CHAPTER 7
NUCLEAR CHANGES
Chapter 7 Section 1:
Radioactivity
What is radioactivity?
◦ Radioactivity is the process by
which an unstable nucleus emits
one or more particles or energy in
the form of electromagnetic
radiation
Nuclear Radiation

Radioactive materials have unstable nuclei

Nuclei change by releasing energy.
 Elements
can transform into isotopes of
the same element or into entirely
different elements
Nuclear Decay
 As
elements “change” or transform they
undergo nuclear decay.
◦ Carbon dating uses rates of nuclear decay
 The
released energy is called Nuclear
Radiation
◦ Can cause damage to living tissue
 When
a radioactive atom decays, the
nuclear radiation leaves the nucleus.
 The
radiation is now able to react with
any surrounding matter
 The
reaction depends on the charge,
mass, & energy of the nuclear
radiation
Types of Nuclear Radiation
4
types of nuclear radiation:
◦ Alpha Particles
◦ Beta Particles
◦ Gamma Rays
◦ Neutron Emission
Alpha Particles a
 Positively
charged & more massive
than any other type of nuclear
radiation

Discovered by Ernest Rutherford (also
discovered the nucleus)

Consist of 2 protons & 2 neutrons
Alpha Particles

Do not travel far through materials (can barely
pass through paper)

The massive size of alpha particles limits
their movement

Remove electrons from surrounding matter
as they pass through it
(_______________)

Ionization causes alpha particles to slow
down
Beta Particles b
 Fast-moving
Electrons produced
by neutron decay
 Negatively
 Travels
charged particles
farther through matter
compared to alpha particles
How can electrons come from a
positively charged nucleus?
 In
the 1930’s, it was discovered that as
a neutron (neutral charge) decays it
forms a proton & electron
 The
electron has very little mass and
gets ejected from the nucleus at a high
speed as a Beta Particle
Gamma Rays
 Very
High Energy Light
 1898-
Marie Curie isolated radioactive
radium
 1900-
Paul Villiard discovered that
radium emitted a new type of nuclear
radiation…
Gamma Rays!
 More
penetrating than Beta particles
 Gamma
rays are not made of matter &
do not have an electric charge
 Gamma
rays are a form of
electromagnetic energy
 Have
 Do
more energy than X-rays
not ionize surrounding matter
 Can
penetrate 60cm of aluminum & 7
cm of lead!
 Not
easily stopped by clothing or
building materials
◦ Can be very harmful
Neutron Emission
 Neutron
Emission is the release of
a high-energy neutron by some
neutron-rich nuclei during radioactive
decay.
 Emitted
from an unstable nucleus
 Do not ionize surrounding matter
 Travel farther through matter than a or
b
Nuclear Decay
 When
an unstable nucleus emits
alpha or beta particles, the # of
protons or neutrons changes.
 Example: Radium-226
changes to
Radon-222 by emitting an alpha
particle
A nucleus gives up 2 protons & 2
neutrons during alpha decay

Nuclear decay processes can be written
similar to a chemical reaction equation

The nucleus before the decay is like the
reactant

The nucleus after the decay is like the
product
A nucleus gains a proton & loses a
neutron during beta decay
 Equation
is formed the same way
except the symbol for a beta particle is
used.
 In
all cases of beta decay, the mass
number before & after the decay
doesn’t change
When writing the equation…
 Write
the original element on the left
side (reactants)
X
is used for the unknown symbol
 A is used for the unknown mass
 Z is used for the unknown atomic #
 Solve
for the unknowns & rewrite the
balanced equation
Radioactive decay rates
A
substance’s half-life is the time in
which half of the radioactive substance
decays
(A measure of how quickly a substance decays)
 After
each half-life passes half of the
sample remains unchanged.
Practice Problem
 Radium
has a half-life of 1599 years.
How long would it take seven-eights
of radium-226 sample to decay?
 1-
7/8 = ______
 ½ x ½ x ½ =______
 How many “half-lives”?______
 Each
half-life is 1599 years
 It
takes 3 half-lives to have 1/8
radium remaining.
3
x 1599 = 4797 years
More Practice! (pg. 228)
Carbon- 14 has a half life of 72
hours. How long would it take
for 15/16 of the carbon
sample to decay?
 Uranium-238
decays very slowly,
with a half-life of 4.47billion
years. What percentage of the
sample would remain after 13.4
billion years?
A
sample of strontium-90
is found to have decayed
to 1/ 8 of its original
amount after 87.3 years.
What is the half-life of
strontium?
A
sample of francium-212
will decay to 1/16 its
original amount after 80
minutes. What is the halflife of francium-212?
 What
is the half-life of a 100.0 g
sample of nitrogen-16 that decays
to 12.5 grams in 21.6 seconds?
 The
half-life of hafnium-156 is
0.025 seconds. How long will it
take a 560 g sample to decay to
one-fourth
of its original mass?
 Potassium-42
has a half-life of 12.4
hours. How much of an 848 g
sample of potassium-42 will be left
after
62.0 hours?
Half-life practice
 Gold-210 is a radioactive
isotope that has a half-life of
12 hours. If a lab starts with a
13milligram sample of gold210, how much will remain
after 37 hours?
Chapter 7 Section 2
Nuclear Fission & Fusion
Nuclear Force

Elements can have both stable & unstable
isotopes.

Example: Carbon-12 is stable
Carbon-14 is unstable &
The stability of a nucleus depends on
the nuclear forces that hold the
nucleus together
Nuclei are held together by a
particular force
 Strong
Nuclear Force is the
interaction that binds protons &
neutrons together in a nucleus

Force is much stronger than the repulsion
force between protons

Strong nuclear forces occur at very short
distances (3 x 10-15 m)
 In
stable nuclei the attractive
forces are stronger than
repulsion forces
 Too
many neutrons or protons
can cause a nucleus to become
unstable & decay
 Nuclei
with too many neutrons or
too few are unstable & will undergo
decay
 Nuclei
with more than 83 protons
are always unstable
◦ Atomic numbers will be greater than
_____
Nuclear Fission
 The
process of producing lighter nuclei
from heavier nuclei is called Fission
 First
observed by Otto Hahn & Fritz
Strassman in 1939
 Their
experiment bombarded
Uranium-235 with neutrons
The Product…
2
lighter nuclei, neutrons, & energy
 Barium-137
 The
& Kryton-84
product includes 15 neutrons
 Uranium
can undergo many types of
fission with different products
Energy is released during Nuclear
Fission
 During
Fission the nucleus breaks into
smaller nuclei
 The
reaction releases large amounts of
energy
 Each
dividing nucleus releases about 3.2
x10-11 J of energy
 Hahn
& Strassman determined
the overall masses of the
elements in their reaction had
decreased after the reaction.
 The
missing mass had changed
to Energy!
The Theory of Relativity
 Presented
by ___________
__________ in 1905
 Describes
the equivalence of mass &
energy observed in nature
 Equivalence
means that matter can be
converted into energy & energy into
matter.
E=
2
mc
Energy = mass x (speed of
2
light)
Neutrons released by fission can start
a chain reaction!
 When
a nucleus splits into lighter
nuclei, they need less neutrons so
the neutrons are emitted.
 The
emitted neutron can then collide
with surrounding nuclei and undergo
fission
Nuclear Chain Reaction
A
series of fission processes
in which the neutrons
emitted by the dividing
nucleus cause the division of
the other nuclei
Chain reaction is the principle
behind the nuclear bomb!
Chain reactions can be controlled
 The
more neutrons that are produced
per reaction the great the chances of
creating a chain reaction
 Specific
materials can be used that will
slow a fission chain reaction by
absorbing neutrons
Nuclear Fusion (Hydrogen Bomb)

Energy can be obtained when light nuclei
are combined into heavy nuclei
 Fusion
is the process in which
light nuclei combine at extremely
high temperatures forming heavier
nuclei & releasing energy
Chapter 7 Section 3
Dangers & Benefits of Nuclear
Radiation
Dangers of Nuclear Radiation
 Background
Radiation is
nuclear radiation that arises
naturally from cosmic rays & from
radioactive isotopes in the soil &
air
◦ Example: Sun, soil, water & plants
 Nuclear
radiation can ionize atoms in
living tissue.
◦ Example: Hemoglobin loses its ability to carry
oxygen when exposed to radiation
 Skin
helps keep low levels of radiation
outside the body.
 Nuclear
radiation can cause burns in the
skin as well as destroy bone marrow
cells.
 Radiation
sickness results from
exposure to high levels of nuclear
radiation.
 Observable
effects from nuclear
radiation exposure (low intensity)
may not appear for days or years.
◦ Example: hair loss, sterility, death of bones,
& cancer.
Nuclear Radiation can cause genetic
mutations
 Long-term
effects of nuclear
radiation appear when DNA
molecules in the body are
damaged.
 Example:
◦ Radioactive wastes from the
Shiprock Uranium Mine
contaminated water that was used
for cattle & sheep.
◦ The radiation damaged the DNA in
their reproductive cells causing
offspring to have birth defects.
Beneficial Uses of Nuclear Radiation
 Many
uses in medicine &
archeological dating
 Smoke
alarms
 Cancer Treatment
Radioactive Tracers
 Used
in agriculture, medicine, & scientific
research
 Radioactive Tracers
are added to
substances so that its location can be
detected later.
 Used
to locate tumors, measure the
speed of a river, track drugs throughout a
body
Nuclear Power
 Nuclear
reactors are used to
generate electricity (uses fission)
 Nuclear
fission has many benefits
◦ Produces not gaseous pollutants
◦ Much more energy than coal & oil
reserves
Disadvantages of Nuclear Fission
 Serious
safety concerns with handling,
treatment, and disposal of the Uranium
 Safe
operation of the nuclear reactor is
also a concern
◦ Power plants can only be active for 40 years
 Nuclear
wastes must be safely stored
Half-life practice
A
radioactive substance has
a half-life of 10 years. What
fraction of a sample of the
substance would be left
after 30 years?
Half-life practice
Iron-109
is a radioactive
isotope that has a half-life of
4 hours. What fraction of
the Iron-109 will remain
after 24 hours?
1. The half-life of a radioactive isotope is
1.0 hr.
a) What fraction of the original sample of
radioactive nuclides is left after 3.0 h?


b) What fraction of the original sample of
radioactive nuclides is left after 1.0 d?

2. A soil sample contains 40 mg of 90Sr. An
atom of 90Sr masses 1.49295 x 10-25kg.
Approximately, how much 90Sr will be in the
sample 150 y from now?

Strontium's half-life = 50 years.

3. In 1898, Pierre and Marie Curie
isolated about 10 mg of 226Ra from 8 tons
of uranium ore. The half-life of Radium is
25 years. If this sample had been placed in
a museum, how much of the radium
would remain in the year 2100?

A radioactive element has a half-life of
20 days. How much of a 16mg sample
would be undecayed after 80 days?