How many half-lives are needed to reduce the radioactivity of a radionuclide to about 1% of its original measurement?

To determine how many half-lives are needed to reduce the radioactivity of a radionuclide to about 1% of its original measurement, it's important to understand the concept of half-life. The half-life is the period it takes for half of a given quantity of a radionuclide to decay. Each subsequent half-life reduces the remaining quantity to half of what it was at the beginning of that period.

When you conduct a series of half-life calculations, you'll notice that after one half-life, 50% of the original quantity remains, after two half-lives, 25% remains, after three half-lives, 12.5%, and after four half-lives, 6.25% remains. Continuing this pattern, after seven half-lives, the remaining quantity is approximately 0.78% of the original amount, which is effectively 1% for practical purposes.

This exponential decay characteristic is fundamental to understanding radioactive decay. Since we want to determine when the radioactivity falls to about 1%, it becomes evident that after seven half-lives, the activity will be reduced to just under 1%. Thus, this reasoning confirms that seven half-lives is the correct answer.