Spontaneous emission of radiation when
the nucleus of an atom breaks down to
form a different element.
The constant level of radioactivity always
present around us
Caused by:
 Cosmic rays
 Naturally occurring uranium
 Radon in the air
 Radioactive isotopes in food and environment
What is your radiation exposure?
National Average: 500 mrem
Background Average: 360 mrem
 Nuclear reactions involve the nucleus
(protons and neutrons)
BUT
chemical reactions involve the
transfer and sharing of
electrons
The bigger the atom
gets and the further
from a 1:1 ratio of
protons and neutrons,
the less stable the
atom is
After element 83 (Bismuth)
the elements are naturally unstable
and may emit decay particles
Electromagnetic Force
 Force of repulsion between positive charges of
the protons in the nucleus
Strong Force
 Force present that holds quarks together and
therefore keeps the protons and neutrons tightly
packed in the nucleus
When the electromagnetic force wins out,
the nuclei break apart!
Decay
Type
Alpha
Beta
Gamma
Reaction
Symbol Charge
Strength
Particle
Can be
stopped
by…
• The superscript indicates the mass number
and the subscript indicates the atomic
number.
Mass Number
Atomic Number
A
X
Z
Element Symbol
1. Write the nuclear symbol for the element
that is given.
2. Draw an arrow.
3. Identify the type of particle that has decayed
and write it after the arrow.
4. Balance the mass number (top) and the
atomic number (bottom).
5. Identify the new element.
 Loss of an alpha particle
239Pu
94
235U
92


4
2α
4
2 He
 Loss of a beta particle (electron)
0
-1 β
42K
19

0
-1 e
 Loss of a beta positive particle (positron)
0
+1 β
42K
19

0
+1 e
 Loss of a gamma particle
 Does
0γ
0
it effect the outcome of the reaction?
 Alpha and
239Pu
94
gamma decay of:

Proton
1p
1
or
Neutron
1n
0
1H
1
Time required for half a sample to decay
The stability of the isotope is what determines
the rate of decay.
Less Stable = Faster Decay
 After each half-life, half of
the sample decays.
 Start = 100%
 40 blue particles are
present
 1 half-life = 50%
 20 blue remain
 2 half-lives = 25%
 10 blue remain
 3 half-lives = 12.5%
 5 blue remain
 4 half-lives = 6.25%
 2.5 blue remain
Amount never
becomes zero!!
 After 10 half-lives sample considered
nonradioactive because it approaches the level of
background radiation.
 Because the amount never reaches zero,
radioactive waste disposal and storage causes
problems.
Would you want radioactive waste stored in your
community?
How can we get rid of nuclear radioactive waste?
Example 1:
The half-life of mercury-195 is 31 hours. If you
start with a sample of 5.00 g, how much of it
will still be left after 93 hours?
Example 2:
How many half lives have passed if there is
only 1.875 g left of a 30 g sample?
If the half life for this sample is 1 hour, how
many total hours have gone by?
Sun is powered by nuclear reactions
Electricity from nuclear power plants
Medical:
 x-rays, cancer treatment, radioactive tracers
Industrial:
 Sterilize equipment, sterilize (irradiate) food, x-
rays of metal equipment (scan for cracks in
airplanes)
Military
Exposure to radiation can cause cell mutations
and illness.
Four factors that affect exposure:
 Dose (strength of radiation)
 Exposure time
 Area exposed
 Tissue exposed
Two light atoms combine to form a heavier
atom of higher energy
This reaction powers the sun!
One heavy atom breaks down into two or more
smaller atoms and produces energy
This becomes a chain reaction
(as one atom splits and hits more, and those split and hit more)
Supercritical:
creates a great
release of energy
- atomic bomb
Ping Pong Video
Particles reacting must have critical mass:
 The minimum amount of mass needed for the
neutron to hit and react with
 No critical mass = no reaction
Fission reactions can also be controlled to
contain the energy.
Non-critical
“controlled”
Super Critical
Controlled Fission: used for nuclear power
Controlled Diagram:
Control Rods to
limit nuclear
fission!
Fission heats
the water that
turns to steam
and moves the
turbines