Contributed by:
The highlights are:
1. The nucleus
2. Mass defect
3. Einstein's equations
4. Binding energy curve
5. Nuclear stability
6. Radioactivity
7. Half-life
1.
NUCLEAR CHEMISTRY
The Basics
2.
The Nucleus
• The nucleus is composed of nucleons
– protons
– neutrons
• A nucleus is characterized by two numbers
– atomic mass number(A; total # of nucleons)
– atomic number (Z; number of protons)
Z E
A
3.
27
13 Al
• total number of nucleons is 27
• total number of protons is 13
• the number of neutrons is 14
4.
Subatomic Particles
one atomic mass unit (u) is defined as 1/12th the mass of
a carbon-12 atom
Particle mass in kg mass in u
-31 -4
electron 9.11 x 10 kg 5.485 x 10 u
-27
proton 1.673 x 10 kg 1.0073 u
-27
neutron 1.675 x 10 kg 1.0087 u
5.
Mass Defect
• Carbon-12 has a mass of 12.000 u
• Its nucleus contains 12 nucleons (6 p & 6n)
• Each nucleon has a mass >1 u
• The mass of a nucleus is slightly less than
the mass of the individual nucleons
• The missing mass is called the mass defect
• mass defect:m = mass nucleons - mass nucleus
6.
Einstein’s Equation
• Energy and mass can be interconverted
• E = mc2
• When protons & neutrons are packed
together to form a nucleus, some of the
mass is converted to energy and released
• This amount of mass is equal to the force of
attraction holding the nucleons together
7.
Einstein’s Equation
• The total energy required to break
up a nucleus into its constituent
protons & neutrons
• binding energy = mc2
• The nuclear binding energy is
measured in MeV which is much
larger than the few eV required to
hold electrons to an atom
8.
Binding Energy Curve
• graph peaks at A=56
• the more BE released
per nucleon, the more
stable the nucleus
• mass number of 56 is
maximum possible
stability
9.
§ Theoretically, all nuclei will try to become larger
or smaller to attain as mass number of nucleons
§ To the right of 56 => want to become smaller
§ To the left of 56 => want become larger
10.
How Many Neutrons?
§ The number of neutrons in a nucleus can vary
§ Range limited by the degree of instability
created by
having too many neutrons
too few neutrons
§ Stable nuclei do not decay spontaneously
§ Unstable nuclei have a certain probability to
decay
11.
Nuclear Stability Facts
§ 270 Stable nuclides
§ 1700 radionuclides
§ Every element has at least one one radioisotope
§ For light elements (Z20), Z:N ratio is ~1
§ Z:N ratio increases toward 1.5 for heavy
elements
§ For Z>83, all isotopes are radioactive
12.
Nuclear Stability Facts
§ The greater the number of protons, the more
neutrons are needed
§ “Magic numbers” of protons or neutrons
which are unusually stable
2, 8, 20, 28, 50, 82, 126
• Sn (Z=50) has 10 isotopes; In (Z=49)& Sb (Z=51)
have only 2
• Pb-208 has a double magic number (126n, 82p) & is
very stable
13.
Band of Nuclear Stability
§ A plot of the known isotopes
on a neutron/proton grid
gives
§ Stable isotopes form a band
of stability from H to U
§ Z:N ratios to either side of
this band are too unstable &
are not known
15.
Nuclear Band of Stability
16.
Radioactivity
• The spontaneous decomposition of an
unstable nucleus into a more stable nucleus
by releasing fragments or energy.
• Sometimes it releases both.
17.
Electromagnetic Radiation
§ Electromagnetic radiation is a form of
energy that can pass through empty space
§ It is not just a particle, and it is not just a
wave. It may be both.
18.
Electromagnetic Radiation
§ The shorter the wavelength, the more energy it
possesses
§ gamma rays are very energetic
§ radio waves are not ver energetic
19.
Some Types of Radioactive
Decay
• Alpha Decay (increases N:Z ratio)
• Beta Decay (decreases N:Z ratio)
• Gamma Decay
20.
Alpha Emission
• ZAX Z-2
A-4Y + 4
2
• Identity of the atom changes
• 92235U 90
231Th + 4
2
• Quick way for a large atom to lose a lot of nucleons
21.
Beta Emission
• Ejection of a high speed electron from the nucleus
• ZAX Z+1 Y + 0
A -1
• 1940K 20 Ca + 0
40 -1
• Identity of atom changes
22.
Gamma Emission
• Emission of high energy electromagnetic radiation
• Usually occurs after emission of a decay particle
forms a metastable nucleus
• Does not change the isotope or element
23.
Radiation Energetics
§ Alpha Particles
relatively heavy and doubly charged
lose energy quickly in matter
§ Beta Particles
much smaller and singly charged
interact more slowly with matter
§ Gamma Rays & X-rays
high energy
more lengthy interaction with matter
25.
Hazards of Radiation Types
§ Alpha Emissions
easily shielded
considered hazardous if alpha emitting material is
ingested or inhaled
§ Beta Emissions
shielded by thin layers of material
considered hazardous is a beta emitter is ingested
or inhaled
§ Gamma Emissions
need dense material for shielding
considered hazardous when external to the body
26.
Radioactive Decay Rates
§ Relative stability of nuclei can be expressed
in terms of the time required for half of the
sample to decay
§ Examples: time for 1 g to decay to .5 g
Co-60 5 yr
Cu-6413 h
U-2384.51 x 109 yr
U-2357.1 x 108 yr
27.
Half-Life
The time required for half of a sample to decay
28.
Half-Life
§ The level of radioactivity of an isotope is
inversely proportional to its half-life.
The shorter the half-life, more unstable the
nucleus
§ The half-life of a radionuclide is constant
§ Rate of disintegration is independent of
temperature or the number of radioactive
nuclei present
30.
Half-Life
Number of Fraction of Amount
Half-Lives Initial Amount Remaining (mg)
Remaining
0 1 20.00 (initial)
1 1/2 10.00
2 1/4 5.00
3 1/8 2.50
4 1/16 1.25
5 1/32 0.625
31.
Half-Life
§ A plot the logarithm of activity vs. the time is a straight line.
§ The quantity of any radioactive element will diminish by a
factor of 1000 during a 10 half-life span
32.
Trying To Reach Nuclear
Stability
§ Some nuclides (particularly those Z>83) cannot
attain a stable, nonradioactive nucleus by a single
emission.
§ The product of such an emission is itself
radioactive and will undergo a further decay
process.
§ Heavy nuclei may undergo a whole decay series of
nuclear disintegrations before reaching a
nonradioactive product.
33.
Decay Series
A series of elements produced
from the successive emission of alpha
& beta particles
35.
The Four Known Decay Series
Parent # of Decay Final Product
Radioisotope Steps of Series
Uranium-238 14 Lead-206
Thorium-232 10 Lead-208
Uranium-235 11 Lead-207
Plutonium-241 13 Bismuth-209
38.
Radon-222
§ Originates from U-238 which occurs naturally
in most types of granite
§ Radon-222 has a half-life of 3.825 days
§ It decays via alpha emissions
§ This isotope is a particular problem because it
is a gas which can leave the surrounding rock
and enter buildings with the atmospheric air
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