Nuclear Chemistry
The Nucleus
• Remember that the nucleus is comprised of
protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together is
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
Stable Nuclei
The shaded region in the
figure shows what
nuclides would be stable,
the so-called belt of
stability.
Most nuclei are stable.
It is the ratio of neutrons
to protons that
determines the stability
of a given nucleus.
Radioactivity
• It is not uncommon for some nuclei to be
unstable, or radioactive.
• There are no stable nuclei with an atomic
number greater than 83.
• Radioisotopes = isotopes that are unstable
and thus radioactive
• There are several ways radionuclides can
decay into a different nuclide.
Radioactive Series
• Large radioactive nuclei
cannot stabilize by
undergoing only one
nuclear transformation.
• They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead).
• Transmutation = the
reaction by which the
atomic nucleus of one
element is changed into
the nucleus of a different
element
Types of Radioactive Decay
Alpha Decay
= Loss of an -particle (a helium nucleus)
4
2
238
92
U
Atomic # increases by 2
# of protons decreases by 2
# of neutrons decreases by 2
Mass # decreases by 4

He
234
90
4
2
Th + He
Types of Radioactive Decay
Beta Decay
= Loss of a -particle (a high energy electron)
0
−1
131
53
Atomic # increases by 1
# of protons increases by 1
# of neutrons decreases by 1
Mass # remains the same
I


0
−1
or
131
54
e
Xe
+
0
−1
e
Types of Radioactive Decay
Positron Emission
= Loss of a positron (a particle that has the
same mass as but opposite charge than an
electron)
0
1
11
6
Atomic # decreases by 1
# of protons decreases by 1
# of neutrons increases by 1
Mass # remains the same
C

e
11
5
B
+
0
1
e
Types of Radioactive Decay
Gamma Emission
= Loss of a -ray (a photon of high-energy
light that has no mass or charge & that
almost always accompanies the loss of a
nuclear particle) 0
0

Artificial Transmutation
= done by bombarding the nucleus with high-energy particles
(such as a neutron or alpha particle), causing transmutation
40
96
20Ca
+ _____ ----->
40
19K
+ 11H
2 H -----> 1 n + _____
Mo
+
42
1
0
**Natural transmutation has a single nucleus undergoing
change, while artificial transmutation will have two reactants
(fast moving particle & target nuclei.**
Nuclear Fission
• Nuclear fission is the type of reaction carried out in
nuclear reactors.
• = splitting of large nuclei into middle weight nuclei and
neutrons
Nuclear Fission
• Bombardment of the radioactive nuclide with a
neutron starts the process.
• Neutrons released in the transmutation strike
other nuclei, causing their decay and the
production of more neutrons.
• This process continues in what we call a nuclear
chain reaction.
Nuclear Fusion
• = the combining of light nuclei into a heavier
nucleus
•
2
1H
+ 21H  42He + energy
• Two small, positively-charged nuclei smash
together at high temperatures and pressures to
form one larger nucleus.
Half-Life
= the time it takes for half of the atoms in a
given sample of an element to decay
- Each isotope has its own half-life; the
more unstable, the shorter the half-life.
- Table T Equations:
fraction remaining = (1/2)(t/T)
# of half-lives remaining = t/T
Key: t = total time elapsed
T = half-life
Sample Half-Life Question 1A
Most chromium atoms are stable, but Cr-51 is an unstable
isotope with a half-life of 28 days.
(a) What fraction of a sample of Cr-51 will remain after 168
days?
Step 1: Determine how many half-lives elapse during 168 days.
Step 2: Calculate the fraction remaining.
Sample Half-Life Question 1B
(b) If a sample of Cr-51 has an original mass of 52.0g, what
mass will remain after 168 days?
Step 1: Calculate the mass remaining:
mass remaining = fraction remaining X original mass
(Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each
half-life.)
Sample Half-Life Question 2
How much was present originally in a sample of Cr-51 if 0.75g
remains after 168 days?
Step 1: Determine how many half-lives elapsed during 168 days.
Step 2: Multiply the remaining amount by a factor of 2 for each half-life.