NUCLEAR CHEMISTRY
THE ULTIMATE IN SPONTANEITY
Review 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
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
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?
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
The energy released when a nucleus is formed from nucleons.
E = mc2
E is for energy unit: Joules (J)=kg.m2/s2
M is for mass unit: kilograms (kg) C is the speed of light (squared)
3.00 x 108 m/s
Binding Energy per Nucleon
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 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
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
Protons 1.007276 amu
Neutrons 1.008665 amu
Electrons 0.0005486 amu
Mass Defect 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
Add these up and subtract by the measured mass
7.05665 amu - 7.01600 amu = 0.04065 amu
This is the mass defect of one atom
Mass Defect
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
Third: Now your ready to calculate the nuclear binding energy!
E = mc2
Plug in your mass defect value in kg
E = (6.7499 x 10-29 kg) (3.0 x 108 m/s)2
= 6.0749 x 10-12 J This is the nuclear binding energy for one atom
Nuclear Binding Energy per Nucleon
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
Nuclear Binding Energy
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?
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?
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?
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
Stable nuclides have certain characteristics
When the number of neutrons are plotted against the number of protons a pattern is observed
Band of Stability
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
Stable nuclides tend to have even numbers of nucleons.
256 stable nuclides 159 have both even protons and
neutrons Only 4 have odd numbers of protons
and neutrons.
Nuclear Shell Model
Nucleons exist in different energy levels, or shells, in the nucleus.
The number of nucleons that represent completed nuclear energy levels, 2, 8, 20, 28, 50, 82, and 126 Called magic numbers
Nuclear Reactions
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
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
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 e + N6 -1 7
Atomic number’s must equal on both sides of the equation
Nuclear Reactions
Try one!
238 4
U He + 92 2 _______
Nuclear Reactions
Try one!
238 4 234
U He + Th92 2 90
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