Nuclear Chemistry
The Band of Stability
Nuclear Stability
• The Band of stability allows us to analyze the
proton to neutron ratio of various nuclides.
• A nucleus with a proton to neutron ratio that is
1:1 is considered to be stable. Anything that
varies that can lead to an unstable nucleus.
• At atomic number 20, more neutrons are needed
than protons to keep the nucleus stable.
• At atomic number 82 virtually no nucleus is
stable.
Particles in Nuclear Equations
• Sub-atomic particles in nuclear equations:
(A proton is also sometimes represented as
.)
• Alpha particles (symbolized α or ) are identical to
helium-4 nuclei, so the helium chemical symbol may also
be used to represent an alpha particle.
• Beta particles (symbolized β, β–, or ) are highspeed electrons. (The emission of electrons from the
nucleus may seem strange, but they result from a
reaction in the nucleus)
• Gamma particles (symbolized γ, or ) are very highenergy photons.
Alpha Decay
• The emission of alpha particles.
• In nuclear equations the sum of the mass
number (symbolized by A) on both sides of the
arrow must be equal.
• The sum of the atomic numbers (symbolized by
Z) must also be equal on both sides. Remember
that only the atomic number identifies the
element.
Beta Decay
• The emission of beta particles (high speed
electrons).
Positron (β+) emission
• Involves the emission of a β+ particle from the nucleus. A key idea of
modern physics is that most fundamental particles have
corresponding antiparticles with the same mass but opposite
charge. (The neutrino and antineutrino are an example.)
• The positron is the antiparticle of the electron. Positron emission
occurs through a process in which a proton in the nucleus is
converted into a neutron, and a positron is expelled. In terms of the
effect on A and Z, positron emission has the opposite effect of β−
decay: the daughter has the same A but Z is one lower (one fewer
proton) than the parent. Thus, an atom of the element with the next
lower atomic number forms. Carbon-11, a synthetic radioisotope,
decays to a stable boron isotope through β+
Electron (e−) capture (EC)
• Occurs when the nucleus interacts with an electron in an orbital from
a low atomic energy level. The net effect is that a proton is
transformed into a neutron:
• (We use the symbol “e” to distinguish an orbital electron from a beta
particle, β.) The orbital vacancy is quickly filled by an electron that
moves down from a higher energy level, and that process continues
through still higher energy levels, with x-ray photons and neutrinos
carrying off the energy difference in each step. Radioactive iron
forms stable manganese through electron capture:
• Even though the processes are different, electron capture has the
same net effect as positron emission : Z lower by 1, A unchanged.
Gamma (γ) emission
• Involves the radiation of high-energy γ photons from an excited
nucleus. Just as an atom in an excited electronic state reduces its
energy by emitting γ photons, usually in the UV and visible ranges, a
nucleus in an excited state lowers its energy by emitting photons,
which are of much higher energy (much shorter wavelength) than
UV photons. Many nuclear processes leave the nucleus in an
excited state, so γ emission accompanies many other (but mostly β )
types of decay. Several γ photons (also called γ rays) of different
energies can be emitted from an excited nucleus as it returns to the
ground state. Some of Marie Curie’s experiments involved the
release of γ rays, such as
• Gamma emission is common subsequent to β− decay, as in the
following:
Nuclear Transmutation
• The formation of different elements by the
bombardment of other particles. For
example the alpha bombardment of
nitrogen-14:
Common Transmutations
Half-lives
• A half-life (t1/2) is the time it takes for a
radioactive sample to decay into half the
original amount.
Nuclear Fission
• The splitting of large unstable nuclei.
• A neutron bombarding a 235U nucleus
results in an extremely unstable 236U
nucleus, which becomes distorted in the
act of splitting. In this case, which shows
one of many possible splitting patterns, the
products are 92Kr and 141Ba. Three
neutrons and a great deal of energy are
released also.
Fission
A chain reaction of uranium-235.
Nuclear Fusion
• The fusion of light nuclei.
• These reactions usually involve isotopes
of hydrogen, helium, and lithium.
• These reactions occur in the sun. They are
being studied in order to harness the
energy produced as a result of a fusion
reaction.
The tokamak design
for magnetic
containment of a
fusion plasma.
• The donut-shaped
chamber of the
tokamak (photo, top;
schematic, bottom)
contains the plasma
within a helical
magnetic field.
Radiation Exposure
• Radiation exposure in relation to tissue damage is
measured in roentgen equivalent for man (rem). The
following table reflects what would occur when there is
whole body radiation exposure.
Common Uses of Radioactive Isotopes
Practice Set 1: Nuclear Equations
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(1) Alpha decay of uranium-234
(2) Electron capture by neptunium-232
(3) Positron emission by nitrogen-12
(4) β− decay of sodium-26
(5) β− decay of francium-223
(6) Alpha decay of bismuth-212
(7) β− emission by magnesium-27
(8) β+ emission by
(9) Electron capture by
(10)β− decay of silicon-32
Practice Set 2: Nuclear Equations
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(11) Alpha decay of polonium-218
(12)Electron capture by
(13)Formation of titanium-48 through positron emission
(14)Formation of silver-107 through electron capture
(15)Formation of polonium-206 through α decay
(16)Formation
of β− through decay
(17)Formation of
β− through decay
(18)Formation of bismuth-203 through α decay
(19)Formation of 186Ir through electron capture
(20)Formation of francium-221 through α decay