Nuclear Chemistry

1
Nuclear Chemistry
2
Contents

• Conservation of Mass-Energy
• Nuclear Binding Energy
• Band of Stability
• Transmutation
• Detecting and Measuring Radiation
• Applications of Radioactivity
• Nuclear Fusion
• Nuclear Fission

3
Conservation of Mass and Energy

• Atomic nucleus is very stable, it is not effected
  at all by any electron events.
• There are some naturally radioactive elements
  which transforms other stable element.
• Nuclear reactions are much more energetic
  compared to atomic and molecular reactions. Here,
  nucleons are rearranged instead of electrons.

4
Conservation of Mass and Energy

• In nuclear reactions mass-energy is conserved.
  Mass and energy are convertible. Einsteins
  relativity theory redefines the mass
• If the particle is at rest (v0), then mm0 (m0
  is rest mass). c is the speed of light which is
  the upper limit of speed. If the particles speed
  approaches c, its mass becomes infinitely large.
• Law of Conservation of Mass-Energy
• The sum of all energy and of all mass (total
  energy) is a constant.

5
Einstein Equation

• Mass is converted into energy by Einstein
  equation
• ?E ?m0 c2
• The energy associated with a chemical reaction is
  proportional to mass exchanged
• CH4 2O2 ? CO2 2H2O ?H -890 kJ
• 890x103(kg.m2.s-2) ?m0(3x108m.s-1)2
• ?m0 8.98x10-9 g (total mass of reactants80
  g 16 g CH4, 64 g O2)

6
Nuclear Binding Energy

• The mass of atomic nucleus of an element is
  always less than the sum of masses of all
  nucleons, neutrons and protons. This energy
  difference is called the binding energy which is
  used to hold nucleons together as the nucleus.

7
Binding Energy per Nucleon as a Function of Mass
Number
8
Example (BE of 4He)

• What is the binding energy of He?
  (1amu1.66x10-27 kg)
• Helium has 2 protons and 2 neutrons.
• BE 4.031875 - 2(1.0073731.008665) 0.030369u
• 0.030369u (1.66x10-27 kg/u) (3x108 m.s-1)2
  4.54x10-12 J
• Energy needed to disintegrate one mole of He
  4.54x10-12 J (6.02x1023 at/mole) 2.73x1012 J

9
Electron Volt as an Energy Unit

• 1 eV 1.6x10-19 J
• 1 keV 103 eV
• 1 MeV 106 eV
• 1 GeV 109 eV

KE1eV
e-
1V
10
BE/u and Nuclear Stability

• Binding Energy/Nucleon (BE/u) is a measure of
  stability of nucleus.
• 1 u 931.5 MeV
• BE(4He)/4 0.03069 u (931.5 MeV/u)/4
• 7.147 MeV/u
• BE(56Fe)/56 8.8 MeV/u (most stable)
• BE(239Pu)/239 7.393 MeV/u

11
Radioactivity

• Nuclear force is the force which holds the
  nucleons in a nucleus. Nuclear forces are short
  distance and stronger than the Coulomb forces
  (repulsive between protons) in the nucleus.
  Number of neutrons increases as the atomic number
  increases. An isotope having more protons than
  neutrons is unstable and decays to a stable
  nucleus (radioactive decay).
• Three important radioactive decay modes Alpha,
  Beta, and Gamma.

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Alpha Decay

• Very heavy nuclei (Agt200, Zgt90) decay by alpha
  particle (4He) emission.
• 241Am ? 237Np 4He
• Coulomb repulsion due to large number of protons
  (Z95) in 241Am is released by the emission of
  alpha particle.

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Energy Released in Alpha Decay

• 241Am ? 237Np 4He Q
• Heat released in alpha emission
• Q M(241Am) - M(237Np) - M(4He)
• (1u 931.5 MeV)
• Q 5.59 MeV

14
Kinetic Energy of Alpha Particle

• 241Am ? 237Np 4He
• Conservation of momentum (p ? pNp)
• M?v? MNpvNp
• Conservation of energy (Q KE? KENp)
• Q 1/2M?v?2 1/2MNpvNp2
• KE? QMNp/( M?MNp)
• KE? (237/241)5.59 5.50 MeV

15
Beta Decay

• One of the neutrons is transformed into a proton
  in the nucleus emitting a beta particle -10e,
  and antineutrino, ?.
• Decay of tritium by beta decay
• Emitted beta particle is like an electron,
  neutrino is chargeless and not detectable

16
Gamma Radiation

• The nucleons of a nucleus are arranged in
  quantized energy levels as electrons in atoms and
  molecules. Alpha and beta particle decays are
  accompanied by a gamma ray emission.
• Gamma radiation is an electromagnetic radiation
  like visible light emitted after excitation of
  electrons in atoms and molecules but at much
  higher energy.

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228Th ? 224Ra 4He(Gamma Emission after Alpha
decay)
18
X-Rays

• X-rays are atomic in origin. They are
  electromagnetic rays created by filling of a
  vacancy created in an inner atomic orbital by an
  outer electron. Electron hole is created by an
  electron beam.

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Example (Beta decay)

• Write the balanced nuclear equation for the decay
  of 9038Sr, a beta emitter.
• Solution
• Conservation of charge 38Z(-1) Z39
• Conservation of mass 90A(0) A90

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Radioactive Disintegration Series

• There are three naturally occuring radioactive
  decay series
• 238U series (half-life4.5x109 years) ? (9 ?
  10?) .. 206Pb.
• 232Th series (half-life 1.4x1010 years) ? (6 ?
  5?) .. 208Pb.
• 235U series (half-life7.0x108 years) ? (12 ?
  9?) .. 207Pb.

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Natural Radioactive Elements

• Synthetic radionuclides

• Naturally occuring radionuclides

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Positron Emission

• Positron has the same mass of electron, but
  positive charge. Radionuclides with excessive
  proton may decay by positron emission. A proton
  is converted into a neutron in the nucleus
  emitting a positron and neutrino.
• Positron is the antimatter of electron, they
  annihilate each other by emitting two photons 511
  keV each.

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Exchange of Matter and Energy
?-ray
Positron
Electron
?-ray
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Electron Capture

• An alternative decay mode to positron emission is
  Electron Capture. A K shell or L shell electron
  is captured by nucleus and proton in nucleus is
  transformed into a neutron. An X-ray is emitted
  after electron capture.
• EC is the only decay mode which depends on
  physical or chemical state of atom. Be in metalic
  form decays 0.3 faster than BeO. K electron is
  closer to nucleus in Be metal.
• (7Be e- ? 7Li)

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Neutron Emission

• A very rare decay mode is by neutron emission.
  Here, the element does not change, but the mass
  number decreases by one.
• 87Kr ? 86Kr 1n
• The net effect of EC is the conversion of p into
  n

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Band of Stability

• When stable isotopes of elements are sorted with
  proton and neutron numbers, a band of stability
  forms. No stable isotope of element 83, bismuth
  is present. Elements beyond 83 are radioactive
  or,
• man made.
• With increasing atomic number, N/Z ratio
  increases beyond 11. The increasing Coulomb
  repulsion with Z is compensated by excess N.

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Stability curve
NZ
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Band of Stability (cont..)

• Isotopes above and the left of stability curve
  have access neutrons. The decay is by beta decay.
  (n ? p e-)
• Isotopes below and right of stability line are
  rich in protons, they decay by positron emission
  or by electron capture. (p ? n e)

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Beta and Positron Decay Around Stability Line
N
Z
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Odd-Even Rule

• When the numbers of neutrons and protons are even
  they are more stable than when both are odd.
• 264 stable isotopes
• 157 even-even
• 5 odd-odd
• 102 even-odd
• When the spins of neutrons or protons are paired
  they are more stable than spins are unpaired.

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Nuclear Energy Levels and Magic Numbers

• Nucleons are located in quantized energy levels
  like electrons of atoms.
• Nucleons with magic numbers are extra stable.
  Magic numbers 2, 8, 20, 28, 50, 82, and 126.
• Extra stable nuclei
• (Double magic)

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Transmutation

• Changing of one element into another is called
  transmutation. Beta decay and nuclear reactions
  are transmutations.
• Nuclear reactions
• Cyclotrons and linear accelerators accelerate the
  nucleus (positive ion) of an atom above the
  Coulomb barrier which induces nuclear reaction on
  a target nucleus.
• 4He 14N ? 18F ? 17O 1p

projectile
target
compound nucleus
residue
evaporated
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Nuclear Reactions

• Formation of a compound nucleus
• Decay of compound nucleus

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Transuranium Elements

• The naturally occurring radioisotopes above Z83
  have very long half lives (gt109 y).
• All elements with Zgt93 (Neptunium) are synthetic.
  They have very short half lives.
• Actinide series elements starting with Thorium
  are synthesized in lab (Z104-111)

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Detecting and Measuring Radiation

• Detection Devices
• Geiger Counter Ionization tube to detect beta
  and gamma rays. Radiation produces ion pairs
  which are collected by the electrodes of Geiger
  counter.
• Scintillation Counters A NaI crystal or a
  plastic scintillator produces a flush of light
  upon the impinge of radiation. The light is
  amplified and counted electronically.

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Detectors (cont..)

• Solid State Detectors (GeLi or SiLi) Intrinsic
  semi conductors made from single crystal of Ge or
  Si doped with Li.
• Film Dosimeters Photographic films darken when
  exposed to radiation. The darkness of film is
  proportional to the dosage received.
• Cloud Chambers When a very high energy charged
  particle passes through a saturated vapor with
  water or alcohol, a trace micro droplets are
  formed.

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Units of Radiation

• Unit of Activity
• Becquerel (Bq) is the unit of disintegrations/seco
  nd
• Curie (Ci) is the activity of 1 g of 226Ra.
• 1 Ci 3.7x1010 Bq
• Unit of Radiation Dose
• Rad and Gray are the absorbed radiation energy by
  a matter
• 1 rad 100 ergs/g
• 1 Gy 1 J/kg
• 1 Gy 100 rad

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Units of Radiation (cont..)

• Units of Dose Equivalent The demage radiation on
  tissue may differ with the type of radiation.
  Same energy alpha particle is more harmful than
  gamma ray when penetrated in tissue. It is
  additive for different types of radiation and
  tissue.
• Sievert (Sv) Equivalent dose, H
• H D Q N
• (D absorbed dose in Gy, Q Quality factor, N
  Other factors)
• Rem is the older dose equivalent. The effect of 1
  Roentgen in human body (creation of 2.1x109
  charges with radiation). D is in rad, H is in rem.

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Radiation Sickness

• Whole body dose of 100 rem causes nausea,
  vomitting, loss of hair, drop in white cell.
• Exposure to 400 rem of radiation will kill half
  of the population. 600 rem kills the whole
  population.
• Limits are set for radiation exposure
• 2 rem for workers in nuclear facilities.
• 0.1 rem for public.

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Radiation Produced Free Radicals

• The major effect of radiation absorbed by the
  body is the creation of ions and radicals.
• Water cation is unstable and breakup path is
• Radicals are very reactive and harmful to DNA

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Background Radiation

• Average radiation doses received

42
Radiation Protection

• Intensity of radiation decreases with the square
  of distance
• If I1 is known at distance d1, then the intensity
  I2 at distance can be calculated as,

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Applications of Radioactivity

• Radioactive tracers
• A radioactive isotope of an element is used to
  trace the distribution of that element in the
  body or can be used in terapy. 131I is used for
  thyroid gland treatment. 99Tc (TcO4-) is used in
  detection of brain tumors.
• Neutron Activation Analysis
• When natural substances are irradiated with
  neutrons in a nuclear reactor stable elements
  capture neutrons and become radioactive. Very low
  concentrations in ppb range can be measured.
• AX 1n ? (A1)X ? (A1)X ?

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Activation of 39K by Neutron Activation
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Radiological Dating

• The half life of radioisotopes are constant over
  time
• Using half lives of some naturally occuing
  isotopes the age of geological samples and
  organic matter can be measured.
• 40K/40Ar (Age of geological formations, t1/2109
  y)
• 14C/12C (Age of organic matter, t1/25700y)
• 210Pb (Sedimentation rate in lake and sea,
  t1/222 y)

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Carbon-14 Dating

• 14C is synthesized in upper atmosphere by the
  interaction of cosmic neutrons and 14N.
• 14C is a beta emitter with a half life of 5700
  years 14C ? 14N ?-
• Recent 14C/ 12C1.2x10-12
• 14C/ 12C(t) 1.2x10-12 e-(0.693/5700)t
• A samples 14C/ 12C3x10-13
• 3x10-131.2x10-12 e-t/8270
• t 12000 y

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Nuclear Fusion

• Two light nuclei fuse to form heavier nucleus
  with a large energy release.
• High energy need to start reaction is provided by
  heat Thermonuclear fusion
• Inertial Confinement heating by laser or X-ray
  beam
• Magnetic confinement Tokamak reactor.

48
Nuclear Fission

• Fission is discovered by Otto Hahn and Fritz
  Strassman in 1939. Soon after Lise Meitner and
  Otto Frish discovered the fission of 235U by
  neutrons
• There are more than 30 possible decay channel.
  b, average number of neutrons is 2.47