Download EST 4 Practice Isotopes, Radioactivity and Half Life Among the

EST 4
Practice
Isotopes, Radioactivity and Half Life
1. Among the elements below, which are isotopes of the same element? Explain your
answer.
2. Which of these representations is impossible? Explain your answer.
3. Natural silver is made up of two isotopes in almost equal proportions. Their atomic
masses are 107 u and 109 u, respectively. What is the relative atomic mass of silver?
4. Boron, which has an atomic mass of 10.81 u, is a mixture of two isotopes, which occur
in a ratio of 20 to 80 percent. The mass number of one of these isotopes is 11, the other
has a mass number of 10. Which of these two isotopes is the most common?
5. Neon is an element with the atomic number 10 and a relative atomic mass of 20.28 u.
Neon has three natural isotopes:

neon-20, with a relative atomic mass of approximately 20 u

neon-21, with a relative atomic mass of approximately 21 u

neon-22, with a relative atomic mass of approximately 22 u
Which of these three isotopes is most commonly found in nature? Explain your answer.
6. Which type of ionizing radiation, though very harmful to human health, can be blocked
with a simple sheet of paper?
7. Food irradiation is a process that kills harmful microorganisms and prolongs the shelf life
of the food. Isotopes like cobalt-60 are used. Since ionizing radiation must travel a certain
distance and penetrate the food completely, which type of radiation would be the most
appropriate for this application? Explain your answer.
8. Scintigraphy is a method of medical imaging used to visualize certain organs as they are
functioning. It involves a radioactive substance, which is injected into the patient. The
radioactive substances used usually have a very short half-life. Explain why they are
preferred over substances with longer half-lives.
9. The Chernobyl nuclear explosion occurred in 1986. Cesium-137 was released into the
atmosphere. It has a half-life of about 30 years. Is it normal that after more than 20 years,
levels of radioactivity in the ground near Chernobyl are still very high? Explain your
answer.
10. Fill in the table demonstrating that the half life of 12 g of uranium takes 35 000 years to
decrease its radioactive substance by half.
Decay Time
(years)
Mass (g) of Uranium remaining
% of Uranium remaining
11. In the last column of the table, write the number of days necessary for each of the
isotopes to weigh less than 1 g. Show your work.
Work:
Isotope
Half-life
Mass
Sodium 24
15 hours
90 g
Phosphorus 32
14 days
50 g
Iodine 131
8 days
120 g
Number of days
12. Carbon-14 is a radioactive isotope often used to date objects discovered on archeological
digs. Its half-life is 5 770 years. An archeologist has discovered a piece of wood.
Analysis of the wood shows that it contains only 10% of carbon-14. What is the
approximate age of the piece discovered? Show your work.
13. Explain when it would be useful to use polonium-216 which has a half life of 0.16
seconds and when you would want to use potassium-40 which has a half life of
1 300 000 000 years.