First slide
Radio activity

A radioisotope X  with a half life 1.4 × 109  years decays to Y  which is stable. A sample of the rock from a cave was found to contain X and Y in the ratio 1: 7 . The age of the rock is


Number of Nuclei at t = 0

Number of Nuclei at time t 

As per question


7N0-7x=x or x=78N0

 Remaining nuclei of isotope X


So three half lives would have been passed

  t=nT1/2=3×1.4×109 years =4.2×109 years 

 Hence, the age of the rock is 4.2×109 years. 

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