The half-life of a radioisotope is a characteristic of its nuclear instability, and it cannot be changed by ordinary chemical or physical means. This is an example of what is known as exponential decay. The amount of the radioactive material thus gets smaller and smaller, but it never disappears entirely.

(In this sense, the word “decay ” does not mean to rot it means to diminish in amount.) If a particular radio-isotope has a half-life of one hour, for example, then at 3 PM there will be only half as many of the original species of radioisotope atoms remaining as there were at 2 PM at 4 PM, there will be only half as many as there were at 3 PM, and so on. In this process, the number of atoms of a radioisotope (a radioactive isotope) is constantly diminishing because the atoms are transforming themselves into other kinds of atoms. Half-lives are most often heard of in connection with radioactive decay. On the other hand, the speeds of many chemical reactions depend on the amounts of the various substances that are present, so their rates cannot be expressed in terms of half-lives more complicated mathematical descriptions are necessary. Processes of this kind are called first-order processes.

The rates of some biological processes, such as the elimination of drugs from the body, can be characterized by their half-lives, because it takes the same amount of time for half of the drug to disappear no matter how much there was to begin with. The faster the process, the shorter the substance ’s half-life is. It is thus an indication of how fast the diminishment process proceeds or of the rate or rapidity of that process. The half-life of a diminishing substance is the time it takes for the amount of substance present to go halfway from some initial value to zero.

What is a half life