Radioactive Decay
What do you know about
Radioactivity?
1. All atoms are made up of __________.
2. What are some radioactive isotopes?
3. Why do some isotopes/atoms break
down and decay?
4. What is given off as atoms break down
and decay?
5. How can we manipulate and use this
process to our advantage?
The Nucleus
• Recall that atoms are composed of
protons, neutrons, and electrons.
• The nucleus of an atom contains the
protons, which have a positive charge, and
neutrons, which have no electric charge.
The Strong Force
• How do you suppose protons and
neutrons are held together so lightly in the
nucleus?
• Another force, called the strong force,
causes protons and neutrons to be
attracted to each other.
The Strong Force
• Protons and
neutrons have to be
close together, like
they are in the
nucleus, to be
attracted by the
strong force.
• The strong force is a
short-range force that
quickly becomes
extremely weak as
protons and neutrons get
farther apart.
Attractions and Repulsion
• Some atoms, such as uranium, have many
protons and neutrons in their nuclei.
• These nuclei are held together less
tightly than nuclei containing only a few
protons and neutrons.
Attractions and Repulsion
• If a nucleus has only a few protons and
neutrons, they are all close enough
together to be attracted to each other by
the strong force.
Forces in a Large Nucleus
• If nuclei have many protons and neutrons,
each proton or neutron is attracted to only
a few neighbors by the strong force.
Radioactivity
• When the strong force is not large enough
to hold a nucleus together tightly, the
nucleus can decay and give off matter and
energy. Larger elements are more
unstable and tend to decay.
• This process of nuclear decay is called
radioactivity.
• All nuclei that contain more than 83 protons are
radioactive.
• However, many other nuclei that contain
fewer than 83 protons also are radioactive.
Radioactivity
• Almost all elements with more than 92
protons don’t exist naturally on Earth.
• They have been produced only in
laboratories and are called synthetic
elements.
• These synthetic elements are unstable,
and decay soon after they are created.
Isotopes
• Nuclei that have the same number of
protons but different numbers of neutrons
are called isotopes.
• These two isotopes of helium each have
the same number of protons, but different
numbers of neutrons.
Stable and Unstable Nuclei
• Nuclei with too many or too few neutrons
compared to the number of protons are
radioactive and decay too.
Nuclear Radiation
• The three types of nuclear radiation
are alpha, beta and gamma
radiation.
• Alpha and beta radiation are particles.
Gamma radiation is an electromagnetic
wave.
Nuclear Radiation
• When an unstable nucleus
decays, particles and energy
called nuclear radiation are
emitted from it.
Alpha Particles
• When alpha radiation
occurs, an alpha
particlemade of
two protons and two
neutrons is emitted
from the decaying
nucleus.
• Notice that the alpha
particle and its
symbol is the same
as a helium nucleus,
• In alpha decay, two
protons and two
neutrons are lost from
the nucleus or Helium
gas is released.
Beta Decay
• Sometimes in an
unstable nucleus a
neutron decays into
a proton and emits
an electron.
• The electron is
emitted from
the nucleus and
is called a beta
particle.
Gamma Rays
• They have no mass
and no charge and
travel at the speed of
light.
• The properties of
gamma rays are
summarized in the
table.
Gamma Rays
• Thick blocks of dense materials, such as
lead and concrete, are required to stop
gamma rays.
Radioactive Half-Life
• Some radioisotopes decay to stable atoms
in less than a second.
• However, the nuclei of certain radioactive
isotopes require millions of years to decay.
• A measure of the time required by the
nuclei of an isotope to decay is called the
half-life.
Radioactive Half-Life
• The half-life of a radioactive isotope is the
amount of time it takes for half the nuclei
in a sample of the isotope to decay.
• The nucleus left after the isotope decays is
called the daughter nucleus.
Radioactive Half-Life
• Half-lives vary widely
among the radioactive
isotopes.
• Some geologists,
biologists, and
archaeologists,
among others, are
interested in the
ages of rocks and
fossils found on
Earth.
Radioactive Dating
• The number of half-lives is the amount of
time that has passed since the isotope
began to decay.
Carbon Dating
• Carbon-14 has a half-life of 5,730 years
and is found in molecules such as carbon
dioxide.
• Plants use carbon dioxide when they
make food, so all plants contain carbon14.
• Only material from plants and animals that
lived with the past 50,000 years contains
enough carbon-14 to be measured.
Common Isotopes
•
•
•
•
Polonium-214 has a half life of .7 sec
Uranium-238 = 4.5 billion years
Carbon-14 = 5,730 years
Lead-212 = 10.6 hours
Problems
• If a human body contained 50 grams of C14 and now had only 25 g, how old is the
body?
• ½ of 50 is 25 so 1 half-life elapsed and
that is 5,730 years
Problems
• If the amount of C-14 is 12.5 g
• 11,460 years
• If the amount of C-14 is 6.25g
• 17,190 years
Graphs of Radioactive Decay
Standard: SPS3a Students will differentiate between alpha, beta
decay and gamma radiation
What is Nuclear Radiation?
Alpha
Decay
What is released?
Radiation is made
of… (Particles or
electromagnetic
waves)
Interesting Fact (Use
pgs 541-543)
Beta
Decay
Gamma
Radiation
Reading Assignment
• Please read
pgs 544-545
on
Radioactive
Half-life.
• As you read
please make
notes over
the following
sections:
Section
Radioactive half-life
Radioactive Dating
Carbon Dating
Uranium Dating
Your Notes