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

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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

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….)

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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

●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

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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

●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

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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

●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

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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

●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