GEOLOGY

Detailed explanation-1: -If a rock contains 25 percent of a parent isotope and 75 percent of its daughter isotope, two half-lives must have passed.

Detailed explanation-2: -After two half-lives, 75% of the original parent atoms have been transformed into daughter products (thus, only 25% of the original parent atoms remain). After three half-lives, only 12.5% of the original parent atoms remain. As more half-lives pass, the number of parent atoms remaining approaches zero.

Detailed explanation-3: -The ratio of parent to daughter after one half-life will be 1:1. After two half-lives, half of the remaining half will decay, leaving one-quarter of the original radioactive parent atoms. Those transformed atoms bring the tally of daughter atoms to three-quarters of the crop of parent plus daughter atoms.

Detailed explanation-4: -After a time interval based on the decay rate, 1/2 of the # of atoms of the parent = 1/2 the # of atoms of the daughter. This = the half life. The decay of parent atoms to daughter atoms continues Page 8 If the half-life of this example is 4 months.