The time required for one half of the nuclei in a sample to decay is called the half-life of the isotope.Half-life is a common way of expressing the rate of radioactive disintegration. Figure 11.17 illustrates what occurs when a radioactive parent decays directly into its stable daughter product. When the quantities of parent and daughter are equal (ratio 1:1), we know that one hall life has transpired When one-quarter of the original parent atoms remain and three-quarters have decayed to the daughter product, the parent/daughter ratio 1:3 and we know that two half -lives have passed. After three half - lives, the ratio of parent atoms to daughter atoms is 1:7(one parent for every seven daughter atoms). If the half- life a radioactive isotope is known and the parent/ daughter ratio can be measured, the age of the sample can be calculated. For example, assume that the half-life of a hypothetical unstable isotope is 1 million years and the parent/daughter ratio in a sample is 1:15. Such a ratio indicates that four half lives have passed and that the sample must be 4 million years old.