The nuclide 31Si is a radioactive isotope that disintegrates by beta decay.

A sample of silicon-31 is found to produce 251 disintegrations/s . Exactly 3.00 hours later, the same sample produces 113 disintegrations/s . What is the half-life of 31Si?
Strategy The relative rate of decay is proportional to the number of atoms present, so we can use Equation 21.3 to calculate the half-life as follows:

Equation 21.3