In radioactivity, the half-life is the time interval required for half of the atomic nuclei in a radioactive sample to decay. If the half-life passes again, half of the remaining mass will remain (1/2, 1/4, 1/8, 1/16, 1/32, ...) The mass gets smaller and smaller, but there is always a little bit left.
This simulation explains the concept of half-life, including the random nature of half-life, in terms of single particles and larger samples. It describes decay processes, including how elements change and emit energy and/or particles. Explains how radiometric dating works and why different elements are used for dating different objects. Also identifies that 1/2 life is the average time for a radioactive substance to decay.