Other methods such as Potassium-argon dating and Isochron dating are based on faulty assumptions and so unreliable as to be useless.Many atoms (or elements) exist as numerous varieties called isotopes, some of which are radioactive, meaning they decay over time by losing particles.Since 1955 the estimate for the age of the Earth has been based on the assumption that certain meteorite lead isotope ratios are equivalent to the primordial lead isotope ratios on Earth.In 1972 this assumption was shown to be highly questionable.
The geological time scale and an age for the Earth of 4.5 b.y.
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For example, in the The decay constant has dimensions of reciprocal seconds.
In the special case in which parent and daughter atoms are present in equal quantities, the age of the specimen is the half-life of the parent isotope: The first assumption, that the amount of the daughter isotope in the original rock is known, is the weakest assumption.
Protons and neutrons together are called nucleons, meaning particles that can appear in the atomic nucleus.