Radioactive Decay
When do you stop counting?
Pretend that the large cylinder is a parent isotope. Now you can visualize the decay of the parent
to the daughter. It's easy to see that the second cylinder is half the size of the first. The third is
half the size of the second. But when the cylinders get small, the differences are much harder to
see. At some point, there is too little of the parent left. That isotope pair is no longer useful for
dating.
Radioactive Decay
Radioactive decay is the breakdown of unstable _______________ into stable elements. To
understand this process, recall that the ______________ of all elements contain the particles
__________________ , __________________, and ___________________ .
Isotopes
An element is defined by the number of _______________ it contains. All ___________ of a
given element contain the same number of protons. The number of _________________ in an
element may vary. Atoms of an element with different numbers of neutrons are called isotopes.
Consider carbon as an example. Two isotopes of carbon are shown below (Figure below ).
Compare their protons and neutrons. Both contain six protons. But carbon-12 has six neutrons
and carbon-14 has eight neutrons.
Radioactive Decay
Isotopes are named for their number of protons plus neutrons. If a carbon atom had seven
neutrons, what would it be named?
Almost all carbon atoms are carbon-12. This is a stable isotope of carbon. Only a tiny percentage
of carbon atoms are carbon-14. carbon-14 is unstable. It is a radioactive isotope of carbon.
Pictured below is carbon dioxide ( Figure below ), which forms in the atmosphere from carbon14 and oxygen. Neutrons in cosmic rays strike nitrogen atoms in the atmosphere. The nitrogen
forms carbon-14. Carbon in the atmosphere combines with oxygen to form carbon dioxide.
Plants take in carbon dioxide during _________________________. In this way, carbon-14
enters food chains.
Carbon-14 forms in the atmosphere. It combines with oxygen and forms carbon dioxide.
How does carbon-14 end up in fossils?
Decay of Unstable Isotopes
Like other unstable isotopes, carbon-14 breaks down, or decays. The original atoms are called
the parent isotopes . For carbon-14 decay, each carbon-14 atom loses an beta particle. It
changes to a stable atom of nitrogen-14. The stable atom at the end is the daughter product (
Figure below ).
Radioactive Decay
Unstable isotopes, such as carbon-14, decay by losing atomic particles. They form different,
stable elements when they decay.
The decay of an unstable isotope to a stable element occurs at a constant rate. This rate is
different for each parent-daughter isotope pair. The decay rate is measured in a unit called the
half-life. The half-life is the time it takes for half of a given amount of an isotope to decay. For
example, the half-life of carbon-14 is 5,730 years. Imagine that you start out with 100 grams of
carbon-14. In 5,730 years, half of it decays. This leaves 50 grams of carbon-14. Over the next
5,730 years, half of the remaining amount will decay. Now there are 25 grams of carbon-14. The
figure below graphs the rate of decay of a substance ( Figure below ).
How many grams will there be in another 5,730 years?
Radioactive Decay
The rate of decay of a radioactive substance is constant over time.
Summary
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A half-life is the time it takes for half of the parent isotopes of an element to change to
the daughter product.
Carbon-14 has a half-life of 5,730 years.
Explore More
Use the resources below to answer the questions that follow.
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What Is Carbon Dating?
o http://www.chem4kids.com/files/atom_isotopes.html
o http://www.geography4kids.com/files/cycles_carbon.html
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5.
What is an isotope?
Describe carbon-14.
What is average atomic mass?
What is radioactive decay?
What is carbon dating?
Review
6. What makes an isotope radioactive? Are all isotopes radioactive?
7. What is a parent isotope and a daughter product?
8. Describe half-life. Use an example.