Question

The half life of a radioactive nuclide is

• A. Half the time needed for a sample to complete decay.
• B. Half the time a sample can be kept before it starts to decay.
• C. The time needed for half a sample to decay.
• D. The time needed for the rest of a sample to decay one half if it has already decayed.

Hint:

Half-life is denoted by \[ T_{1/2} \] and is defined as the time taken for the radioactive nuclei in a sample to reduce to half their original number.

Answer 1

The Correct Option is C

Step 1: Understand the meaning of half-life.
Half-life is an important concept in radioactivity. It represents the time required for the number of undecayed radioactive nuclei in a sample to reduce to half of its initial value.
If initially the number of nuclei is \[ N_0, \] then after one half-life, \[ N=\frac{N_0}{2} \]

Step 2: Analyze the given options.

Option (1):
This is incorrect because radioactive decay theoretically never becomes completely zero.

Option (2):
This is incorrect because radioactive decay starts immediately and spontaneously.

Option (3):
This correctly defines half-life as the time required for half of the radioactive sample to decay.

Option (4):
This statement is not the standard definition of half-life.

Step 3: Final conclusion.
Hence, the correct definition of half-life is \[ \boxed{\text{The time needed for half a sample to decay}} \]