Answer. com after three half-lives, what fraction of a radioactive sample has decayed?

Thus the fraction decayed is

Explanation: Radioactive decay follow first order kinetics.

Now, to calculate the amount decayed after n half lives, we use the formula:

where,

a = amount of reactant left after n-half lives

= Initial amount of the reactant

n = number of half lives  = 3

Putting values in above equation, we get:

The fraction decayed after 3 half lives will be of the initial amount.

Thus the fraction decayed is