1. Why are some elements radioactive? In other words, why do they undergo nuclear decay?
2. Can we predict when a particular atom of a radioactive substance will decay?
Although nobody really knows the exact answer to the first question, many scientists do feel that certain combinations of protons and neutrons make the nucleus of an atom unstable, and therefore make it tend to be radioactive.
The answer to the second question is no. No one can tell when a particular atom of a radioactive substance will decay. However, when we are dealing with large numbers of radioactive atoms we can predict how long it will take for a certain number or percentage of the atoms to decay. For each radioactive isotope, we can predict a certain half-life.
The half-life is the time it takes for one-half of the atoms originally present in a sample to decay.
The half-life of a radioactive isotope may be short or long. For example, the half-life of 238/92U is 4.5 billion years, whereas the half-life of 257/103Lr is 8 seconds.
Example. The half-life of strontium-90 is 28 years. If we have 100 grams of strontium-90 today, how many grams of strontium-90 will be left in our sample in 84 years?
Since the half-life of strontium-90 is 28 years, this means that half of our strontium-90 will decay in 28 years. After 84 years, our sample will contain 12.5 grams of strontium-90.
It should be noted that strontium-90 has a chemical similarity to calcium (notice that Ca and Sr are in the same chemical family), which causes it to replace calcium in bone, and stay there, undergoing its radioactive decay. When nations test atomic bombs, strontium-90 gets into the atmosphere and begins to float around the earth. Gradually it falls to the ground. Since cows eat the grass from the fields, the strontium-90 gets into milk and other dairy products. This radioactive isotope accumulates in our bones side by side with calcium. It then bombards the nearby tissues and organs with beta rays. That 28-year half life means that it stays around for a long time.
If a person gets a particularly large dose, it can cause bone cancer. That's why thoughtful people were happy in the summer of 1963 when Russia, the United States, and Great Britain (3 of 5 nations at the time with nuclear weapons) signed the treaty banning atmospheric nuclear tests. Unfortunately, China and France didn't sign. In late spring of 1974 India exploded a nuclear device underground, and became the sixth nation to join the nuclear club. They had not signed a test-ban treaty.
Selected Radioisotopes and Their Half-Life