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Chemistry: The Central Science
Chapter 21: Nuclear Chemistry
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A nuclear reaction involves changes in the nucleus of an atom
Nuclear chemistry – the study of nuclear reactions, with an emphasis in their uses in
chemistry and their effects on biological systems
21.1: Radioactivity
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Two types of subatomic particle: proton and neutron
o The particles are referred to as nucleons
Mass numbers – the total number of nucleons in the nucleus
o Atoms with the same atomic number but different mass numbers are known
as isotopes
 Isotopes are labeled using chemical symbols such as
,
, and
 The superscript is the mass number
 The subscript is the atomic number
 Different isotopes have different natural abundances
Nuclide – a nucleus with a specified number of protons and neutrons
o Nuclei that are radioactive are called radionuclides
 Atoms containing radionuclides are called radioisotopes
Nuclear Equations
o Radionuclides are unstable and spontaneously emit particles and
electromagnetic radiation
 Emission of radiation is one of the ways in which an unstable nucleus is
transformed into a more stable one with less energy
 The emitted radiation I the carrier of the excess energy
 E.g. Uranium-238
 Uranium-238 undergoes nuclear reaction in which helium-4
nuclei are spontaneously emitted
o The helium-4 particles are known as alpha (α) particles
 A stream of these particles is called alpha
radiation
 Nuclear equation:
o Uranium-238 splits into an α particle and thorium-234
nucleon
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o When a nucleus decomposes in such way as the uranium-238, it is said to have
decayed or to have undergone radioactive decay
 Because an alpha particle is involved in the decay of uranium-238, the
process is also called alpha decay
o Mass and atomic numbers must be balanced in all nuclear equations
 Radioactive properties of the nucleus are independent of the chemical
state of an atom
 In writing nuclear equations, we are not concerned with the
chemical form
Types of Radioactive Decay
o Three most common kinds of radioactive decay: alpha (α), beta (β), and
gamma (γ) radiation
 Alpha radiation consists of a stream of helium-4 nuclei
 Beta radiation consists of streams of beta (β) particles, which are highspeed electrons emitted by an unstable nucleus
 Represent by
or
o Subscript -1 represents the negative charge of the
particle
 Beta emission is equivalent to the conversion of a neutron to a
proton, thereby increasing the atomic number by 1:
o
 The electron comes into being only when the nucleus
undergoes a nuclear reaction
 Gamma radiation (or gamma rays) consists of high-energy photons
 It changes neither the atomic number nor the mass number of a
nucleus
 Represented as or merely γ
 Generally, gamma rays are not shown when writing nuclear
equations
o Two other types of radioactive decay are positron emission and electron
capture
 A positron is a particle that has the same mass as an electron, but an
opposite charge
 Represent by
 The emission of positron has the effect of converting a proton
to a neutron, thereby decreasing the atomic number of the
nucleus by 1:
o
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Electron capture is the capture by the nucleus of an electron from the
electron cloud surrounding the nucleus
 Electron capture, like positron emission, has the effect of
converting a proton to a neutron
o
21.2: Patterns of Nuclear Stability
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Neutron-to-Proton Ratio
o At close distances, a strong force of attraction, called the nuclear force, exists
between nucleons
 Neutrons are intimately involved in this attractive force
o The neutron-to-proton ratios of stable nuclei increase with increasing atomic
number
o The color band in the figure below is the area within which all stable nuclei are
found
 This area is known as the belt of stability
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 The belt of stability ends at element 83
 All nuclei with 84 or more protons are radioactive
o The type of radioactive decay that a particular radionuclide undergoes
depends largely on how its neutron-to-proton ratio compares to those of
nearby nuclei within the belt of stability
 Nuclei above the belt of stability – these neutron-rich nuclei can lower
their ratio and move toward the belt of stability by emitting beta
particle
 Nuclei below the belt of stability – these proton-rich nuclei can increase
their ratio by either positron emission or electron capture
 Nuclei with atomic number ≥ 84 – these heavy nuclei tend to undergo
alpha emission
 Emission of alpha particle decreases both the number of
neutrons and protons by two, moving the nucleus diagonally
toward the belt of stability
Radioactive Series
o A series of nuclear reactions that begins with an unstable nucleus and
terminates with a stable one is known as radioactive series, or a nuclear
disintegration series
 Three of such series occur in nature
 Begins with uranium-238, ends with lead-206
 Begins with uranium-235, ends with lead-207
 Begins with thorium-232 and ends with lead-208
Further Observations
o Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, 126 neutrons
are generally more stable than nuclei that do not contain these numbers of
nucleons
 These numbers of protons and neutrons are called magic numbers
o Nuclei with even numbers of both protons and neutrons are generally more
stable than those with odd numbers of nucleons
21.3: Nuclear Transmutations
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A nucleus can change identity if it is struck by a neutron or by another nucleus
o Nuclear reactions that are induced in this way are known as nuclear
transmutation
Nuclear transmutations are sometimes represented by listing, in order, the target
nucleus, bombarding particle, the ejected particle, and the product nucleus
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o The alpha particle, proton, and neutron are abbreviated as α, p, and n
respectively
Acceleration Charged Particles
o Charged particles, such as alpha particles, must be moving very fast to
overcome the electro static repulsion between them and the target nucleus
 The use of particle accelerators, bearing such names as cyclotron and
synchrotron, help accelerate the nucleus
 The projectile particles are introduced to a vacuum chamber within the
cyclotron
 The particles are then accelerated by making the dees (the
hollow D-shaped electrodes) alternately positively and
negatively charged
Using Neutrons
o Most synthetic isotopes used in medicine and scientific research are made
using neutrons as projectiles
 Neutrons are neutral and are not repelled by the nucleus
Transuranium Elements
o Artificial transmutations have been used to produce the elements with atomic
number above 92
 These elements are known as the transuranium elements because
they occur immediately following uranium in the periodic table
21.4: Rates of Radioactive Decay
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Decay of radioisotopes can be very fast or very slow
Radioactive decay is a first-order kinetic process
o First-order process has a characteristic half-life, which is the time required for
half of any given quantity of a substance to react
Radiometric Dating
o Because the half-life of any particular nuclide is constant, the half-life can
serve as a nuclear clock to determine the ages of different objects
 The method of dating objects based on their isotopes and isotope
abundances is called radiometric dating
o E.g. Carbon-14 has been used to determine the age of organic materials
 The procedure is based on the formation of carbon-14 by capture of
solar neutrons in the upper atmosphere
 The carbon-14 is incorporated into carbon dioxide then into more
complex carbon-containing molecule through photosynthesis
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When animals eat the plants, the carbon-14 moolecules
becomes incorporated within them
o Because a living plant or animal has a constant intake of
carbon compounds, it is able to maintain a ratio of
carbon-14 to carbon-12
 Once the organism dies, it no longer ingests carbon compound
o The ratio of carbon-14 to carbon-12 decreases as a result
 By measuring this ratio and comparing it to that
of the atmosphere, we can estimate the age of an
object
Calculations Based on Half-life
o
 The first-order rate constant, , is called the decay constant
o The rate at which a sample decays is called its activity, and it is often
expressed as the number of disintegration observed per unit time
 The Becquerel (Bq) is the SI unit of expressing the rate at which
nuclear disintegrations are occurring
 A becquerel is defined as one nuclear disintegration per second
 An older, but still widely used, unit of activity is the curie (Ci)
 A curie is defined as 3.7 × 1010 disintegrations per second
o As a radioactive sample decays, the amount of radiation emanating from the
sample decays as well
o A first-order rate law can be transformed into the following equation
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is the time interval of decay
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is the decay constant
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is the number of nuclei at time zero
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is the number remaining after the time interval
o From the equation above, we can obtain the relationship between the decay
constant, , and half-life,
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21.6: Energy Changes in Nuclear Reactions
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Energies associated with nuclear reactions can be considered with the aid of
Einstein’s famous equation relating mass and energy
o
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stands of energy
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stands for mass
stands for speed of light, 2.9979 × 108 m/s
This equation states that the mass and energy of an object are
proportional
 If a system loses mass, it loses energy
 If a system gains mass, it gains energy
The mass changes in chemical reactions are too small to detect
o Thus, it is possible to treat chemical reactions as though mass is conserved
The mass changes and the associated energy changes in nuclear reactions are much
greater than those in chemical reactions
o E.g.
 The nuclei in this reaction have the following masses:
, 238.003
amu;
, 233.9942 amu; and
, 4.0015 amu
 The mass change, Δm, is the total mass of the products minus the total
mass of the reactants
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 The energy change per mole associated with this reaction can be
calculated using Einstein’s equations:
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is converted to kilograms to obtain
in joules
Nuclear Binding Energies
o Masses of nuclei are always less than the masses of the individual nucleons of
which they are composed
 The mass difference between a nucleus and its constituent nucleons is
called the mass defect
o The energy required to separate a nucleus into its individual nucleons is called
the nuclear binding energy
 The larger the binding energy, the more stable is the nucleus toward
decomposition
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o The binding energies per nucleon can be used to compare the stabilities of
different combinations of nucleons
 The binding energy per nucleon at first increases in magnitude as the
mass o= number increases, reach about 1.4 × 10-12 J for nuclei whose
mass numbers are in the vicinity of iron-56
 It then decreases slowly to about 1.2 × 10-12 J for very heavy
nuclei
o Fission – the process in which heavy nuclei gain stability and therefore give off
energy if they are fragmented into two mid-sized nuclei
o Fusion – The process in which very light nuclei are combined or fused
together to give more massive nuclei
21.7: Nuclear Power: Fission
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Uranium-235, uranium-233, and plutonium-239 undergoes fission when struck by a
slow-moving neutron
o A heavy nucleus can split in many different ways
On average, 2.4 neutrons are produced by every fission of uranium-235
o If one fission produces two neutrons, these two neutrons can cause two
additional fissions and so forth
 The number of fissions and the energy release quickly escalate and the
result is a violent explosion
 Reactions that multiply in this fashion are called chain reactions
For a fission chain reaction to occur, the sample of fissionable material must have a
certain minimum mass
o The amount of fissionable material large enough to maintain the chain
reaction with a constant rate of fission is called the critical mass
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o A mass in excess of a critical mass is referred to as a supercritical mass
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Nuclear Reactors
o Nuclear fission produces the energy generated by nuclear power plants
o Rods composed of materials such as cadmium or boron control the fission
process by absorbing neutrons
 These rods regulate the flux of neutrons to keep the reaction chain
self-sustaining, while preventing the reactor core from overheating
o Steam is used to drive a turbine connected to an electrical generator
 The steam must be condensed
21.8: Nuclear Power: Fusion
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Fusion reactions are known as thermonuclear reactions
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o Fusion reaction requires high energies to overcome the repulsion between
nuclei
An apparatus called tokamak uses the magnetic fields to contain and to heat the
reaction
o Temperatures of over 100,000,000 K have been achieved in a tokamak