of nitrogen-14 is not predictable.

B, The ratio of these two elements doesn’t change.

C, Nitrogen-14 has an unpredictable decay rate.

D, Carbon-14 tends to evaporate too quickly.

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Rosemarie T. | Published Freelance Author and ScientistPublished Freelance Author and Scientist
It is not predictable simply because it leaks out of a dead organism unpredictably, NOT because it has an unpredictable decay rate.  In other words, what can't be predicted ISN'T the nature of N-14's decay, but exactly how quickly/thoroughly the N-14 will evacuate the organism once that organism dies.


Thank you, Rosemarie!!
Its hard to understand but I understood that N-14 is not unpredictably because it leaks out of a dead organism.