Nuclear Chemistry
http://www.skanschools.org/webpages/rallen/
• The study of the structure of atomic nuclei and
the changes they undergo.
Nuclear Chemistry
Chemical Reactions
Nuclear Reactions
Occur when bonds are broken
Occur when nuclei emit particles
and/or rays
Atoms remain unchanged, though,
they may be rearranged
Atoms often converted into atoms of
another element
Involve ONLY valence electrons
May involve protons, neutrons and
electrons
Associated with small energy changes
Associated with large energy changes
Reaction rate influenced by T, particle
size, concentration and catalysts
Reaction rate is NOT normally
influenced by T, P and catalysts
How do Chemical and nuclear
reactions compare?
• 1895-1898
• Roentgen found that invisible rays were emitted when electrons
bombarded the surface of certain materials
• Becquerel accidentally discovered that phosphorescent uranium
salts produced spontaneous emissions that darkened
photographic plates
Discovery of Radioactivity
• Marie Curie
• Isolated the components (uranium atoms) emitting rays
• Radioactivity process by which particles give off rays
• Radiation the penetrating rays and particles emitted by a
radioactive source
• Identified 2 new elements, Polonium and Radium on the basis of
their radioactivity
Discovery of Radioactivity
• Marie Curie’s discovery contradicted Dalton’s theory of
invisible atoms
• Remember isotopes are atoms of the same element that
have different numbers of neutrons
Discovery of Radioactivity
• isotopes of atoms with unstable nuclei
• Too many or too few neutrons
Radioisotopes
• These unstable nuclei lose energy by emitting
radiation to attain more stable atomic
configurations
• Spontaneous process
Radioactive Decay
• The Neutron: Proton ratio
• The nucleus contains all the positively charged protons that should
repel each other, BUT neutrons act as a “glue” to hold them
together
• Neutrons exert a force of attraction known as the strong nuclear
force (it is much stronger than the force of gravity)
Patterns of Nuclear Stability
• Stable nuclei (up to atomic number 20) contain about equal
number of protons and neutrons
• Above atomic number 20, more neutrons are required to keep the
particles held together (so the ratio of neutrons to protons goes
above 1:1)
• The optimum ratio of
neutrons to protons is
called the “belt of
stability”…outside of
this range, atoms are
radioactive.
• Notice how the belt
ends at 83 protons
• Above element 83 (bismuth), all nuclei are
radioactive…there are so many protons in
the nucleus that no number of neutrons can
hold them together due to the strong
positive-positive charge repulsion
• The conversion of an atom of one element
into an atom of another element (radioactive
decay is one way that this occurs)
Transmutation
• Alpha Emission
• nucleus emits an alpha (α) particle
• An alpha particle is a particle composed of 2 protons and
2 neutrons
• In
• So it is the same as a Helium nucleus
Types of Radioactive Decay
• Alpha particles are large, slow moving, and easy to stop
• They cannot penetrate through a single sheet of paper/clothing
• Example
Types of Radioactive Decay
• Alpha Decay
• Example
What product is formed when Ra-226 undergoes α decay?
Types of Radioactive Decay
• Beta Emission- nucleus emits a beta (β-) particle
• A beta particle is a high speed electron!!!
• In
• Since it is the same as an electron, it could also be
Types of Radioactive Decay
• Beta particles are almost massless, negatively charged, fast
moving, and about 100x more penetrating than α particles
• They cannot penetrate more than 3 mm of aluminum/wood
• Beta particles are formed in the nucleus when a neutron is
converted into a proton and an electron…then the electron is
emitted from the nucleus
Neutron proton + beta
Types of Radioactive Decay
• Example
Atomic # increases by 1
Remember n e + p
(then e is emitted)
Types of Radioactive Decay
• Positron Emission-nucleus emits a beta (β+) particle
• There are 2 kinds of beta particles
• β- which is called a beta particle
• β+ which is called a positron
• Positrons are almost massless, positively charged particles
Types of Radioactive Decay
• Positron particles are formed in the nucleus when a
proton is converted into a neutron and a positron…then
the positron is emitted from the nucleus
• Positrons are short lived because when they leave the
nucleus, nucleus they quickly collide with electrons in the
electron cloud to produce gamma rays
Types of Radioactive Decay
• Therefore, positrons don’t really penetrate matter because
they don’t get a chance
• Example
Atomic # decreases by 1
Remember p: n + e
(then e is emitted)
Types of Radioactive Decay
• Nucleus “absorbs” an electron
• During electron capture, an inner shell electron is pulled into
the nucleus where it combines with a proton to form a
neutron
• So you lose an electron and proton
Electron Capture
• An X-Ray is emitted as an electron moves from a higher
energy level to a lower energy level to fill the vacancy
left by the “captured” electron
• Example
Electron written as a
reactant
Electron Capture
Atomic # decreases by 1
No change in mass
• Nucleus emits high energy photons (energy!!!!)
• Since it is just an emission of energy, there is no change in mass or
atomic number
• Gamma emission accompanies almost every other radioactive
emission
• Gamma emission is the most penetrating radiation…10,000x more
penetrating than alpha particles
• Blocked by lead/concrete (thick)
Gamma Radiation
Radioactive
Source
α β- γ
(+) (-)
neutral
+
-
Beta
Gamma
Alpha
Radiation in an electric or magnetic field
• Isotopes- atoms of the same element with different
number of neutrons
• Isotopes are named by giving the name of the element
followed by the sum of the neutrons and protons
• Example: Carbon Atom
• 6 protons, 6 neutrons Carbon-12 (stable)
• 6 protons, 7 neutrons Carbon-13 (stable)
• 6 protons, 8 neutrons Carbon-14 (radioactive)
Radioactive Decay & Half Life
• when the nucleus of a radioactive isotope gives up its
extra energy
• Can take the form of alpha particles, beta particles or
gamma rays
• Radioactive decay- process of emitting the radiation
Ionizing Radiation
• Decay Chain-when several decays are required to make a nuclei
stable
• Scientists have studied decay chains extensively
• Know how many decays it will take for each radioactive isotope to
become stable and how much energy will be released with each
decay
Decay Chain
• Scientists have not been able to determine exactly when a
single radioactive atom will decay
• Half Life- the time required for ½ of a radioisotope’s
nuclei to decay into products
• The half life can vary substantially from one isotope to
another
Half Life
For any radioisotopes
# of Half Lives
% Remaining
0
100%
1
50%
2
25%
3
12.5%
4
6.25%
5
3.125%
6
1.5625%
Half-Life
Half-Life
• Strontium-90 has a half life of 29 years. What does this mean?
• Suppose you have 10.0 g of strontium-90
• 1) Can use a table format:
# of Half Lives
Time (years)
Amount (g)
0
0
10
1
29
5
2
58
2.5
3
87
1.25
4
116
0.625
Half-Life Example
Or
• 2) Can use a calculation
Amount remaining = ½ t/T x initial amount
Where: t = time elapsed , T =half-life
(t/T= # of half life periods)
Half-Life
• Example:
• If gallium-68 has a half life of 68.3 minutes, how much of a
160.0 mg sample is left after
• 1 half-life?
80 mg (1/2 of 160)
• 2 half-lives? 40 mg (1/2 of 80)
• 3 half- lives? 20 mg (1/2 of 40)
Half-Life
• Example:
• Cobalt-6-, with a half-life of 5 years, is used in cancer
radiation treatments. If a hospital purchases a supply of 30.0
g, how much would be left after 15 years
5 yrs
5 yrs
5 yrs
30 15  7.5  3.75 g
OR
X = ½ t/T
X= ½ 15/5 x 30
X = ½ 3 x 30
Half-Life
X = 3.75 g
• It is possible, under the right conditions, for us to transform one
element into another
• This type of transmutation is done by “slamming” a particle into a
nucleus
• This process causes the nucleus to change and as a result the identity
of the atom is changed
Artificial Nuclear Transmutations
• Example:
• Alpha particles can be used to transform one nucleus into another
• Since the nucleus of the alpha particle is positive and the nucleus of
the atom being bombarded is also positive, the particles will
naturally repel each other
• In order to overcome this repulsion, the reaction must occur at very
high speeds (speed of light).
• These speeds are reached using a particle accelerator
Artificial Nuclear Transmutations
• Nuclear Fission- the splitting of a nucleus into smaller nuclei
• A large nuclei captures a slow moving neutron
• Resulting neutrons can then be used to split other nuclei- chain reaction
Fission and Fusion
Equation:
Fission
• Man made not common form of natural decay
• Releases a very large amount of energy
• Produces energy for nuclear power and drives the explosion for
nuclear weapons
Fission
• The total nuclear mass of the products is less than the total nuclear
mass of the reactants
• How can that be?
Mass Defect
Some of the mass is converted into energy
We can calculate the amount of energy using Einstein’s equation:
E= mc2
Energy
Fission
Mass
Speed of Light
(3.8 x 108 m/s)
• Einstein’s theories on relativity became a
convenient target for Nazi propaganda.
• In 1931, the Nazi’s enlisted other physicists to
denounce Einstein and his theories as "Jewish
physics."
• Passed a law barring Jews from holding any
official position, including teaching at
universities.
• Einstein also learned that his name was on a list
of assassination targets
• 1932moved to US, taught at Princeton
• 1939wrote a letter to FDR telling him of the
possibility of a Nazi bomb
• He was not invited to work on the Manhattan
Project
Albert Einstein
Mass Defect
Amu= atomic mass unit
Reactants:
235.043924 amu
+ 1.008665 amu
236.052589 amu
Fission
Total mass of REACTANTS
Reactants
Mass Defect
Products:
140.914363 amu
Krypton
91.926270 amu
Barium
+ 3.025995 amu
235.866628 amu
Fission
3 neutrons
Total Mass of PRODUCTS
Products
• Can be controlled by absorbing the neutrons released form the
reaction
• Where can controlled fission?
Nuclear Power Plant
Controlled Fission
• Fission continues until nuclei are split
• Where can uncontrolled fission reactions be used? Nuclear Weapons
Uncontrolled Fission
• Weapons
• WWII: The US Army Air Force received received orders to drop
these weapons anytime after August 3, 1945. On August 6, “Little
Boy” fell on Hiroshima . Three days later, “Fat Man” destroyed
Nagasaki.
Nuclear Bombs
Little Boy Hiroshima-0815
Size
10 ft Long
Weight
8.900 lbs (132 lbs > 90% U-235)
Height of Blast
1900 ft
Yield
15-16 TNT
Casualities
100,000 immediate deaths
200,00 total deaths
Fat Man- Nagasaki- 1102 August 9, 1945
Size
10.5 ft long, 5 ft diameter
Weight
10.300 lbs (12 lbs. Pu 239 of
which 2 lbs underwent fission
Height of blast
1650 ft
Yield
22 TNT
Casualties
70, 000 immediate deaths
140,000 total deaths
• The secret to controlling a chain reaction is to control the neutrons
• If the neutrons can be controlled, then the energy can be released in a
controlled way. That’s what scientists have done with nuclear power
plants
• United States has about 100 nuclear reactors, producing a little
more than 20% of the country’s electricity.
Nuclear Power Plants
• In France, almost 80% percent of the country’s electricity
is generated through nuclear fission
• How do nuclear plants make electricity?
Nuclear Power Plants
Nuclear Power Plant
• The fuel for nuclear reactors is held in fuel rods
• Fuels rods usually contain mostly non-fissionable U-238
and about 3% fissionable U-235
• A critical mass of fissionable U-235 is present (but not
nearly enough for a nuclear explosion)
Fuel for Reactors
• A chain reaction fission reaction of U-235 occurs in the reactor
core
• During the fission of U-235 a small amount of mass is
converted into a large amount of energy
• The energy generated heats water in the reactor core
• Heated water in the reactor core (under high pressure so it
remains a liquid) transfers heat to a second loop of water
• Heated water in the second loop changes to steam
• This steam spins the turbines of giant generators that produce
electrical energy (electricity)
Energy Production
• Steam that has moved past the turbines is cooled by water
taken from a nearby body of water
• Cooled steam condenses to liquid water and re-circulates
• So much heat is generated that some must be released
into the air through cooling towers
Cooling the Reactor
• Moderator= something which slows down the high speed
neutrons
• Water often acts as the moderator in nuclear reactors
• A moderator slows down high-speed neutrons making
them more readily captured by other nuclei
Facilitating the Reaction
• Controlling the neutrons in a reactor allows for control of the
nuclear chain reaction
• Control rods rods made of neutron absorbing material (such as
cadmium or boron)
• Control rods are inserted between fuel rods to control the
reaction
• When all the control rods are pushed all the way into the reactor
core, all neutrons are absorbed and the chain reaction stops
Controlling the Reaction
• Nuclear Power Plants have CERTAIN ADVANTAGES:
• No fossil fuels are burned (saving fossil-fuel resources for
producing plastics and medicine)
• No combustion products like carbon dioxide and sulfur dioxide
to pollute the air and water
Advantages
1. Cost: expensive to build and operate
• The electricity that’s generated by nuclear power costs about twice as
much as electricity generate through fossil fuel or hydroelectric plants
2. Supply of fissionable U-235 is limited
• Of all the naturally occurring uranium, only about 0.75% is U-235.
• A vast majority is non-fissionable U-238
• At current usage levels, we’ll be out of naturally occurring U-235 in
fewer than 100 years
• But there’s a limit to the amount of nuclear fuel available in the earth,
just as there’s a limit to the amount of fossil fuels
Problems
3. Accidents (safety)
4. Disposal of nuclear wastes
Problems
Three Mile Island (1979)
• A combination of operator error and equipment failure
caused a loss of reactor core coolant.
• The loss of coolant led to a partial meltdown and the
release of a small amount od radioactive gas.
• There was no loss of life or injury to plant personnel or
the general public
Accidents
Chernobyl (Ukraine 1986)
• Human error, along with poor reactor design and engineering, contributed to a
tremendous overheating of the reactor core, causing it to rupture
• Two explosions and a fire resulted, blowing apart the core and scattering nuclear
material into the atmosphere. A small amount of this material made its way to
Europe and Asia
•
•
•
•
The area around the plant is still uninhabitable
The reactor is encased in concrete and must remain that way for 100s of years
100s of people died
Many others felt the effects of radiation poisoning
• Instances of thyroid cancer, possibly caused by the release of I-13, have risen
dramatically in the towns surrounding Chernobyl
Accidents
• The fission process produces large amounts of radioactive isotopes, and
some of the half lives of radioactive isotopes are rather long
• Those isotopes are safe after ten half lives
• The wastes are basically buried and guarded at the sites. High level
wastes pose a much larger problem
• They’re temporarily being stored at the site of generation, with the plans to
eventually seal the material in glass and then in drums. The material will
then be stored underground in Nevada
• At any rate, the waste must be kept safe and undisturbed for at least
10,000 years!!!
Disposal of Nuclear Watse
• The process in which light weight nuclei combine to form a
heavier, more stable nucleus
• Nuclear fusion produces more energy than fission
• Ex. Hydrogen Bombs
Fusion
• Fusion Reactions power the sun and other stars.
• The reaction that occurs in the sun can be represented as
follows
Fusion
2.0140 amu
+ 3.01605 amu
• Reactants:
5.03005 amu
• Products:
Total mass of Reactants
4.00260 amu
+ 1.008665 amu
5.011265 amu
Fusion
Total mass of Products
• Mass Loss
5.03005 amu- 5.011265 amu = .018785 amu
Where it went E = mc2
Energy of Stars and H-bombs
Fusion
• Equation
• A fusion reaction with 1 g of the fuel composed of
deuterium and tritium will produce the energy equivalent
to 8 tons of petroleum
Fusion