Radioactivity
(2nd half of Ch 39)
39.5 Radioactive Half-Life
• Radioactive isotopes decay at different rates.
• The radioactive decay rate is measured in
terms of a half-life.
• The half-life of a substance is the time needed
for half of the radioactive atoms to decay.
• Some half-lives are less than a millionth of a
second. But some, like Uranium-238, has a
half-life of 4.5 billion years.
39.5 Half-Life cont.
• If half-lives are so long, how do scientists
measure them?
• They can measure the rate at which a
substance decays, using radiation detectors.
• The half-life of an isotope is related to its rate
of disintegration (a half-life can be computed
from the rate of disintegration.)
• Stop: answer question 1 on pg 616!
39.6 Transmutation
• When a nucleus emits an alpha or beta
particle, a different element is formed.
• The changing of one element to another is
called transmutation.
• For example: Uranium has 92 protons. When
an alpha particle is ejected during decay, the
nucleus is reduced by two protons and two
neutrons. The new element is Thorium
(atomic number 90).
Transmutation cont.
• The Thorium is also radioactive, and decays by
Beta particles.
• When a beta particle is ejected, one of the
neutrons spontaneously changes into a proton.
• The new element is now Protactinium (atomic
number 91).
• Eventually, Uranium decays to harmless and nonradioactive lead (atomic number 82). See Figure
39.15 pg 619.
• STOP: Answer question 2 on pg 618.
39.7 Artificial Transmutation
• Ernest Rutherford in 1919 was the first to
succeed in artificially transmuting an element.
• He bombarded nitrogen nuclei with alpha
particles and found oxygen and hydrogen
atoms that were not there before.
• Artificial transmutation can be used to convert
harmful radioactive waste into nonradioactive
elements.
39.8 Carbon Dating
• Taking carbon out to Applebees. (hahaha) 
• When cosmic rays from outer space hit our upper
atmosphere, transmutation takes place and
protons, neutrons and electrons are created.
• The protons combine with electrons to make
hydrogen, but the neutrons find their way down
to Earth’s lower atmosphere and react with
Nitrogen to form Carbon-14 and Hydrogen.
39.8 Carbon Dating cont.
• Plants take in some of this radioactive Carbon-14.
• Animals eat plants (or eat other animals that eat
plants), so animals get Carbon-14 in them.
• Carbon-14 is a beta emitter, and decays back into
nitrogen.
• When an animal or plant dies, the amount of
Carbon-14 it has stops being replenished.
• The Carbon-14 in the dead plant or animal decays at a
known rate of 5730 years for a half-life.
• The ratio of regular Carbon-12 to Carbon-14 can then
be used to determine how long ago the organism lived.
• STOP: Answer question 2 on pg 622.
39.9 Uranium Dating
• The dating of older, non-living things is
accomplished with uranium instead of
Carbon-14.
39.10 Radioactive Tracers
• Small amounts of radioactive elements can be
added to things to determine how well that
thing is working.
• Examples: it can be added to fertilizer to see
how much is being taken up into the plants,
medicine to see how much is being absorbed,
sewage lines to see where the leak in the
underground pipe is, etc.
• See Figure 39.19 pg 624.
39.11 Radiation and You
• Radiation has been around since before
humans.
• Radiation is what warms the planet and keeps
the core molten.
• Even the helium in a balloon is the product of
radioactivity.
39.11 Radiation and You cont.
• Much of the radiation we are exposed to is
cosmic radiation from space.
• The atmosphere stops a lot of this radiation.
• At higher elevations, radiation is more intense.
For example, in Denver, you get twice the
radiation than you do at sea level.
• A couple of flights between NY and San
Francisco expose you to as much radiation as
a chest X-ray.
39.11 Radiation and You cont.
• Radiation damages cells.
• Cells can repair most kinds of molecular
damage if the radiation the cells are exposed
to is not too intense.
• But if you are exposed to intense radiation, or
moderate radiation over a long period of time,
cancer and a shorter life expectancy can
result.
Assignment
• Worksheet!