Half-life is defined as the time it takes for half of the atoms in a radioactive material to decay.

• A. The Rate of Decay
• B. The Energy Released Per Decay
• C. Time It Takes For Half Of The Atoms In A Radioactive Material To Decay
• D. Time For All Atoms To Decay

Answer: (C)

Half-life is a measure of time for a fixed portion of a radioactive sample to decay. For any given isotope, after one half-life interval has passed, half the original undecayed atoms remain. That’s why the statement “the time it takes for half of the atoms in a radioactive material to decay” is the defining description of half-life. This time constant is intrinsic to the isotope and does not depend on how much material you start with.

The rate of decay, or activity, is how many decays occur each second, and it changes as the number of undecayed atoms decreases. The energy released per decay is the energy per individual decay event, a property of the decay process itself, not a measure of how long it takes for half the atoms to decay. And the idea of all atoms decaying in a finite time conflicts with exponential decay, which approaches zero remaining undecayed atoms but never reaches zero in a finite interval. The relationship can be summarized by N(t) = N0 × (1/2)^(t/T_half), illustrating how the number of undecayed atoms halves every half-life period.