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Nuclear Chemistry
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The NucleusSection 1
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●Atomic nuclei made of protons and neutrons●Collectively called nucleons●In nuclear chemistry, atom is called nuclide●Nuclide identified by number of protons and
neutrons in nucleus
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Mass Defect and Nuclear Stability●b/c atom made of protons, neutrons and electrons,
you would expect mass to be same as mass of equal numbers of protons, neutrons and electrons●Not so
●Consider 42Helium
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42 Helium●2 protons: (2 x 1.007276 amu) = 2.014552 amu●2 neutrons: (2x1.008665amu) = 2.017330amu●2 electrons: (2x0.0005486amu)=0.001097amu●= 4.032979 amu●Atomic mass of 42 Helium is 4.00260 amu●0.03038 less than calculated mass●Mass defect ! difference between mass of atom and sum
of masses of protons, neutrons and electrons
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Nuclear Binding Energy●What causes loss in mass?●According to E=mc2 mass can be converted into energy
and vice versa●Mass defect is caused by conversion of mass to energy
upon formation of nucleus●Mass units of mass defect can be converted to energy
units using Einstein’s equation
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●First convert 0.03038 amu to kg to match unit for energy, kg·m2/s2
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●Then calculate energy equivalent
E = mc2
= 4.54 x 10-12 kg·m2/s2 = 4.54 x 10-12 J
●This is nuclear binding energy ! energy released when nucleus is formed from nucleons
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●Can be thought of as energy needed to break apart nucleus
●Also a measure of the nucleus’ stability
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Binding Energy per Nucleon●Used to compare the stability of different nuclides●Binding energy per nucleon ! binding energy of the
nucleus divided by the number of nucleons it contains●Higher BEPN = more tightly nucleons held together●Elements with intermediate atomic masses have
greatest BEPN so are most stable
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Nucleons and Nuclear Stability
●Stable nuclides have certain characteristics●When number of protons in stable nuclei plotted
against number of neutrons, you get band of stability●Band of stability ! stable nuclei cluster over a
range of neutron-proton ratios
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●Low atomic numbers: most stable nuclei have n:p ratio of about 1:1●Ex. He – 2n, 2p●As atomic number
increases, stability increases to 1.5:1●Ex. Pb – 124 n, 82 p
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●This trend can be explained by relationship between nuclear force and electrostatic forces between protons●Protons in nucleus repel all other protons through
electrostatic repulsion●Short range of nuclear force allows them to
attract only protons close to them
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●As number of protons in nucleus increases, electrostatic force between protons increases faster than nuclear force●More neutrons required to increase nuclear force
and stabilize nucleus●Beyond atomic number 83 (Bi), repulsive force of
protons so great that no stable nuclides exist
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●Stable nuclei have even numbers of nucleons●Of stable nuclides, more than half have even
numbers of protons and neutrons●Only 5 have odd numbers of both●Shows that stability is greatest when nucleons are
paired
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●Most stable nuclides have 2, 8, 20, 28, 50, 82, or 126 protons, neutrons, or total nucleons●Extra stability supports theory that nucleons exist at
certain energy levels●Nuclear shell model – nucleons exist in different
energy levels, or shells, in the nucleus●Magic numbers – numbers of nucleons that
represent completed outer energy levels (2, 8, 20….)
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Nuclear Reactions●Unstable nuclei go through
spontaneous changes that change the numbers of protons and neutrons●Give off large amounts of energy to
increase stability●Nuclear reaction – reaction that
changes the nucleus of an atom
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●In equations for nuclear reactions, total of atomic numbers and total of mass numbers equal on both sides
●When atomic number changes, identity of element changes●Transmutation – change in identity of nucleus
as result of change in number of protons
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Radioactive DecaySection 2
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●1896 Henri Becquerel studied possible connection between light emission of some uranium compounds after exposure to sunlight and X-ray emission●Wrapped photographic plate in lightproof
covering and placed uranium compound on top●Placed this in sunlight
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●Photographic plate exposed even though it was protected from visible light●Suggests exposure by x-rays
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●Tried to repeat – cloudy weather prevented exposure to sunlight●Plate was still exposed●This meant sunlight wasn’t needed to produce
rays that exposed the plate●Rays were made by radioactive decay
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●Radioactive decay – spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation, or both●Nuclear radiation exposed the plate –
particles or electromagnetic radiation emitted from nucleus during radioactive decay
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●Uranium is a radioactive nuclide – unstable nucleus that undergoes radioactive decay●Marie and Pierre Curie found of the known elements in
1896, only uranium and thorium were radioactive●1898 they discovered two new radioactive metal
elements – polonium and radium●Since then many others discovered●All nuclides beyond atomic #83 are unstable and so
radioactive
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Types of Radioactive Decay●Nuclide’s type and rate of decay depend on
nucleon content and energy level of nucleus1. Alpha particle2. Beta particle3. Positron4. Gamma ray
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Alpha Emission●Alpha particle (α) – two protons and two neutrons
bound together and emitted from nucleus during some kinds of radioactive decay●Alpha particles = He nuclei●Charge = +2●Emission restricted almost completely to heavy nuclei
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●In heavy nuclei, both number of neutrons and protons are reduced to increase stability of nucleus●Ex. Po-210 and Pb-206
●Atomic number decreases by two●Mass number decreases by four
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Beta Emission●Nuclides above band of stability unstable b/c
neutron/protons ratio too big●there are too many neutrons, so it splits into a
proton and electron●Electron emitted from nucleus as beta particle●Beta particle (β) – electron emitted from nucleus
during some kinds of radioactive decay
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●Ex. Decay of into N-14●Atomic number increases by one●Mass number stays the same
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Positron Emission●Nuclides below band of stability are unstable b/c their
neutron/protons ratio is too small (too many protons)●To decrease number of protons, one can be converted
into a neutron by emitting a positron●Positron – particle that has the same mass as an
electron, but has a positive charge, and is emitted from the nucleus during some kinds of radioactive decay
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●Ex. one proton from K is converted to a neutron so atomic number decreases by one●positive charge from proton (positron) emitted
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Electron Capture●Neutron/proton ratio too small (too few
neutrons)●Inner orbital electron captured by nucleus of its
own atom●Inner orbital electron combines with proton,
forming neutron
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Gamma Emission●Gamma rays (γ) – high energy electromagnetic
waves emitted from nucleus as it changes from excited to ground energy state
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●According to nuclear shell model, gamma rays are made when nuclear particles undergo transitions in nuclear-energy levels●Similar to emission of photons when electron
drops to lower energy level (photoelectric effect)●Gamma emission usually occurs immediately
after other types of decay, when other types of decay leave nucleus in excited state
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Half-Life●No two radioactive isotopes decay at the same
rate●Half-life – time required for half of the atoms of
a radioactive nuclide to decay
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Decay Series●One nuclear reaction is not always enough to
make a stable nuclide●Decay series – series of radioactive nuclides
made by successive radioactive decay until a stable nuclide is reached●Heaviest nuclide of each decay series – parent
nuclide●Nuclides produced by decay of parent nuclides
– daughter nuclides
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●All naturally occurring nuclides with atomic numbers >83 are radioactive and belong to one of three natural decay series●Parent nuclides – U-238, U-235,
Th-232
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●Locate parent nuclide U-238●As it decays it emits an alpha
particle
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●Mass number decreases by four, so its position on graph moves down vertically by four
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●The atomic number decreases by two, so horizontal position goes down two●The daughter nuclide is
isotope of thorium
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●The half-life of Th-234 is about 24.1 days●It decays giving off beta particles●This increases its atomic number, so
horizontal position●Mass number/vertical position stay
the same
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●In the final step, Po-210 loses alpha particle to form Pb-206
●This is a stable, nonradioactive isotope of lead●It contains 82 protons – magic number
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Artificial Transmutations●Artificial radioactive nuclides are not found
naturally on Earth●Made by artificial transmutation –
bombardment of nuclei with charged and uncharged particles●b/c neutrons have no charge, they can penetrate
nucleus●Positively charged alpha particles, protons, etc.
are repelled by nucleus●Great amounts of energy are needed
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●Energy may be supplied by accelerating particles in magnetic or electrical field of particle accelerator
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Artificial Radioactive Nuclides●Radioactive isotopes of all natural
elements have been made by artificial transmutation●Production of technetium and
promethium by artificial transmutation filled gaps in periodic table
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●Transuranium elements – more than 92 protons in nuclei●Currently 19 named
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Nuclear RadiationSection 3
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●In Becquerel’s experiment nuclear radiation from uranium compound penetrated lightproof covering of film●Different types of radiation have
different abilities to penetrate●Alpha particles, beta particles,
gamma rays
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●Alpha particles can travel a few cm in air●Have low penetrating ability due to large
mass and charge●Cannot penetrate skin●Can cause damage inside body if
substance that emits alpha particles is ingested/inhaled
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●Beta particles (emitted electrons) travel at close to the speed of light●Penetrating ability 100 times
greater than alpha particles●Can travel a few meters in air
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●Gamma rays have highest penetrating ability
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Radiation Exposure●Nuclear radiation can transfer energy
from nuclear decay to electrons of atoms or molecules and cause ionization●Roentgen(R) – unit used to measure
nuclear radiation exposure●Equal to amount of gamma and X ray
radiation that produces 2 x 109 ion pairs when it passes through 1 cm3 of dry air
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●Ionization damages living tissue●Radiation damage to human tissue measured
in rems (roentgen equivalent, man)●Rem – unit used to measure the dose of any
type of ionizing radiation that factors in the effect that the radiation has on human tissue●Long-term exposure can cause DNA mutations
that result in cancer and other genetic defects●DNA mutated through direct radiation or
contact with ionized molecules
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●Everyone is exposed to environmental background radiation●Average (for people in US) is 0.1 rem per
year●Maximum allowed dose is 0.5 rem per year●Airline crews and people at high altitudes
have increased levels b/c of increase cosmic ray levels at high altitudes
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●Radon-222 trapped inside homes causes exposure●Released from certain rocks and
moves up through soil into homes through holes in foundation●Increases risk of lung cancer,
especially in smokers
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Radiation Detection●Three devices used to measure
radiation:1. Film badges2. Geiger-Muller counter3. Scintillation counter
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Film badges●Use exposure of film to measure
approximate radiation exposure of people working with radiation
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Geiger-Muller counter●Instruments that detect radiation
by counting electric pulses carried by gas ionized by radiation●Typically used to detect beta-
particles, X rays, gamma radiation
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Scintillation counters●Radiation can be detected when it
transfers energy to substances that scintillate or absorb ionizing radiation and emit visible light●Instruments that convert
scintillating light to electric signal for detecting radiation
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Applications of nuclear radiation●Many applications based on fact that
physical and chemical properties of stable isotopes are the same as radioactive isotopes of the same element
1. Radioactive dating2. In medicine3. In agriculture
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Radioactive dating●Process by which the approximate age
of an object is determined based on amount of certain radioactive nuclides present●Based on half-life●Measure amount of accumulating
daughter nuclides or disappearance of parent nuclide
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Carbon-14●Half-life 5715 years●Can be used to estimate age of organic
material up to 50,000 years old●Nuclides with longer half-lives are used to
date older objects●Radioactive dating has been used to date
minerals and lunar rocks more than 4 billion years old
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In medicine●Cobalt-60 used to destroy certain types
of cancer cells●Radioactive tracers – incorporated into
substances so that movement of the substances can be followed by radiation detectors●Can be used to diagnose cancer and
other diseases
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In agriculture●Radioactive tracers in fertilizers determine
effectiveness of fertilizer●Amount of tracer absorbed by plant
indicates amount of fertilizer absorbed●Nuclear radiation also used to prolong
shelf life of food●Gamma rays from cobalt-60 can be used
to kill bacteria and insects that spoil food
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Nuclear Fission and Fusion●During fission, a larger heavier nucleus
splits into two or more lighter nuclei●Products include nuclei as well as
nucleons form from fragments’ radioactive decay●Fission powers nuclear reactors
including those on nuclear powered submarines and aircraft carriers
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Fusion●Opposite of fission●Very high temps and pressures
used to combine light atoms●Primary process that fuels stars●Creating and maintaining fusion
harder than fission
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●Both fission and fusion release enormous amounts of energy that can be converted into heat and electric energy●Both produce nuclear waste●Fission produces more waste than fusion●As new processes are developed to use
energy from fission and fusion develop, how to contain, store, dispose of the waste?
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Containment●Every radioactive substance has half-life●Waste from medical research has half-life
of few months or less●Waste produced in nuclear reactor will
takes 100s-1000s of years to decay●Needs to be contained so living organisms
are shielded from radioactivity●On-site storage and off-site disposal
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Storage●Most common waste from
nuclear power plants is the fuel rod
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●Fuel rods can be contained above ground in water pools or dry casks
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●When pools are full, rods are moved to dry casks made of concrete and steel●Both pools and casks only
temporary storage
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Disposal●Done with intention of never
retrieving the material●Needs careful planning●In US there are 131 disposal
sites in 39 states
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Nuclear Fission and Nuclear Fusion
Section 4
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Nuclear Fission● Very heavy nucleus splits into more stable nuclei of
intermediate mass● Releases tons of energy● Can occur spontaneously or when nuclei are bombarded with
particles● U-235 bombarded with slow neutrons – nucleus captures a
neutron becoming unstable● Nucleus splits into medium-mass nuclei with emission of
more neutrons● Mass of product less than reactants – missing mass
converted to energy
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Chain Reaction●When fission by neutrons produces more
neutrons, a chain reaction can occur●Chain reaction – reaction in which the
material that starts the reaction is also one of the products and can start another reaction●2-3 neutrons emitted when U-235 fission
occurs
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●Fission continues until all U-235 atoms are split or until neutrons fail to strike another atom●If mass of U-235 sample is below minimum,
too many neutrons escape without hitting another and the chain reaction stops●Critical mass – minimum amount of nuclide
that provides the number of neutrons needed to sustain a chain reaction●Uncontrolled chain reactions provide explosive
energy of atomic bombs●Nuclear reactors – use controlled-fission
chain reactions to produce energy and radioactive nuclides
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Nuclear Power Plants●Convert heat made by nuclear fission into
electrical energy●5 main components1. Shielding2. Fuel3. Control rods4. Moderator5. Coolant
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Shielding●Radiation-absorbing material
used to decrease emission of radiation, especially gamma rays, from the reactor
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Fuel and Coolant●U-235 is fuel●Fission produces energy as heat●Heat absorbed by coolant
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Control Rods●Neutron-absorbing rods that help
control the reaction by limiting the number of free neutrons
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Moderator●b/c fission of U-235 is induced by
slow neutrons, moderator used to slow down fast neutrons made by fission
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●Nuclear power plants provide competitively priced electricity WITHOUT emitting greenhouse gases or particulates
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92
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Nuclear Fusion●Low mass nuclei combine to form heavier,
more stable nucleus●Releases even more energy per gram of
fuel than fission●In sun and stars, hydrogen nuclei combine
at extremely high temp (10,000,000°C) and pressure to form helium nucleus with loss of mass and release of energy
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●If fusion reactions could be controlled they could be used to generate energy●Researchers studying ways to contain
reacting plasma required for fusion●Plasma – extremely hot mixture of positive
nuclei and electrons●No known material withstand initial temp
(10,000,000°C) to induce fusion
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●Scientists use strong magnetic fields to suspend plasma inside container but away from the walls●Large amounts of energy is also needed
to initiate fusion reactions●For fusion to be practical energy source,
more energy must come out than needs to be put in