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Half-life is a fundamental concept in radioactive decay. Option B is correct: The half-life of a radioactive element is the time required for exactly half of the radioactive atoms in a sample to undergo decay. For example, if Carbon-14 has a half-life of 5730 years, half of a C-14 sample will decay in 5730 years, and half of the remaining half will decay in the next 5730 years, and so on. Option A is incorrect: No radioactive substance fully decays to zero in a finite time because the decay is exponential; theoretically, some atoms always remain. Option C is incorrect: Radioactive decay is a disintegration process; atoms do not double. The number only decreases exponentially. Option D is incorrect: Radioactive decay of individual atoms is a random process governed by probability; it is impossible to predict when a single atom will decay. Half-life is used in carbon dating, nuclear medicine, and determining the age of rocks.
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