Ch. 9: RADIOACTIVITY AND
NUCLEAR REACTIONS
Section 1-RADIOACTIVITY
What is an atom?
 An atom is the smallest piece of
matter.
 Atoms are composed of protons,
neutrons, and electrons.
 Ex. The element silver is
composed of only silver atoms.
The element hydrogen is composed
of only hydrogen atoms.
 Nucleus =
protons  + charge
neutrons  no charge
 Outside the nucleus =
electrons  - charge
(neutral)
Atomic Number = # of protons
Atomic Mass = # of protons & # of neutrons
Element
Atomic # 
Uranium
92
U
Symbol 
Atomic Mass 
238.029
PROTONS
 The total amount of charge in a nucleus is
determined by the number of protons, which also is
called the atomic number.
 Atomic number = number of protons
ELECTRONS
 Negatively charged electrons are
electrically attracted to the
positively charged nucleus and
swarm around it.
 An electron has a charge that is
equal, but opposite to a proton’s
charge.
 Atoms usually contain the same
number of protons as electrons.
 # of protons = # of electrons
PROTONS AND NEUTRONS IN THE
NUCLEUS
 Protons and neutrons
are packed together tightly in the nucleus.
 The region outside the nucleus in which the electrons
are located is large compared to the size of the
nucleus.
Nucleus/Atom VS. Marble/Stadium
 The size of a
nucleus in an
atom can be
compared to a
marble sitting in
the middle of an
empty college
football
stadium.
Where is the majority of an atom’s
mass?
 The nucleus contains almost all the mass of the
atom, because the mass of one proton or neutron is
2,000 times greater than the mass of an
almost
electron.
MASSES
PARTICLE RELATIVE RELATIVE ACTUAL
CHARGE MASS
MASS
ELECTRON
1-
PROTON
1+
NEUTRON
0
1
1836
9.11 X 10-28
1
1.674 X 10-24
1
1.675 X 10-24
HOW ARE PROTONS AND NEUTRONS HELD
TOGETHER SO TIGHTLY IN THE NUCLEUS?
 Usually only positive (+) and negative (-) charges
attract, but another force, called the STRONG
FORCE, causes protons and neutrons to be attracted
to each other.
(STRONG FORCE HOLDS NUCLEUS TOGETHER)
FOUR BASIC FORCES
 1) GRAVITY
 2) ELECTROMAGNETIC
 3) STRONG (NUCLEAR) The strong force is one of the
four basic forces and is about 100 times stronger
than the electric force.
 4) WEAK (NUCLEAR)
SHORT-RANGE FORCE
 Protons and neutrons have to be close together, like
they are in the nucleus, to be attracted by the strong
force (SHORT-RANGE FORCE)
(ONLY WORKS OVER SHORT DISTANCES—small
atoms)
LONG-RANGE FORCE
 The electric force is a long-range force, so protons
that are far apart still are repelled by the electric
force.
(ONLY WORKS OVER LONG DISTANCES—LARGE
ATOMS)
RADIOACTIVITY
 SMALL ATOMS
 LARGE ATOMS
 SF >(greater) EF
 EF >(greater) SF
 STABLE
 Unstable
 Particles repel; give off matter
and energy.
 This process of nuclear decay is
called radioactivity.
LARGE ATOMS
 LARGE nuclei tend to be UNSTABLE and can break
apart or decay.
 ALL nuclei that contain more than 83 protons ARE
radioactive (ATOMIC NUMBER >greater than 83)
ISOTOPES
 Nuclei that have the same number of protons, but
different number of neutrons are called isotopes.
 EXAMPLE—
U-235, U-238
Same element (URANIUM) At. # = 92
Different # of neutrons (U-235 = 143 n)best
(U-238 = 146 n)
CHAPTER 9: RADIOACTIVITY AND
NUCLEAR REACTIONS
Section 2—NUCLEAR DECAY
WHAT IS NUCLEAR RADIATION?
 When an UNSTABLE nucleus decays,
particles and energy are emitted from the
decaying nucleus.
 These particles and energy are called
NUCLEAR RADIATION. (Also known as
NUCLEAR DECAY AND RADIOACTIVITY)
WHAT ARE THE 3 TYPES OF
NUCLEAR RADIATION?
 The 3 types of nuclear radiation are alpha,
beta, and gamma.
 Alpha and beta radiation are particles
(matter)
 Gamma radiation is not a particle; it
behaves like a wave; similar to light;
electromagnetic radiation; most dangerous
ALPHA PARTICLES
 Made of protons and
neutrons
 Cannot pass through
low density materials,
such as a sheet of
paper
 SMOKE DETECTORS
some give off alpha
particles; smoke
disrupts process and
alarm goes off
(Americium-241)
What happens when an atom no longer
has the same number of protons?
 It is no longer the same element. (The
number of protons identifies the element as
a particular element)
 Transmutation is the process of changing
one element to another through nuclear
decay.
BETA PARTICLES
Made of protons, neutrons, and
electrons
Caused by another basic force—the
WEAK FORCE
Much faster and can pass through
low density materials, such as paper,
but are stopped by aluminum foil
GAMMA RAYS
They are NOT made of protons,
neutrons, or electrons
They are a form of radiation called
electromagnetic waves
They are able to travel through low
and medium density materials, such
as paper and aluminum foil; but are
blocked by high density materials,
such as lead and concrete
Is RADIATION harmful?
 Damage from alpha and beta particles and
gamma rays can cause cells in your body to
quit functioning properly, which can lead to
illness and disease.
 How does someone protect themselves?
1. TIME (around material)
2. DISTANCE (from material)
3. SHIELDING (safety equipment)
4. ROUTINE MONITORING
RADIOACTIVE HALF-LIFE
The half-life of a radioactive isotope is
the amount of time it takes for half of
the nuclei in a sample to decay.
Some decay to stable atoms in less
than a second.
Some require millions of years to
decay.
RADIOACTIVE DATING
 Some geologists, biologists, and
archaeologists are interested in the ages of
ROCKS and FOSSILS found on Earth.
 CARBON DATING –carbon-14 is often used
to find the ages of objects once living. (Also,
potassium-40)
 URANIUM DATING –uranium-235 can be
used to estimate the ages of rocks.
CHAPTER 9: RADIOACTIVITY AND
NUCLEAR REACTIONS
Section 3: DETECTING RADIOACTIVITY
Because you can’t see or feel alpha and beta particles or gamma rays,
you must use instruments to detect their presence.
 A Geiger counter is a
device that measures
radioactivity by
producing an electric
current when
radiation is present.
The intensity of
radiation present is
determined by the
number of clicks or
flashes of light per
second.
BACKGROUND RADIATION
This type of radiation is not
produced by humans. It
comes mainly from Earth’s
rocks, soils, and atmosphere
(building materials—bricks,
wood, and stones; food,
water, and air)
RADON GAS
 The largest source of
background radiation
comes from radon gas.
Radon is produced in
Earth’s crust by the
decay of Uranium238. Radon gas can
seep into houses and
basements from the
surrounding soil and
rocks.
CHAPTER 9: Radioactivity
and Nuclear Reactions
Section 4: NUCLEAR REACTIONS
What is the first step in a game
of pool?
 A similar result occurs
when a neutron is shot
into the large nucleus of a
uranium-235 atom. The
nucleus is split.
 The process of splitting 1
nucleus into 2 nuclei with
smaller masses is called
nuclear fission. The word
“fission” means to
divide.
 FISSION RHYMES WITH
DIVISION
The total mass
of the products
is slightly less
than the mass
of the original
nucleus and the
neutron. The
small amount of
missing mass is
converted into a
tremendous
amount of
energy.
A chain
reaction is
an ongoing
series of
fission
reactions.
(Billions of reactions
can occur each
second)
↑ REACTIONS
= ↑ ENERGY
Nuclear fission is used to generate electricity
and is used in nuclear weapons.
 The critical mass is the
amount of fissionable
material required so
that each fission
reaction produces
approximately 1 more
fission reaction (keeps
chain reaction going—
so won’t die out or
grow out of control).
 U-235 (1 nucleus)
 Dynamite (1 molecule)
 U-235 30 million
times more energy
1 gram of U-235
2 tons of COAL
 Even more energy can
be released in another
type of nuclear
reaction called nuclear
fusion.
 In nuclear fusion, 2
nuclei with low masses
are combined to form
one nucleus of larger
mass.
 Extremely high
temperatures are
found in the center of
stars, including the
Sun.
 These high temps
(millions of degrees
Celsius) are related to
nuclear fusion.
 A small amount of mass is changed into an enormous
amount of ENERGY.
 Earth receives a small amount of this energy as HEAT
and LIGHT.
 As the Sun ages, HYDROGEN nuclei are converted to
HELIUM. (ONLY 1% OF SUN’S MASS HAS BEEN
CONVERTED TO ENERGY—ENOUGH FOR 5
BILLION MORE YEARS)
(SUN)
(RHYMES WITH DIVISION)