Final answer:
The half-life of a radioactive substance is the time it takes for half of the original nuclei to decay and is a measure of its rate of decay, not a measure of the threat it poses to living organisms.
Step-by-step explanation:
The statement that the half-life of a radioactive substance represents the length of time it poses a threat to living organisms is false. The half-life (t₁/₂) of a radioactive substance is the time it takes for half of the original nuclei in a sample to decay or the time at which half of the nuclei remain. It is a measure of the rate at which a radioactive substance undergoes radioactive decay, not directly a measure of its threat or safety to living organisms.
The half-life is a characteristic constant unique to each isotope and does not depend on the sample's size or external conditions like temperature or pressure. For example, Carbon-14 has a half-life of 5,730 years, while Uranium-235 has a much longer half-life of 704 million years. In contrast, medical isotopes used for imaging, such as Iodine-131, have a relatively short half-life of 8.02 days to ensure they do not remain in the body for too long.
Understanding a radioactive isotope's half-life is crucial for various applications, including estimating how long a radioactive isotope will be active or determining how long it must be stored before its radiation level decreases sufficiently to no longer be a concern. However, the risk a radioactive substance poses is determined by several factors, including its half-life, the type of radiation emitted, and how it interacts with living tissue.