Half-life is the time it takes for half the quantity of a substance to decay. Fermium (atomic number 100) has 19 isotopes, most of which have half-lives of less than a second.

The concept of half-life is fundamentally used in the context of radioactive decay, which is a process by which an unstable atomic nucleus loses energy by emitting radiation. In the case of Fermium (atomic number 100), which has 19 isotopes, most of these isotopes are unstable and exhibit radioactive decay, converting into more stable atoms over time.

For a given isotope of Fermium, the half-life is the duration required for one-half of the amount of that isotope to undergo radioactive decay. If most of the isotopes of Fermium have half-lives of less than a second, it means that within one second, half of the original atoms of these isotopes would have decayed into other elements or isotopes.

As an example, if you started with 10 grams of a hypothetical Fermium isotope with a half-life of less than a second, you would have less than 5 grams remaining after less than a second has passed. In the next interval of less than a second, you'd have less than half of that remaining, and so on, until virtually all the original Fermium has decayed.

This quick rate of decay makes these Fermium isotopes highly unstable and challenging to study, as they persist for such a brief period before changing into other elements.