NUCLEAR CHEMISTRY
THE ULTIMATE IN SPONTANEITY
Review
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Atomic number (Z) – number of protons
Mass number (A) – sum of the protons and
the neutrons
Nuclides– atoms with the same atomic
number but different mass numbers, different
numbers of neutrons. (Isotopes)
Nucleons – the particles that make up the
nucleus. (protons and neutrons = mass #)
Facts about the nucleus
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Very small
Very dense
Held together by the nuclear strong
force
Location of the protons and neutrons
Most of the mass of an atom is located
Mass Defect
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The difference between the calculated
mass and the actual mass is known as
mass defect.
calculated mass is the sum of all of the
subatomic particles that make up the
atom.
What causes the lost mass?
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According to Albert Einstein, mass and
energy can be converted into each
other.
Some of the mass is lost during the
formation of the nucleus.
The amount of energy can be calculated
using Einstein’s famous equation.
Nuclear Binding Energy
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The energy released when a nucleus is
formed from nucleons.
E = mc2
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E is for energy unit: Joules (J)=kg.m2/s2
M is for mass
unit: kilograms (kg)
C is the speed of light (squared)
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3.00 x 108 m/s
Binding Energy per Nucleon
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The binding energy per nucleon is used
to compare the stability of different
nuclides.
It is the binding energy of the nucleus
divided by the number of nucleons that
are in the nucleus.
Binding Energy
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The higher the binding energy per
nucleon, the more tightly packed the
nucleons are held together, the more
stable the nuclide.
"A is for atom" (1952) video
Mass Defect Example Problem
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What is the nuclear binding energy per
nucleon for lithium-7. The measured
mass of lithium-7 is 7.01600 amu.
First determine the calculated mass of
lithium -7 from the mass of the protons,
neutrons and electrons
Mass Defect
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Protons
1.007276 amu
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Neutrons
1.008665 amu
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Electrons
0.0005486 amu
Mass Defect
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3 protons x 1.007276 amu = 3.021828 amu
3 electrons x 0.0005486 amu =0.0016458 amu
4 neutrons x 1.008665 amu = 4.03466 amu
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Add these up and subtract by the measured mass
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7.05665 amu - 7.01600 amu = 0.04065 amu
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This is the mass defect of one atom
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Mass Defect
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The mass defect is in the wrong unit
Second: convert amu into kg (the SI unit for mass)
1 amu = 1.6605 x 10-27 kg
0.04065 amu x 1.6605 x 10-27 kg
1 amu
= 6.7499 x 10-29 kg
Nuclear Binding Energy
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Third: Now your ready to calculate the
nuclear binding energy!
E = mc2
Plug in your mass defect value in kg
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E = (6.7499 x 10-29 kg) (3.0 x 108 m/s)2
= 6.0749 x 10-12 J
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This is the nuclear binding energy for one atom
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Nuclear Binding Energy per
Nucleon
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Last: divide the nuclear binding energy
by the number of nucleons (mass #)
6.0749 x 10-12 J
7
= 8.6784 x 10-13 J/nucleon
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Nuclear Binding Energy
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Elements with intermediate atomic
masses have the greatest binding
energies per nucleon and are therefore
the most stable. Iron is the most stable
isotope.
Binding Energy per Nucleon
How does the nucleus stay
together?
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Relationship between the nuclear strong
force and the electrostatic forces
between protons.
Like charges repel each other through
electrostatic repulsion
How does the nucleus stay
together?
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The nuclear strong force allows protons
to attract each other at very short
distances.
As protons increase in the nucleus so
does the electrostatic forces, faster than
nuclear forces.
Why do atoms want more
neutrons than protons?
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More neutrons are required to increase
the nuclear force and stabilize the
nucleus.
> 83 the repulsive forces of protons is
so great that no stable nuclides exist.
Band of Stability
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Stable nuclides have certain
characteristics
When the number of neutrons are
plotted against the number of protons a
pattern is observed
Band of Stability
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The neutron-proton ratio of stable
isotopes cluster around a narrow band
called the band of stability.
For atoms with low atomic numbers the
ratio is 1 : 1
As the atomic number increases, the
ratio increases to 1.5 : 1
Band of Stability
Magic Numbers
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Stable nuclides tend to have even
numbers of nucleons.
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256 stable nuclides
159 have both even protons and neutrons
Only 4 have odd numbers of protons and
neutrons.
Nuclear Shell Model
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Nucleons exist in different energy
levels, or shells, in the nucleus.
The number of nucleons that represent
completed nuclear energy levels,
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2, 8, 20, 28, 50, 82, and 126
Called magic numbers
Nuclear Reactions
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Unstable nuclei undergo spontaneous
changes that change the number of
protons and/or neutrons.
Give off large amount of energy by
emitting radiation during the process of
radioactive decay.
Nuclear Reactions
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Eventually unstable radioisotopes of
one element are transformed into
stable, non-radioactive, isotopes of a
different element.
Total of mass number and atomic
number must be equal on both sides of
a reaction.
Nuclear Reactions
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When the atomic number changes, the
identity of the element changes.
A transmutation is a change in the
identity of a nucleus as a result of a
change in the number of protons.
Nuclear Reactions
Mass Number’s must equal on both sides of the
equation.
14
0
14
C
6
e

-1
+
N
7
Atomic number’s must equal on both sides of the
equation
Nuclear Reactions
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Try one!
238
4
U  He +
92
2
_______
Nuclear Reactions
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Try one!
238
4
234
U  He +
92
2
Th
90