John A. Schreifels

1
Chapter 21

• Nuclear Chemistry

2
Overview

• Radioactivity and Nuclear Bombardment Reactions
• Radioactivity
• Nuclear Bombardment Reactions
• Radiations and Matter Detection and Biological
  Effects
• Rate of Radioactive Decay
• Applications of Radioactive Isotopes
• Energy of Nuclear Reactions
• Mass Energy Calculations
• Nuclear Fission and Nuclear Fusion

3
Nuclear Chemistry

• In this chapter we will look at two types of
  nuclear reactions.
• Radioactive decay is the process in which a
  nucleus spontaneously disintegrates, giving off
  radiation.
• Nuclear bombardment reactions are those in which
  a nucleus is bombarded, or struck, by another
  nucleus or by a nuclear particle.

4
Nuclear Reactions and their characteristics

• Nuclear Chemistry study of changes in structure
  of nuclei and subsequent changes in chemistry.
• Radioactive nuclei spontaneously change
  structure and emit radiation.
• Differences between nuclear and chemical
  reactions
• Much larger release in energy in nuclear
  reaction.
• Isotopes show identical chemical reactions but
  different nuclear reactions.
• Nuclear reactions not sensitive to chemical
  environment.
• Nuclear reaction produces different elements.
• Rate of nuclear reaction not dependent upon
  temperature.

5
NUCLEAR STRUCTURE Stability

• nucleon any nuclear particle, e.g. protons, p,
  and neutrons, n.
• Nucleus held together by strong attractive
  forces but electrostatic repulsion causes large
  atoms (gt83 protons) to be unstable.
• Let Z atomic ( of protons) and A Z of
  neutrons. Isotopes represented as .
• has 8 p, 8 e?, and 8 n
• has 8 p, 8 e?, and 9 n
• has 8 p, 8 e?, and 10 n.
• Structure deduced from emission of radiation from
  unstable particles
• .a ray attracted towards negatively charged
  plate Þ Positively charged.
• .b ray attracted towards positively charged
  plate Þ Negatively charged.
• .g ray not attracted to either plate Þ Neutral.

6
NUCLEAR REACTIONS

• Radioactivity nucleus unstable and spontaneously
  disintegrates.
• Nuclear Bombardment causes nuclei to
  disintegrate due to bomdarbment with very
  energetic particles.
• Particles in nuclear reactions
• Positron positively charged particle with same
  mass as electron.
• Gamma ray Very high energy photon (l 10-12 M
  Visible l 10-7M).
• Nuclear reaction written maintaining mass and
  charge balance.
• E.g. g..

7
RADIOACTIVITY

• Types of Radioactive decay
• Beta emission Converts neutron into a proton by
  emission of energetic electron atomic
  increases
• E.g. Determine product for following reaction
• Alpha emission emits He particle.
• E.g. Determine product
• Positron emission Converts proton to neutron
• E.g. Determine product of
• Gamma emission no change in mass or charge but
  usually part of some other decay process.
• E.g.
• Electron capture electron from electron orbitals
  captured to convert proton to neutron.
• E.g. Determine product

8
NUCLEAR STRUCTURE and STABILITY

• Shell model of nucleus protons and neutrons
  exist in energy levels which have optimum of
  each in each shell.
• Magic of nuclear particles in particular
  shell (similar to 2,8,18 etc. for electrons.)
• Protons 2, 8, 20, 28, 50, 82
• Neutrons 2, 8, 20, 28, 50, 82 and 126.
• E.g. ?-particles ( ) are doubly
  magic.
• Nuclei with even of protons and neutrons most
  stable. ( Largest of stable isotopes).
• Nuclei with odd of protons and neutrons least
  stable. (Least of stable isotopes).

9
Band of Stability

• Band of stability stable isotopes. (above Z
  82 ? - or ? - emission.)
• above beta emission
• below electron or positron emission

10
NUCLEAR BOMBARDMENT (Transmutation)

• Bombard nuclei with nuclear particles to convert
  element to another one.
• Rutherford discovered
• E.g.1. Identify product for electron capture
• E.g.2. Identify products for neutron bombardment
  of Fe
• E.g.3 Identify the product of

11
RATE OF DISINTEGRATION

• Rate of disintegration proportional to number of
  nuclei present.
• Rate kN or
• Half-life-time required for half of original
  nuclei to undergo decay.
• At t1/2 N 1/2No and , t1/2 0.693/k or
• E.g.1 The half-life of Cobalt-60 is 5.26 years
  how much of the original amount would be left
  after 21.04 years?
• E.g.2 Tritium decays by beta emission with a
  half-life of 12.3 years. How much of the
  original amount would be left after 30 years?
• E.g.3 If a 1.0 g sample of tritium is stored
  for 5.0 years, what mass of that isotope remains?
  k 0.563/year.

12
RATE OF DISINTEGRATION2

• Dating ancient objects Carbon-14 is generated
  naturally from cosmic rays.
  .
• is unstable with a half-life of 5730 yr.
• Rate of disintegration measured and is
  proportional to the concentration of 14C
• E.g. Charcoal from a tree killed by the volcanic
  eruption that formed the crater in Crater Lake
  (in Oregon) gave 7.0 disintegrations of 14C
  min.?1g?1 of total carbon. Present-day carbon
  (in living matter) gives 15.3 disintegrations
  min.?1g?1 of total carbon. Determine the date of
  the volcanic eruption.

13
RADIATION DETECTION

• Geiger counters detect charged particles produced
  from interaction of gas with particles emitted
  from radioactive material.
• Scintillation counters detect particles from
  radioactive material by measuring intensity of
  light when these particles hit phosphor.
• Units 1 curie (Ci) 3.7x1010 disintigrationss-1

14
Energy Changes During Nuclear Reactions

• Most nuclear reactions give off a large amount of
  energy.
• The energy required to break an nucleus its
  individual protons and neutrons is called the
  binding energy, Eb.
• The total mass changes upon combination of
  protons and neutrons.
• E.g. determine the mass change during the
  formation of Helium nuclei.
• Measured mass of He nuclei (excluding electrons)
  4.00150 amu (?m 0.03038 g/mol called the
  mass defect).
• Energy change calculated from the mass change
  (decrease) using the Einstein equation ?E
  ?mc2.
• E.g. determine the binding energy for 1 mol He.
• E.g. determine the mass change during the
  combustion of butane ?2878 kJ/mol

15
Binding Energies

• 56Fe has highest Eb and is most stable isotope.
• Energy sources
• Fission for large radioactive elements, such as
  U-235
• Fusion for two deuterium producing He. Not yet
  accomplished.