Radioactive dating isotopes used
Some isotopes are radioactive; that is, they are unstable because their nuclei are too large.
Geologists regularly use five parent isotopes to date rocks: uranium-238, uranium-235, potassium-40, rubidium-87, and samarium-147.
This process of changing one element (designated as the parent isotope) into another element (referred to as the daughter isotope) is called radioactive decay.
The parent isotopes that decay are called radioisotopes.
These parent radioisotopes change into daughter lead-206, lead-207, argon-40, strontium-87, and neodymium-143 isotopes, respectively.
Thus geologists refer to uranium-lead (two versions), potassium-argon, rubidium-strontium, or samarium-neodymium dates for rocks.
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So, after only half an hour, half the sand should be in the top bowl, and the other half should be in the bottom bowl.