The half-life of a radioactive species is the amount of time it takes the quantity of the species to decrease by half. After one half-life, ½ of the initial quantity of material will remain, after two ¼ will remain, after three 1/8 will remain, and so on. Half-lives of radioactive nuclei vary widely, and we’ll list just a few values below to give an idea of the range of timescales covered.