Nuclear Chemistry
L. Scheffler
The Nucleus
The nucleus is comprised of the two nucleons:
protons and neutrons.
The number of protons is the atomic number.
The number of protons and neutrons together is
effectively the mass of the atom.
Isotopes
Not all atoms of the same element
have the same mass due to
different numbers of neutrons in
those atoms.
 There are three naturally occurring
isotopes of uranium:

Uranium-234
 Uranium-235
 Uranium-238

Radioactive Nuclei
The nuclides of some of the
isotopes of certain isotopes
unstable, or radioactive.
 They are known as radionuclides.
 Radionuclides can decay into a
different nuclide by several
different methods.

Spontaneous Radioactivity



Spontaneous radioactivity involves
unstable nuclei which decay with the
eventual formation of a stable nuclei.
Thus some nuclei are stable and others
are not, for example 12C and 14C , 3H
and 40K. The isotopes in these
examples have the same number of
protons but differ in the number of
neutrons.
Stability Factors
1.
2.
Two important factors determine
nuclear stability
The mass number (which is the total
number of nucleons in the nucleus)
the neutron to proton ratio.
Stability Factors
180


In the nucleus,
positively charged
protons repel each
other.
As the number of
protons in the nucleus
increases, the forces
of repulsion between
the protons increases
substantially.
More neutrons are
required to maintain
stability
140
120
# neutrons

160
100
80
60
40
20
0
0
50
100
# protons
150
Alpha Emitters



If a radionuclide has a lower neutron to proton
ratio than for a stable nucleus, it falls to the right
of the stable nucleus
To increase the neutron to proton ratio back to a
stable ratio, it undergoes alpha decay.
Releasing an alpha particle raises the neutron to
proton ratio since the number of neutrons is
greater than or equal to the number of protons
Beta Emitters
If a radionuclide has a higher neutron to proton
ratio than for a stable nucleus, it falls to the left of
the stable nucleus
To reduce the number of neutrons back to the
stable ratio it undergoes beta decay.
A neutron disintegrates with the emission of a high
speed electron known as a beta particle:



A proton is produced from the neutron. This raises
the number of protons by one and reduces the
number of neutrons by one.
The neutron to proton ratio is lowered until it
reaches a stable value. Then no further radioactive
occurs

Other Forms of Nuclear Decay

If a radionuclide has a lower neutron to
proton ratio, a proton can also be
transformed into a neutron by either
positron emission or electron capture.
Heavy Nuclei Are Always
Unstable




If the total number of nucleons exceeds 209, the
limit for stable nuclei, the nucleus always lies
beyond the stable limit
The nucleus is always radioactive.
Several types of radioactive decay are involved
in order to reach the stability.
For example, the radioisotope 238U undergoes a
sequence of fourteen radioactive decay steps
before forming the final product 206Pb.
Nuclear Binding Energy
When a nuclear reaction occurs the mass
of the particles is not conserved.
 The early quantum physicists were able to
demonstrate that there is a small change
in mass when a particle is moving. It is
given by the formula

.

.
.

.
.

.
.

.
Fission is the release of
energy by splitting heavy
nuclei such as Uranium-235
and Plutonium-239
How does a nuclear plant work?
• Each fission releases 2 or 3
neutrons
• These neutrons are slowed down
with a moderator to initiate more
fission events
• Control rods absorb neutrons to
keep the chain reaction in check
Controlled Fission Chain Reaction
D-T Fusion
D
T
Fusion is the release of
energy by combining two
light nuclei such as
deuterium and tritium
4He
3.52 MeV
Neutron
14.1 MeV
• The goal of fusion research is to
confine fusion ions at high
enough temperatures and
pressures, and for a long enough
time to fuse
• This graph shows the
exponential rate of progress over
the decades
Confinement Progress
There are two main confinement approaches:
The energy from the
reaction drives a steam
cycle to produce
electricity
Nuclear Power Plant
Nuclear Power produces no greenhouse gas emissions; each year
U.S. nuclear plants prevent atmospheric emissions totaling:
•5.1 million tons of sulfur dioxide
•2.4 million tons of nitrogen oxide
•164 million tons of carbon
Nuclear power in 1999 was the cheapest
source of electricity costing 1.83 c/kWh
compared to 2.04 c/kWh from coal
• Magnetic Confinement uses strong
magnetic fields to confine the
plasma
• This is a cross-section of the
proposed International Thermonuclear Experimental Reactor
(ITER)
• Inertial Confinement uses powerful
lasers or ion beams to compress a
pellet of fusion fuel to the right
temperatures and pressures
• This is a schematic of the National
Ignition Facility (NIF) being built at
Lawrence Livermore National Lab