Define and discuss half life of radioactive isotopes. Compare the half life of radium, uranium and carbon to indicate the vast differences in theirs half life. Determine that after 20 half life's the amount of radioactivity becomes absent. Point out that radioactive decay is not affected by external conditions. Discuss how the half life is used to determine the age of an organic substance. Introduce the formula used to arrive at the age of an organic material. Work the formula amount remaining = original amount (No) x (1/2) ^t to arrive at the age of a substance. Discuss radioisotope use in diagnosis and treatment of some endocrine disorders.