# 35.2: Introduction


Radioactive decay is the process of an atom changing into a different type of atom; it is a change in the number of protons.  Half-life is the time it takes for one half of a sample of radioactive atoms to decay, so that half of the original number of atoms have decayed.  Each radioactive isotope has its own half-life.  The half-life of an isotope of carbon may be used to determine the age of a fossil.  The amount of a radioactive isotope remaining in a sample indicates the approximate age of the sample.

Table $$\PageIndex{1}$$: Radioactive Isotope and Half-life

Half-life

$$\ce{_{94}^{239}Pu}$$

24,400 years

$$\ce{_{92}^{238}U}$$

4.51 x 109 years

$$\ce{_{84}^{214}Po}$$

0.00016 seconds

$$\ce{_{83}^{210}Bi}$$

5 days

$$\ce{_{82}^{210}Pb}$$

20.4 years

$$\ce{_{6}^{14}C}$$

5,730 years