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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. 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. 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.
