# Radioactive dating using uranium

(Do not confuse with the highly radioactive isotope, strontium-90.) Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium-86.If three different strontium-containing minerals form at the same time in the same magma, each strontium containing mineral will have the same ratios of the different strontium nuclides, since all strontium nuclides behave the same chemically.Rubidium-Strontium dating: The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87.Strontium-87 is a stable element; it does not undergo further radioactive decay.If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.Contrary to creationist claims, it is possible to make that determination, as the following will explain: By way of background, all atoms of a given element have the same number of protons in the nucleus; however, the number of neutrons in the nucleus can vary.

F, the fraction of K40 remaining, is equal to the amount of potassium-40 in the sample, divided by the sum of potassium-40 in the sample plus the calculated amount of potassium required to produce the amount of argon found. In spite of the fact that it is a gas, the argon is trapped in the mineral and can't escape.

The sum of protons plus neutrons is the mass number.

We designate a specific group of atoms by using the term "nuclide." A nuclide refers to a group of atoms with specified atomic number and mass number.

Because of radioactivity, the fraction of rubidium-87 decreases from an initial value of 100% at the time of formation of the mineral, and approaches zero with increasing number of half lives.

At the same time, the fraction of strontium-87 increases from zero and approaches 100% with increasing number of half-lives.