If the half-life of a sample of a radioactive substance is 30 seconds, how much would be left after 60 seconds? a. one-fourth b. half c. one-eighth d. you would need to know how many atoms you are starting with.

A. one-fourth

Explanation:

Half life is the amount of time taken by a radioactive material to decay to half of its original value.

Expression for rate law for first order kinetics is given by:

where,

k = rate constant  

t = age of sample  = 60 sec

a = let initial amount of the reactant  = 100 g

a - x = amount left after decay process =

Thus as 25 g of radioactive substance would be left after 60 seconds, which is of the initial amount.

particle   p is an electron and particle q is a neutron