Which values do scientists compare when using potassium-argon dating? 1) the number of stable and unstable parent atoms 2) the number of non-decayed elements left in the rock 3) the amount of parent and daughter material trapped in the rock 4) the amount of gas from the decayed material that has escaped from rock

The correct answer is 3) the amount of parent and daughter material trapped in the rock.

Potassium–argon dating (K–Ar dating), is a method which is used in archaeology and it is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). The decay product Ar is able to escape the liquid rock. But, when the rock solidifies (recrystallizes) Ar starts to accumulate. To calculate time since recrystallization you must measure  

the ratio of the amount of Ar accumulated to the amount of K that remained.

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