Transcript Nuclear Chemistry

Remember the easy stuff!!
nucleus
Electrons
We don’t care about the electrons
right now…
What particles make up the nucleus?
Both protons and neutrons are called NUCLEONS
protons
neutrons
Recall some stuff about the nucleus:
Notation:
238
Mass
number
92 U
Atomic
number
element
Isotopes: nuclei with the same number of
protons, but not necessarily the same number
of neutrons
Isotopes
• Isotopes have the same number of
protons, different number of neutrons
• Another way to show an isotope is to
have the mass number follow the name of
the element (Carbon-14)
12C
and 13C differ in their
(a) number of neutrons.
(b) number of protons.
(c) number of electrons in orbit around the nucleus.
(d) chemical behavior.
(e) all of the above.
What is the charge on a proton? The neutron?
What is the nature of the force between the
nucleons?
How does the nucleus hold itself together??!!
The Nucleus is a delicate balance between
[1] The attractive nuclear strong force between
all of the nucleons, and
[2] The repulsive electromagnetic force
between all of the protons
It turns out that our delicate balance of forces
tends to break down when nuclei get large:
The electromagnetic forces eventually
overcome the nuclear ones and the nucleus
spontaneously breaks apart
This “instability” is called radioactivity:
All nuclei having atomic number larger than
83 (bismuth) are radioactive.
Radioactivity
•
Radioactivity is the process in which an
•
Any atom containing an unstable nucleus
is called a radioactive isotope or
radioisotope for short
unstable atomic nucleus emits charged
particles and energy
•As more protons are added (the nucleus
gets heavier) the proton-proton repulsion
gets larger.
•The heavier the nucleus, the more
neutrons are required for stability.
•The belt of stability deviates from a 1:1
neutron to proton ratio for high atomic
mass.
Ratio of
neutrons to
protons in
nucleus
determines
if nuclei are
stable
Too many
neutrons
Too few
neutrons
nuclei with
>than 83
protons all
unstable
Radioactive decay
is how nuclei get to
belt of stability
sometimes an
unstable nucleus
goes through a
series of nuclear
reactions before it
gains stability.
This sequence is
called a
radioactive series
3 Types of Spontaneous Decay:
1. Alpha particle emission
2. Beta particle emission
3. Gamma ray emission
High energy photon (light)
A little bit of
History
Lead
block
Uranium
Florescent
Screen
Gold Foil
Remember this?
This led to the discovery of
the nucleus…
Ernest Rutherford’s Other Experiment:
• No study on radiation is complete without giving
credit to the Curies (Marie, Pierre and Irene)• Their discoveries include radium, polonium and
positron.
• Marie-died from
Overexposure
To radiation
Pierre Curie
(1859-1906)
Marie Curie
(1867-1934)
Types of Nuclear Radiation
1. Alpha
2.Beta
3. Gamma

alpha particles
and
alpha decay
What is an alpha-particle?
It has two protons and two
neutrons
4
2 He
What is alpha-decay?
It is when a nucleus emits an
alpha particle
4
2 He
Alpha Decay = Alpha Particle Emission
Nuclear Equations -represent nuclear decay to
become stable - they must balance.
1) mass #
2)atomic #
The total number of p+and n0 before a nuclear reaction
must be the same as the total number of nucleons after
reaction.
1. Alpha emission
Nuclides that undergo alpha decay
have too many protons for stability
( >83 protons and mass > 209 )
Remember: This is the
mass number, which
is the number of
protons plus neutrons
Radium
Ra226
88
88 protons
138 neutrons
Alpha Particles ()They have a mass of 4 and charge of +2
Radon
Rn222
n p
p n
+
86
86 protons
136 neutrons
 (4He)
2 protons
2 neutrons
•The alpha-particle () is a Helium nucleus.
•It’s the same as the element Helium, with the
electrons stripped off !
•The alpha particles go in the air collide with air
molecules and become Helium – not dangerous.
Example: Write the nuclear equation for the alpha
decay of uranium 238:
238
92 U
234
90 X
+
4
2 He
Now look at the chart to find this one:
THORIUM
Nuclear Equation Practice:
Alpha Decay
How would alpha decay equations be
written for these atoms:
•
238
92
U
•
209
84
Po 


beta particles
and
beta decay
Beta Decay
Electron!
14
6 C
14
7 N
+ electron (beta)
200link
2.Beta emission
A neutron turns into a proton and an electron.
The beta particle is the electron,
written as
0
-1
e
or
0
-1

Beta Particles ()-
are electrons, “no” mass and charge of –1.
The decay of Carbon 14
Carbon
C14
6
6 protons
8 neutrons
Nitrogen
N14
7
7 protons
7 neutrons
e-
+
0Electron
-1
(beta-particle)
We see that one of the neutrons from the C14 nucleus
“converted” into a proton, and an electron was ejected.
The remaining nucleus contains 7p and 7n, which is a
nitrogen nucleus.
In symbolic notation, the following process occurred:
n  p + e
Write out the nuclear equations in each case
and identify the daughter nuclei:
Alpha decay of
222Rn:
86
Beta decay of
3
1 H:
Beta decay of
90Sr
38
Beta Decay
• A beta particle is an
electron emitted by an
unstable nucleus
• Beta particles can be
stopped by a thin sheet of
metal such as aluminum
Nuclear Reaction Equation Practice:
Beta Decay
0 
• A beta particle is written 0-1 e or
-1
• During beta decay, the mass remains the same
and the atomic number increases by one
•21482 Pb

•21884 Po

38
90Sr
is a radioactive isotope that decays by beta-decay. Its
daughter nucleus is
(a) 3686Kr.
(b) 3788Rb.
(c) 3789Rb.
(d) 3989Y.
(e) 3990Y.

gamma rays
and
gamma decay
Gamma decay occurs because the nucleus is at too
high an energy. The nucleus falls down to a lower
energy state and, in the process, emits a high
energy photon known as a gamma particle. Here's a
diagram of gamma decay with helium-3:
Gamma decay is only energy emission,
not particle emission.
Gamma particles (g)
In much the same way that electrons in atoms can be in an
excited state, so can a nucleus.
Neon
Ne20
10 protons
10 neutrons
(in excited state)
Neon
Ne20
+
10 protons
10 neutrons
(lowest energy state)
gamma
A gamma is a high energy light particle.
It is NOT visible by your naked eye because it is not in
the visible part of the EM spectrum. It has neither
Charge nor mass. They are highly penetrating.
Gamma decay
• A gamma ray is a penetrating
ray of energy emitted by an
unstable nucleus
• Gamma rays are energy
waves that travel through
space at the speed of light
Gamma decay
• During gamma decay, the atomic
number and mass remain the
same, but the energy of the
nucleus decreases
• Gamma rays can be stopped by
several centimeters of lead or by
several meters of concrete
Other Types
There are other types of
radioactive decay we will
not go into:
- positron emission
- neutron emission
Measuring
Radiation
The unit of radiation exposure is called the
REM (Radiation Equivalent in Man)
This is a direct measure of the number of
damaged cells.
The average dose for one year is about
0.3 Rem
Sudden dose of 1000 rems causes
death in 30 days!!
Exposure to ionizing radiation:
All of these are
IONIZING RADIATION
alpha
beta
gamma
Ionizing radiation causes cell damage that can lead
to DNA damage that can lead to CANCER.
Radiation Doses
• Normal
– 1 XRay (medical)
0.04 rem = 40 mrem
– CAT Scan
3 rem
– Portland  New York 1 mrem (4 hr flight)
(0.001 rem)
– Cosmic Rays
30 – 50 mrem/year
– Food (40K, Ra)
~40 mrem/year
Bad News
(acute radiation exposures)
• 1 rem
 More cancer ~ 100 extra cases per 106 people
(+0.0001 chance)
• 25-50 rem
 Lose white blood cells (+ above)
• 50-200
rem
 Sick, low white blood cell count, Get Leukemia
in 10-30 years, Genetic mutations in progeny.
• 450 rem
 50% death in 30 days (with “heroic” medical
intervention)
• 1000 rem
 Death, 100%
• 5000 rem
 Can’t shoot back
Detecting Radiation
• Devices used
to detect
radiation
include
Geiger
counters and
film badges
Penetrating Radiation
The penetrating power
of radiation is really a
function of mass.
g-radiation (zero mass)
penetrates much further
than -radiation, which
penetrates much further
than -radiation.
http://library.thinkquest.org/3471/radiation_effects_body.htm
l
“Background” Radiation
•
Background radiation is nuclear radiation that
occurs naturally in the environment
–
–
–
Radioisotopes in the air, water, rocks, plants, and
animals all contribute to background radiation
Cosmic rays (streams of charged particles) from outer
space that collide with the Earth’s atmosphere also
contribute
Background radiation levels are low enough to be safe
Half-Life
RATES OF NUCLEAR DECAY
•
A half-life is the time required for one half of a
sample of radioactive sample of a radioisotope to
decay
– Unlike chemical reactions, nuclear decay rates are
constant regardless of temperature, pressure or
surface area
A very important concept in nuclear chemistry
and its applications is the idea of
HALF-LIFE
Half-Life: the TIME it takes for half of a given amount
of a specific isotope to decay
This depends on the relative stability of the the
isotope in question: semi-stable nuclei last longer
than very unstable ones.
1 Kg
Example: 14C (half-life
6,000 years).
How much is left after 6,000 years?
12,000 years?
18,000 years?
24,000 years?
30,000 years?
36,000 years?
1 Kg
Example: 14C (half-life
6,000 years).
There must be an
easier way!
How much is left after 6,000 years?
12,000 years?
18,000 years?
We divided by
2 six times
24,000 years?
30,000 years?
36,000 years?
We can do the process in reverse to see how
much time it took a substance to decay:
Radioactive Dating:
Applications
Let’s meet some of the unstable
isotopes in our neighborhood:
• Radon -222
–
–
–
–
–
–
Forms as part of the Uranium Series
Decomposition of Uranium in many rocks-granite
Migrates into basement cracks
Dense and collects in lower part of house
Readily inhaled
Its decomposition product is polonium, if it decays
in the lung it emits an alpha particle
– The polonium solid decays resulting in lung
disease
Radium 226
• 1st radioactive element associated with
biological damage
• Some are phosphorescent (glow)
• Believed to cause the reaction that caused
leukemia that killed Marie Currie
Uranium-238
• Used to estimate age of earth
• We use its half life to estimated how long it
has been since the rock solidified.
Potassium 40
• Light element (one of the few)
• Emits positron to form Ar-40
• Most of argon in atmosphere is Ar-40
Artificial Radioactive isotopesTransmutation
• Artificial radioactive
Isotopes are produced
by bombarding a target
element with nuclei of
other elements
• Carried out in cyclotron
• Many new and useful
Isotopes have been
created.
Practical uses of Radioisotopes
H-3
Am-241
Co-60
Cr-51
Tritium
Americium-241
Cobalt-60
Chromium-51
U-235-
Uranium-235
U-238
Uranium-238
Archaeological dating
Smoke detectors
Cancer treatment
Determination of
blood volume
Nuclear reactors and
weapons
Archaeological dating
Artificial Radioactive isotopes may be
used as tracer materials.
Example: We might like to know how
phosphorus in fertilizers is used by
plants. By incorporating a small
amount of radioactive Phosphorus in
the fertilizer, chemists can measure the
time it takes for plants to utilize the
Phosphorus. They also can track where
the P goes.
Dating Archeological samples
Example:
U-238 decays slowly through the uranium
disintegration series to lead-206. The
half-life of this conversion is 4.5 X 109
years. To determine the age of a rock,
we need to determine the amount of
uranium present when the rock
solidified, No and the amount of
uranium in the rock today, Nf
• The amount of U-238 present in the rock
today can be measured. However, we can’t
calculate the original. The solution lies in the
fact that each atom of uranium ends up as an
atom of lead. Therefore, if we take a rock
sample and determine the number of atoms
of U-238 and the number of atoms of Pb-206,
we can say that
• No = atoms U-238 + atoms Pb-206
• Nt = atoms 238-U
• **Errors will exist, the original rock may have
contained some Pb-206 and some of the
original U-238 may not end up as Pb-206
Fusion
&
Fission
FISSION AND FUSION
• Fission is the splitting of an atomic
nucleus into two smaller parts
• Fusion is a process in which the nuclei of
two atoms combine to form a larger
nucleus
Fission
•
•
A chain reaction is a chain of fission reactions
triggered by neutrons released during the fission of
a nucleus
About 20% of the electricity in the US comes from
fission reactions
Fission
• A tremendous
amount of
energy is
produced
during a
fission
reaction
Fission
• Advantages to using
fission reactions is the
lack of air pollution.
• Disadvantages include
the risk of exposure
and radioactive waste
Nuclear Power Plant
FG21_020.JPG
http://ull.chemistry.uakron.edu/genobc/animations/chain.mov
FG21_017.JPG
Atomic Bomb
2 subcritical
masses
come
together
when
detonated
Fusion
• Fusion reactions
can release huge
amounts of
energy
• Fusion reactions
occur in the sun
and stars
Nuclear Fission-the process that yields
two nuclei of almost equivalent mass.
• It does not occur spontaneously
• It is a chain reaction
• It is usually used to generate electricity.
Heat is generated in fission reactions.
• Nuclear Fusion- The combination of two
nuclei into a larger atom is called nuclear
fusion.
• The reactions within the sun are fusion
reactions that combine Hydrogen nuclei to
form a helium atom
• It takes simple, nonradioactive materials and
produces helium, so it should be a clean and
inexpensive source of energy.
• Several designs for fusion reactors are being
tested. Some success, it will be some time
before this cheap, nonpolluting energy source
will become a reality.
Nuclear fusion
Tokamak- uses magnetic fields to
contain fusion rxn
• Fusion of tritium and deuterium requires about
40,000,000K:
2 H + 3 H  4 He + 1 n
1
1
2
0
• These temperatures can be achieved in a
nuclear bomb or a tokamak.
• A tokamak is a magnetic bottle: strong
magnetic fields contained a high temperature
plasma so the plasma does not come into
contact with the walls. (No known material can
survive the temperatures for fusion.)
• To date, about 3,000,000 K has been achieved
in a tokamak.
Fusion reaction in sun provides all
our energy makes smaller elements
fusion in supernova made elements
larger than Fe
Fat Man
Produced this cloud
after its detonation at
Nagasaki on August
15th, 1945, killing at
least
80,000 civilians
instantly.
140,000 civilians
perished in Hiroshima
when Little Boy was
dropped just two weeks
earlier.
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