Learning Goal 5
– The number of protons in an atom (# of protons
= # of electrons).
– The total number of protons and neutrons in an
Atoms with the same atomic number, but
different mass numbers.
– Atoms have the same number of protons, but
a different number of neutrons.
Isotopes of an element have the same
chemical properties but different weights.
The amount of time it takes for half of the atoms
to decay into a more stable form.
Cobalt-60 has a half-life of 5.3 years and is
made in a reactor. Cobalt-60 is used for
radiation therapy of cancer patients.
Over 3,500 isotopes are known, and most are
made in the lab.
Smoke Detectors and
How many of us have smoke detectors in our house? Chances are
that a great number of homes have had one or more of these
devices installed as an early warning system in case of fire. What
most consumers don't know is that many of these units contain a
small amount of americium-241. By utilizing the radioactive
properties of this material, smoke from a fire can be detected at a
very early stage. This early warning capability has saved many lives.
In fact, studies have shown that 80% of fire injuries and 80% of fire
fatalities occur in homes without smoke detectors.
Significant progress has been made in this field of study since the discovery of
radioactivity and its properties. One application is carbon-14 dating. Recalling that all
biologic organisms contain a given concentration of carbon-14, we can use this
information to help solve questions about when the organism died. It works like this.
When an organism dies it has a specific ratio by mass of carbon-14 to carbon-12
incorporated in the cells of it's body. (The same ratio as in the atmosphere.) At the
moment of death, no new carbon-14 containing molecules are metabolized, therefore
the ratio is at a maximum. After death, the carbon-14 to carbon-12 ratio begins to
decrease because carbon-14 is decaying away at a constant and predictable rate.
Remembering that the half-life of carbon-14 is 5700 years, then after 5700 years half
as much carbon-14 remains within the organism.
Example : If an organism such as a tree contained 1 gram of carbon-14 while it was
living, then after 5700 years it would contain half that amount, or 0.5 grams of carbon14.
This method of dating using carbon-14 is only good for organisms or artifacts that are
biological by nature and on the order of tens of thousands or years old.