Dating rocks using radioactive isotopes
It is the number of neutrons in their nuclei that varies, but too many neutrons make the nuclei unstable, as in carbon14.Each atom is understood to be made up of three basic parts.The nucleus contains protons (tiny particles each with a single positive electric charge) and neutrons (particles without any electric charge).So, for example, every carbon atom contains six protons and six electrons, but the number of neutrons in each nucleus can be six, seven, or even eight.
Therefore, carbon has three isotopes, which are specified as carbon12, carbon13 and carbon14 (figure 1). Comparison of stable and unstable atoms of the element carbon.
Geologists regularly use five parent isotopes as the basis for the radioactive methods to date rocks: uranium238, uranium235, potassium40, rubidium87, and samarium147.
Dating rocks using radioactive isotopes comments 

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How is radioactive dating used to determine the age of an object.
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Video 1.6 Using radioactivity to date rocks  Dr. Ray Burgess .
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