nuclear chemistry - wordpress.com · 2016-03-15 · nuclear radiation can transfer energy from...
TRANSCRIPT
Nuclear Chemistry
The NucleusSection 1
●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
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
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
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
●First convert 0.03038 amu to kg to match unit for energy, kg·m2/s2
●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
●Can be thought of as energy needed to break apart nucleus
●Also a measure of the nucleus’ stability
10
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
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
●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
●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
●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
●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
●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….)
19
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
●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
Radioactive DecaySection 2
●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
●Photographic plate exposed even though it was protected from visible light●Suggests exposure by x-rays
●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
●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
●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
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
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
●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
31
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
●Ex. Decay of into N-14●Atomic number increases by one●Mass number stays the same
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
●Ex. one proton from K is converted to a neutron so atomic number decreases by one●positive charge from proton (positron) emitted
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
37
Gamma Emission●Gamma rays (γ) – high energy electromagnetic
waves emitted from nucleus as it changes from excited to ground energy state
●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
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
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
●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
●Locate parent nuclide U-238●As it decays it emits an alpha
particle
●Mass number decreases by four, so its position on graph moves down vertically by four
●The atomic number decreases by two, so horizontal position goes down two●The daughter nuclide is
isotope of thorium
●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
●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
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
●Energy may be supplied by accelerating particles in magnetic or electrical field of particle accelerator
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
●Transuranium elements – more than 92 protons in nuclei●Currently 19 named
Nuclear RadiationSection 3
●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
●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
●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
●Gamma rays have highest penetrating ability
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
●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
●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
●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
Radiation Detection●Three devices used to measure
radiation:1. Film badges2. Geiger-Muller counter3. Scintillation counter
Film badges●Use exposure of film to measure
approximate radiation exposure of people working with radiation
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
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
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
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
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
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
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
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
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
●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?
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
Storage●Most common waste from
nuclear power plants is the fuel rod
●Fuel rods can be contained above ground in water pools or dry casks
●When pools are full, rods are moved to dry casks made of concrete and steel●Both pools and casks only
temporary storage
Disposal●Done with intention of never
retrieving the material●Needs careful planning●In US there are 131 disposal
sites in 39 states
Nuclear Fission and Nuclear Fusion
Section 4
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
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
●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
Nuclear Power Plants●Convert heat made by nuclear fission into
electrical energy●5 main components1. Shielding2. Fuel3. Control rods4. Moderator5. Coolant
Shielding●Radiation-absorbing material
used to decrease emission of radiation, especially gamma rays, from the reactor
Fuel and Coolant●U-235 is fuel●Fission produces energy as heat●Heat absorbed by coolant
Control Rods●Neutron-absorbing rods that help
control the reaction by limiting the number of free neutrons
Moderator●b/c fission of U-235 is induced by
slow neutrons, moderator used to slow down fast neutrons made by fission
●Nuclear power plants provide competitively priced electricity WITHOUT emitting greenhouse gases or particulates
92
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
●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
●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