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CHM 120 CHAPTER 24 Nuclear Chemistry

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From the half-life equation, we can write. FISSION AND FUSION ... useful because of its short half-life :- used in bone scans :- heart imaging ... – PowerPoint PPT presentation

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Title: CHM 120 CHAPTER 24 Nuclear Chemistry


1
CHM 120CHAPTER 24 Nuclear Chemistry
  • Dr. Floyd Beckford
  • Lyon College

2
  • Chemical properties are determined by the
  • electronic distribution
  • - they are only indirectly affected by the
  • nucleus
  • Nuclear reactions involve changes in the
  • composition of the nucleus

3
NUCLEAR REACTIONS
  • Nuclear reactions involve a change in
  • the nucleus of the atom
  • Chemical reactions involve only a change
  • in the distribution of the valence electrons
  • 2.Isotopes have similar chemical properties
  • Isotopes have very different nuclear
  • properties

4
  • 3. Reaction rates of nuclear reactions are
  • not affected external factors
  • Ordinary chemical reactions are affected
  • 4. Large amounts of energy are released
  • or absorbed in a nuclear reaction
  • - millions of kJ are involved
  • Chemical reactions involve relatively small
  • amounts of energy

5
  • Recall that an atom can be represented by
  • its nuclide symbol

A mass number Z atomic number
  • Some nuclides spontaneously emit radiation
  • They are said to be radioactive and are
  • called radionuclides
  • The process of emitting radiation is referred
  • to as radioactivity

6
NUCLEAR STABILITY
  • Most naturally occurring nuclides have even
  • numbers of protons and even numbers of
  • neutrons
  • In nuclear chemistry stable implies that
  • the half-life of the isotope can be measured
  • Radionuclides whose half-life cannot be
  • measured are considered unstable

7
  • The reason for radioactivity has to do with
  • the ratio of neutrons to protons in the nucleus
  • A plot of of neutrons vs. of protons
  • shows a band of stability
  • For low atomic numbers most stable nuclides
  • have equal numbers of protons and neutrons
  • Above Z 20, nuclides have more n than p
  • All isotopes heavier than Bi-209 are
  • radioactive

8
ENERGY CHANGES
  • It is observed that the mass of an atom is
  • always less than the sum of the masses of its
  • constituent particles
  • This difference is called the mass defect,
  • ?m
  • Reflects the nuclear binding energy, BE
  • - energy holding the nucleons together

9
  • From Einsteins equation
  • BE (?m)c2 c speed of light
  • Binding energy have different units
  • 1 MeV 1.60 x 10-13 J 1J 1 kgm2/s2
  • Also expressed as MeV/nucleon
  • A higher binding energy per nucleon
  • corresponds to greater stability

10
RADIOACTIVE DECAY
  • Nuclei whose neutron-to-proton ratio lie
  • outside the stable region undergo spontaneous
  • radioactive decay by emitting one or more
  • particles and/or electromagnetic radiation
  • Type of decay depends on whether the
  • nuclide is above or below the band of stability
  • Three common modes alpha, beta and gamma

11
  • Neutron-rich nuclei above the band
  • Too high a n/p ratio they undergo decays
  • that decrease the ratio
  • The common decay mode is beta emission
  • A beta particle ?- is an electron ejected
  • from the nucleus when a neutron is converted
  • into a proton

12
  • Beta emission results in a simultaneous
  • increase in the number of protons and
  • decrease in the number of neutrons
  • Notice that the equations are balanced
  • A, Z (products) A, Z (reactants)
  • This is true for all nuclear reactions

13
  • Neutron-poor nuclei below the band
  • Most of these nuclei increase their n/p ratio
  • by undergoing alpha emission
  • Alpha particles are helium nuclei has two p
  • and two n

14
  • Two other less common types of decay for
  • these nuclei are positron emission and
  • electron capture
  • A positron is essentially a positive electron
  • - positrons are emitted when a proton
  • is converted into a neutron

15
  • Positron emission results in a decrease in
  • the atomic number and an increase in the
  • number of neutrons
  • - there is no change in the mass number
  • Same effect is accomplished by electron
  • capture
  • - an electron is captured from the n 1
  • shell

16
RADIOACTIVE DECAY RATES
  • Radionuclides have different stabilities and
  • decay at different rates
  • Radioactive decay is a first order process
  • - the rate is proportional to the number of
  • radioactive nuclei in the sample, N
  • Decay rate (A) kN
  • k decay constant

17
  • Recall that for a first-order reaction, the
  • integrated rate law is
  • From the half-life equation, we can write

18
FISSION AND FUSION
  • Fission a heavy nuclide splits into lighter
  • nuclei
  • Fusion the combination of light nuclei to
  • form a heavier nuclei
  • Nuclear fission is usually induced
  • - the heavy nuclide is struck with
  • neutrons to cause it to split

19
  • The production of the neutrons can set up a
  • nuclear chain reaction

20
  • Nuclear reactors
  • Controlled fission reactions generate
  • electricity
  • Key to nuclear reactors is the control rods
  • - absorb and therefore regulate the flow of
  • neutrons
  • Fission energy ? heats the coolant ?
  • produces steam ? drives a turbine ?
  • produces electricity

21
  • Fuel for the reactor is usually UO2 enriched
  • with 3 235U
  • A moderator is also used
  • - to slow the neutrons
  • - water (or sometimes graphite)
  • Fusion produces more energy than fission
  • The biggest fusion reactor? the sun
  • - fusion of hydrogen to form helium

22
APPLICATIONS
  • Radioactive dating
  • The ages of articles of organic origin can
  • be estimated by radiocarbon dating
  • Method is based on the slow and continuous
  • production of 14C in the upper atmosphere

14C O2 ? 14CO2
23
  • This 14CO2 is removed from the atmosphere
  • by synthesis
  • Since 14CO2 is continuously produced, there
  • exist an equilibrium between the atmosphere
  • and a living organism
  • After death the concentration of 14CO2 will
  • decrease with time

24
  • Measuring the 14C activity of a sample
  • allows for determination of the age of the
  • organism
  • Method accurate for relatively young
  • species (1000 20 000 years)
  • The potassium-argon and uranium-lead
  • methods are used for dating older objects

25
  • Medical uses
  • In vivo uses used to assess the
  • functioning of an organ or system
  • 2. Therapeutic uses a radionuclide is used
  • to kill diseased tissue
  • e.g. 60Co used to kill tumors
  • 3. Imaging uses diagnostic information
  • e.g. 131I used to study thyroid and brain
  • function

26
  • Metastable technetium-99, 99Tc-m is very
  • useful because of its short half-life
  • - used in bone scans
  • - heart imaging
  • The usefulness of 60Co and other
  • radionuclides for killing tissue is related to
  • emission of gamma rays, ?

27
  • A ? ray is a stream of high-energy photons
  • They have very short wavelength and no
  • mass
  • Always accompanies ? and ? emission
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