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
atom
 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 decay.
Elements with an atomic number greater
than 83 contain radioactive isotopes.
Rutherford’s Experiment
All nuclei with atomic numbers greater than 82 are
radioactive.
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

radioactive
When a nucleus is ______________________,
it gives off decay
one element to another
particles and changes from
________________________.
This is
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
________________________________________________________

natural radioisotopes
II. Four Modes of Decay

spontaneous disintegration
Natural Radioactivity: _________________________________
nucleus
of the _________________
of an atom, with the emission of
_________________________________
Particles and/or energy

O
Modes of Decay (See Table ______)
Penetration: How far into a material the radioactive particle will
go

Natural Transmutations
Transmutation = When the atomic nucleus of one
element is converted into the atomic nucleus of another
element. Transmutations can occur naturally or
artificially.
When an unstable nucleus decays, it emits radiation
in the form of:
1. Alpha decay
2. Beta decay
3. Positron Emission
4. Gamma Rays
ALPHA DECAY:
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
nucleus!!!!!!
Types of Decay

Alpha Radiation
Nucleus loses 2 protons and 2 neutrons (alpha
particle)
 Has a +2 charge
 Can be blocked by paper or clothing
Example:
226 Ra → 222 Rn + 4 He
88
86
2

Particle
Alpha
Beta
Positron
Gamma
Mass
Charge
Symbol
Penetrating Power
4 amu
2+
Low
0 amu
1-
Moderate
0 amu
1+
Moderate
0 amu
none
High
b

Beta Radiation
A neutron turns into a proton and gives off an electron
(beta particle)
 Has a –1 charge
 Can be blocked by metal foil
 Since a proton is formed, the atomic number
increases by…1
Example:
14 C → 14 N
0 β
+
6
7
-1

An Equation for Beta Decay:
POSITRON EMISSION
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 RADIATION
Gamma rays have no mass and no
charge.
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
equation.
Types of Decay continued…

Gamma Radiation



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
radiation
Not completely blocked by lead or concrete
(BUT GAMMA, IT’LL KILL YOU!!!)
Example:
238 U → 234 Th + 4 He + 2 0 γ
92
90
2
0
Types of Decay

Identify type of decay:

A. alpha, B. beta, C. gamma, D. positron, E.
electron capture
 147N + 0-1β
 115B + 01β
228
4 He + 224 Po
Rn

86
2
84
14 C + 0 e → 14 N
6
-1
5
14 C
6
11 C
6
Nuclear Reaction Equations

Nuclear Reaction Equations:
Calculating the new compound or types of
decay involved.
Practice Problems:
12 N 
0 β
?
+
7
1
14 N →
0 β
?
+
7
-1
226 Ra →
4 He
?
+
88
2

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
reactants.
Write this as an equation.
SUMMARY:
**Natural Transmutations will have
only one reactant.
**Artificial transmutations will have
two reactants.
Fission and
Fusion
FISSION = a reaction involving the splitting
of a heavy nucleus to produce two smaller
nuclei.
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.
STEP 1:
STEP 2:
STEP 3:
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
reaction.
Fission can occur
when a nucleus of
a heavy atom
captures a
neutron, or it can
happen
spontaneously.
FUSION
Fusion = the combining of two light nuclei
to produce a heavier nucleus.
Nuclear fusion
fuels both the sun
and the stars. We
are not able to
reproduce these
reactions here on
Earth because they
require such HIGH
temperatures and
pressures.
Fusion reactions DO NOT have radioactive products
like fission reactions. This makes it a safer energy
source.
MASS DEFECT
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
nuclear fusion.
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
occur.
HALF-LIFE
HALF LIFE
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?
Rn-222?
Which of the above isotopes do you think is more
stable? Why?
Answer = Carbon-14
The shorter the half-life the less stable an isotope
is.
Set up a Table: Carbon -14
Time
# of Half
Lives
Amount
0 years
0 half lives
100g
5730 years
1 half life
50g
11460 years 2 half lives
25g
Sample Problem:
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
undergone?
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
REVIEW QUESTIONS:
1. What does the term “half-life” mean in nuclear
chemistry?
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
common isotopes?
5. If there was a 100g sample of Ra-226, how many
grams would remain after 3200 years? Why?
RISKS AND USES
OF RADIATION
RISKS
There are inherent risks
involved with radioactivity and
the use of radioactive isotopes.
Risks include:
- biological exposure
- long-term storage and disposal
- nuclear accidents
EFFECTS OF RADIATION ON
LIVING TISSUE
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
reactions
Remember this…
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
and water.
Chernobyl: aftermath of the explosion and meltdown. Thirty-one
firefighters died trying to control the blaze in Unit 4. (1986)
Uses
Radioactive isotopes also have
many beneficial uses.
Medicine
- Radioisotopes with short
half-lives that are quickly
eliminated from the body are
important tracers in medical
diagnosis.
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)
Radioactive dating
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
25,000 years!
What are some pro’s and con’s of
C-14 dating and U-238 dating?
Industrial measurements
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.