Nuclear Unit Notes Powerpoint presentation
Transcript Nuclear Unit Notes Powerpoint presentation
Structure of the Atom
Introduction and Vocabulary
Nuclear chemistry involves reactions that
occur in the nucleus of an atom.
Therefore, we will be studying changes in the
number of protons and neutrons of an atom.
Nuclear reactions often occur in nuclei that
are unstable due to a disproportionate ratio of
neutrons to protons.
Compare the ratio of protons to neutrons in
Carbon-12 and Uranium 238.
Introduction and Vocabulary
Unstable = liable to change or be altered; not
firm or fixed
Transmutation = a change in the nucleus of an
The number of protons and neutrons
determines the mass of a given nucleus.
The ratio of protons to neutrons in all nuclei
with atomic numbers greater than 83 makes
the nuclei unstable.
In order to become stable, isotopes of these
Elements with an atomic number greater
than 83 contain radioactive isotopes.
All nuclei with atomic numbers greater than 82 are
An isotope that is radioactive is called a radioisotope.
Radioisotope-tagged "Smart Bullets" – Monoclonal antibodies
target malignant cells for diagnosis and treatment.
I. Natural Radioactivity and Stability
Nuclear Stability: the larger (more massive) a nucleus is, the
harder it is for it to stay together
When a nucleus is ______________________,
it gives off decay
one element to another
particles and changes from
known as ___________________________
Natural decay or natural transmutation
Atoms with an atomic number of 1 through 83 have at least one
stable isotope, but…
All isotopes of elements above 84 are more reactive and are
II. Four Modes of Decay
Natural Radioactivity: _________________________________
of the _________________
of an atom, with the emission of
Particles and/or energy
Modes of Decay (See Table ______)
Penetration: How far into a material the radioactive particle will
Transmutation = When the atomic nucleus of one
element is converted into the atomic nucleus of another
element. Transmutations can occur naturally or
When an unstable nucleus decays, it emits radiation
in the form of:
1. Alpha decay
2. Beta decay
3. Positron Emission
4. Gamma Rays
When an unstable nucleus emits an alpha particle,
the nucleus is called an alpha emitter.
An alpha particle is the same thing as a helium
Types of Decay
Nucleus loses 2 protons and 2 neutrons (alpha
Has a +2 charge
Can be blocked by paper or clothing
226 Ra → 222 Rn + 4 He
A neutron turns into a proton and gives off an electron
Has a –1 charge
Can be blocked by metal foil
Since a proton is formed, the atomic number
14 C → 14 N
An Equation for Beta Decay:
When a proton becomes a neutron, the atom
loses a positive charge (positron).
This is called positron emission.
Positron emission tomography shows that limbic regions in the brains of in-treatment
cocaine users are activated by watching cocaine-related videos. Watching videos of
nature scenes does not result in activation of these regions. This suggests that cueinduced craving for cocaine reflects activity in specific regions of the brain.
(Greater activation is denoted by colors at the top end of the scale to right.)
Gamma rays have no mass and no
It is the energy lost from settling
within the nucleus after a change.
Since gamma rays do not affect the
atomic number or mass number, it is
generally not shown in the nuclear
Types of Decay continued…
Each gamma ray has no mass and no charge
and no particles; it’s just pure energy!
Account for almost all the mass lost in
Not completely blocked by lead or concrete
(BUT GAMMA, IT’LL KILL YOU!!!)
238 U → 234 Th + 4 He + 2 0 γ
Types of Decay
Identify type of decay:
A. alpha, B. beta, C. gamma, D. positron, E.
147N + 0-1β
115B + 01β
4 He + 224 Po
14 C + 0 e → 14 N
Nuclear Reaction Equations
Nuclear Reaction Equations:
Calculating the new compound or types of
14 N →
226 Ra →
Contain only one reactant because the
element is naturally giving off a particle.
Examples of natural transmutations:
alpha decay, beta decay, and positron
emission occur in nature as a result of
unstable neutron to proton ratios.
ARTIFICIAL TRANSMUTATION = a
change in the nucleus of an atom caused by
bombardment (addition) of a particle (such
as a neutron)
Sine you are adding a particle to the
nucleus, artificial transmutations have 2
Write this as an equation.
**Natural Transmutations will have
only one reactant.
**Artificial transmutations will have
FISSION = a reaction involving the splitting
of a heavy nucleus to produce two smaller
Fission will always yield 3 neutrons!!
When a nucleus undergoes fission, it splits into
several smaller fragments. These fragments, or
fission products, are about equal to half the
original mass. Three neutrons are also emitted.
We can induce an atom of uranium-235 to undergo fission by
bombarding it with neutrons. Does this mean we have to
keep bombarding a lot of U-235 with a lot of neutrons in
order to get any useful energy out of it? No, we let the
uranium do it for us. This is called an uncontrolled chain
Fission can occur
when a nucleus of
a heavy atom
neutron, or it can
Fusion = the combining of two light nuclei
to produce a heavier nucleus.
fuels both the sun
and the stars. We
are not able to
reactions here on
Earth because they
require such HIGH
Fusion reactions DO NOT have radioactive products
like fission reactions. This makes it a safer energy
In both fission and fusion reactions, the mass of
the products is less than the mass of the
reactants! This contradicts the Law of
Conservation of Mass.
The missing mass or “MASS DEFECT” is
matter that is turned into energy!
This missing mass can be determined
by Albert Einstein’s equation:
E = mc2
The power that fuels the
sun and the stars is
In a hydrogen bomb, two
isotopes of hydrogen,
deuterium and tritium are
fused to form a nucleus
of helium and a neutron.
This fusion releases 17.6
MeV of energy. Unlike
nuclear fission, there is
no limit on the amount
of fusion that can
Radioactive substances decay at a constant rate.
This rate of decay is random and is not dependent on,
or affected by, temperature or pressure.
Half Life = the time it takes for half of the atoms in a
given sample of an element to decay.
Every isotope has its own half life.
See Reference Table N.
What is the half-life of C-14?
Which of the above isotopes do you think is more
Answer = Carbon-14
The shorter the half-life the less stable an isotope
Set up a Table: Carbon -14
# of Half
0 half lives
1 half life
11460 years 2 half lives
Most chromium atoms are stable, but Cr-51 is an unstable isotope
with a half-life of 28 days.
(a) After 168 days, how many half-lives will a sample of Cr-51 have
Ans. = 6 half-lives
(b) What fraction of a sample of Cr-51 will remain after 168 days?
Ans. = 1/64
(c) If a sample of Cr-51 has an original mass of 52.0 g, what mass
will remain after 168 days.
Ans. = 0.8125g
Answer for (C)
After 1 half-life, mass = 26.0g
After 2 half-lives, mass = 13.0g
After 3 half-lives, mass = 6.5g
After 4 half-lives, mass = 3.25g
After 5 half-lives, mass = 1.63g
After 6 half lives, mass = 0.815g
1. What does the term “half-life” mean in nuclear
2. Do any factors affect the rate of decay?
3. Explain the relationship between half-life and
stability of an isotope.
4. Which reference table lists the half lives of
5. If there was a 100g sample of Ra-226, how many
grams would remain after 3200 years? Why?
RISKS AND USES
There are inherent risks
involved with radioactivity and
the use of radioactive isotopes.
- biological exposure
- long-term storage and disposal
- nuclear accidents
EFFECTS OF RADIATION ON
High penetrating radiation (i.e. gamma
radiation) goes directly into living tissue.
This energy strips electrons from
molecules forming ions and high energy
molecules called free radicals.
These free radicals and ions can cause
severe tissue damage and also rapid
growth of unhealthy cells.
This is one way cancer
is formed in the body.
Nuclear power plants
Many nuclear reactors use
uranium as fuel for fission
When the fuel rods no
longer have enough uranium
to be useful, they contain
many unstable decay
products with long half-
It is difficult to store and
dispose of these products.
Nuclear power plants
themselves can be safe
when used properly,
however, they can be deadly
if something goes wrong.
In the event of a nuclear
accident, harmful radiation
can be released into the air
Chernobyl: aftermath of the explosion and meltdown. Thirty-one
firefighters died trying to control the blaze in Unit 4. (1986)
Radioactive isotopes also have
many beneficial uses.
- Radioisotopes with short
half-lives that are quickly
eliminated from the body are
important tracers in medical
Ex: Iodine-131 is used to detect
and treat thyroid conditions.
Ex: Technetium-99 (tc-99)
accumulates in tumors and can be
used to identify tumors on a scan.
*** Both I-131 and tc-99 have short
half-lives and are quickly eliminated
from the body to minimize damage.
-Cobalt-60 emits gamma rays which
can be used in cancer treatment.
-Although this is dangerous, the
gamma rays will likely affect the
rapidly growing cancer cells
more than the normal cells.
Other forms of gamma radiation can
be used to kill bacteria in food
(I.e. raw meats and produce)
These include cesium-137 (Cs-137)
And cobalt-60 (Co-60)
Carbon-14 is the best known isotope
used in radioactive dating. Its halflife is 5730 years. After 4 halflives, too little C-14 remains for it
to be accurately measured.
Therefore C-14 is not used to date
things older than 25,000 years old.
A pre-excavation photograph of
one the Arabian hearths
sampled for charcoal, for
Carbon 14 dating.
Uranium-238 eventually decays
into Pb-206 in a series of steps.
The ratio of U-238 compared to Pb206 can be used to date
geological formations way over
What are some pro’s and con’s of
C-14 dating and U-238 dating?
Gamma rays are absorbed by different
substances in different amounts.
Thicker materials will absorb more
radiation than thinner materials.
Industries can measure the thickness
of products such as plastic wrap,
garbage bags, etc. by measuring the
amount of radiation they absorb.