Download Section 18.3: Uses of Nuclear Chemistry

Section 18.3 Notes—Uses of Nuclear Chemistry
Objectives:
1. Define the half-life of a radioactive nuclide, and explain how it can be used to
determine an object’s age.
2. Describe some of the uses of nuclear chemistry.
3. Compare acute and chronic exposures to radiation.
Half-life
Half-life is the time required for half of a sample of radioactive substance to
disintegrate by radioactive decay or natural processes. A radioactive substance
decays at a constant rate that is unique to the particular substance. The rate of decay
is not influenced by external conditions, such as temperature or pressure.
Radioactive
Isotope
Half-life
Polonium-215
0.0018
seconds
Bismuth-212
60.5 seconds
Sodium-24
15 hours
Iodine-131
8.07 days
Cobalt-60
5.26 years
Radium-226
1600 years
Uranium-238
4.5 billion
years
Radioactive Dating
The fact that radioactive decay is constant and is not influenced by external
conditions makes it possible to determine the age of an object based on the amount
of radioactive isotope that it contains. An example of radioactive dating involves
the radioactive isotope carbon-14. Nearly all of the carbon on earth is present as the
stable carbon-12 isotope. However, there is also a small percentage is the unstable
carbon-14 that decays to form nitrogen-14. The ratio between carbon-12 and
carbon-14 in the environment remains constant through time.
Both carbon-12 and carbon-14 have the same electronic configuration, so they
undergo the same chemical reactions (have the same chemical properties. Both
isotopes are in carbon dioxide, which is used by plants in photosynthesis. As a
result, all plants and animals that eat plants contain the same ratio of carbon-14 to
carbon-12. Any carbon-14 that decays while the organism is alive is replaced
through photosynthesis or eating. But when a plant or animal dies, it stops taking in
carbon-containing substances, so the carbon-14 that decays is not replaced and the
amount decreases over time.
The half-life of carbon-14 is 5715 years. By analyzing organic material left from
dead plants and animals, scientists can estimate the age of an object by comparing
the amount of carbon-14 present to the amount in a living organism. Carbon-14 is
useful for dating specimens less than 50,000 years old (or about nine half-lives).
Scientists can date ancient rocks on the order of geologic time using Potassium-40,
which has a half-life of 1.28 billion years.
Other Uses of Nuclear Chemistry
Smoke Detectors
Today, nuclear reactions have become part of modern living. Smoke detectors
contain a small amount of americium-241, which decays to form neptunium-237
and alpha particles:
241
95𝐴𝑚
→
237
93𝑁𝑝
+ 42𝐻𝑒
The alarm is set off when the alpha particles contact the smoke particle.
Art Forgeries
A process called neutron activation analysis to determine whether artwork is a
forgery. In this process, a tiny sample of the artwork is bombarded by neutrons.
After some of the atoms in the sample absorb neutrons, they become radioactive
isotopes and emit gamma rays as they decay. Scientists can then identify each
element in the sample by the characteristic gamma ray pattern that the sample’s
atoms emit. This creates a “fingerprint” of the elements in the sample. If the
fingerprint matches materials not available when the artwork was supposedly
created, then the artwork is a fake.
Nuclear Medicine
Nuclear medicine includes the use of nuclear reactions both to diagnose certain
conditions and to treat a variety of diseases. There are many uses of radioactive
isotopes in medicine, such as:
 Doctors can create images of bones using x-rays. But x-rays cannot produce
images of softer tissues such as muscle. Doctors can take a close look at a
person’s heart using a thallium stress test. The person is given an intravenous
injection of thallium-201, which acts chemically like calcium and collects in
the heart muscle. As the thallium-201 decays, low-energy gamma rays are
emitted and are detected by a special camera that produces images of the heart
muscle.
 The radioactive isotope technetium-99 emits low-energy gamma rays. Bones
that are repairing themselves absorb minerals at a certain rate and take up
technetium at the same time. Because of this, doctors can monitor diseases
that cause fracture, infection, arthritis, or an invading cancer.
 Doctors use a process called positron emission tomography (PET) to create
detailed three-dimensional pictures of a person’s organs. In this procedure, the
patient is injected with a radioactive isotope that that has a short half-life and
ejects positrons. As the positrons collide with electrons, gamma rays are
produced, which are detected by a scanner.
Radiation Exposure to the Sun
Exposure to radiation can affect a person’s health. The affect on humans is
measured using a unit called a rem. A rem (Roentgen Equivalent Man) is a unit
used to measure a dosage of ionizing radiation.
Effect of Whole-Body Exposure to a Single Dose of
Radiation
Dose (rem)
probable effect
0-25
no observable effect
25-50
slight decrease in white blood
cell count
50-100
marked decrease in white blood
cell count
100-200
nausea, loss of hair
200-500
ulcers, internal bleeding
> 500
death
Exposure to a single large dose of radiation are said to have experienced an acute
radiation exposure. More than 230 people suffered acute radiation sickness and 28
died when a meltdown occurred in 1986 at the Chernobyl nuclear power plant in the
Ukraine.
The effects of nuclear radiation on the body can add up over time. Exposure to
small doses of radiation over a long period can be as dangerous as a single large
dose. Some scientific studies have shown a correlation between chronic radiation
exposure and certain types of cancer.