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The amount of strontium-86 in a given mineral sample will not change.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.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.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.(Note that this does not mean that the ratios are the same everywhere on earth.
The number of protons in the nucleus of an atom is called its atomic number.
Strontium-86 is a stable element that does not undergo radioactive change.
In addition, it is not formed as the result of a radioactive decay process.
Therefore the amount of argon formed provides a direct measurement of the amount of potassium-40 present in the specimen when it was originally formed.
Because argon is an inert gas, it is not possible that it might have been in the mineral when it was first formed from molten magma.
Rubidium-Strontium dating: The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87.