Radioactive elements "decay" (that is, change into other elements) by "half lives." If a half life is equal to one year, then one half of the radioactive element will have decayed in the first year after the mineral was formed; one half of the remainder will decay in the next year (leaving one-fourth remaining), and so forth.The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life (in other words raised to a power equal to the number of half-lives).The corresponding half lives for each plotted point are marked on the line and identified.It can be readily seen from the plots that when this procedure is followed with different amounts of Rb87 in different minerals, if the plotted half life points are connected, a straight line going through the origin is produced. The steeper the slope of the isochron, the more half lives it represents.The decrease in the amount of potassium required to form the original mineral has consistently confirmed the age as determined by the amount of argon formed.Carbon-14 dating: See Carbon 14 Dating in this web site.The creationist "argon escape" theory does not support their young earth model.) The argon age determination of the mineral can be confirmed by measuring the loss of potassium.In old rocks, there will be less potassium present than was required to form the mineral, because some of it has been transmuted to argon.
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Therefore the relative amounts of rubidium-87 and strontium-87 can be determined by expressing their ratios to strontium-86: Rb-87/Sr-86 and Sr87/Sr-86 We measure the amounts of rubidium-87 and strontium-87 as ratios to an unchanging content of strontium-86.Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements.By "age" we mean the elapsed time from when the mineral specimen was formed.An atom with the same number of protons in the nucleus but a different number of neutrons is called an isotope.
For example, uranium-238 is an isotope of uranium-235, because it has 3 more neutrons in the nucleus.
It has the same number of protons, otherwise it wouldn't be uranium.