Nuclear Chemistry

Atomic Nucleus  Very small  Very dense x g/cm Ping-Pong ball of nuclear matter = 2.5 billion tons of mass  Large magnitude of energy in nucleus  Composed of neutrons and protons  Neutrons & protons made of quarks

Nuclear Vocabulary Nucleons: particles in the nucleus: – p + : proton – n 0 : neutron. Mass number (A): the number of p + + n 0. Atomic number (Z): the number of p +. Isotopes: same number of p + ; different numbers of n 0. Nuclide: unique atom: A Z X

Why Are Some Nuclei Unstable? Proton has high mass and high charge. Proton-proton repulsion is large. In nucleus protons are very close to each other. Strong nuclear forces: cohesive forces in nucleus. Neutrons involved with strong nuclear force. As more protons are added the proton-proton repulsion gets larger.

Neutron-to-Proton Ratio The heavier the nucleus, the more neutrons are required for stability. The belt of stability deviates from a 1:1 neutron to proton ratio for high atomic mass.

Radioactive Decay  Many nuclei are unstable (out of 2000 nuclides, 279 are stable).  Light nuclides stable when n  : p + = 1  More stability with even numbers of n  and p +  Super stable: magic numbers 2, 8, 20, 28, 50, 82, 126  Nuclides with 84 or more protons are unstable. Radioactivity: decomposition of a nucleus to form another nucleus plus particle(s). In nuclear equations, number of nucleons is conserved:  U  Th He

Packet Example Page 7  Which of the following nuclei are especially stable: helium-4, calcium-40, or technetium-98?  helium-4 and calcium-40, because 1:1 proton:neutron ratio and even numbers  technetium-98, NO! Few nuclei are stable with odd numbers of protons and neutrons, plus above Z = 84

Three Types of Radioactive Decay Three types: –  -Radiation: loss of 4 2 He from nucleus –  -Radiation: loss of an e - from nucleus –  -Radiation: loss of high-energy photon from nucleus Write all particles with their atomic and mass numbers: 4 2 He and 4 2  represent  -radiation

Alpha-particle Production   particle is a helium nucleus: 4 2 He  Very common decay for heavy radioactive nuclides  Decaying nucleus changes sheds  particle, changes mass U  Th He

Practice Problems p 3  1. What product is formed when radium-226 undergoes alpha decay?  2. What element undergoes alpha decay to form lead-208?

Beta-radiation  Neutron splits into p + and e -  (Electron is created by release of energy)  High-speed electron is  particle   -radiation is loss of electron from nucleus and change of a neutron to a proton  Mass of decaying nucleus remains constant 1 0 n  1 1 p e -

Gamma Ray Production   -ray is high energy photon  Not a particle!  Part of EM spectrum  Accompanies some nuclear decays  Release of  -rays allows relaxation of excited nucleus to its ground state

Spontaneous Fission  Another decay process  Spontaneous fission: splitting of heavy nuclide into 2 lighter nuclides with similar mass numbers  Mass number changes as with  -decay  Occurs at very slow rate for most nuclides.

Positron Emmision  Occurs for nuclides whose neutron/proton ratio if too small  Positron: particle with mass of electron but opposite charge  Positron emission changes a proton to a neutron, causing a higher neutron/proton ratio than original atom. 1 0 p +  1 0 n e +

Positron Annihilation  Positron is the antiparticle of the electron.  Collision of electron and positron changes matter to EM gamma radiation e e +  

Electron Capture  Inner-orbital electron captured by nucleus  Proton captures electron and forms neutron  Occurs very slowly  Gamma rays are produced 1 1 p e -  1 0 n

Nuclear Stability At Bi (the belt of stability ends and all nuclei are unstable. Nuclei above belt have  - emission. Nuclei below belt have  + - emission or electron capture. Nuclei with atomic numbers greater than 83 usually have  -emission.

Packet Examples Pages 4-5  Write nuclear equations for the following processes:  a. Mercury-201 undergoes electron capture  b. Thorium-231 decays to form protactinium-231

Packet Examples Page 6  Predict the mode of decay of  carbon-14  xenon-118

Short-Hand Notation Nuclear transmutations can occur using high velocity  -particles:  14 N + 4   17 O + 1 p. The above reaction is written in short- hand notation: 14 N( ,p) 17 O.

Example Packet Page 7  Write the balanced equation for the process summarized as 27 Al(n,  ) 24 Na

Patterns of Nuclear Stability A nucleus usually undergoes more than one transition on its path to stability. Radioactive series: series of nuclear reactions that accompany path to stability. Daughter nuclei: nuclei resulting from radioactive decay.

Radioactive Series For 238 U, first decay is to 234 Th (  -decay). 234 Th undergoes  -emission to 234 Pa and 234 U. 234 U undergoes  -decay (several times) to 230 Th, 226 Ra, 222 Rn, 218 Po, and 214 Pb. 214 Pb undergoes  -emission (twice) via 214 Bi to 214 Po which undergoes  -decay to 210 Pb. The 210 Pb undergoes  - emission to 210 Bi and 210 Po which decays (  ) to the stable 206 Pb.

Nuclear Transmutations Nuclear transmutations: change of one element into another by collisions between nuclei. First observed by Rutherford in 1919 (N to O) Irene & Frederic Joliot-Curie in 1933 (Al to P) Can occur using high velocity  -particles: 14 N + 4   17 O + 1 p To overcome electrostatic forces, charged particles need to be accelerated in particle accelerators before they react.

Kinetics of Radioactive Decay  Rate law for radioactive decay is first-order.  ln(N/N 0 ) = -kt  Half-life (t ½ ): time required for the number of nuclides to reach half the original value.  Remember: t ½ = 0.693/k k = decay constant

Half-lives Each isotope has a characteristic half-life. Half-lives not affected by temperature, pressure or chemical composition. Half-lives of natural radioisotopes longer than half-lives of synthetic radioisotopes.  Half-lives range from fractions of a second to millions of years.

Units of Radioactivity  Activity: rate of decay (disintegrations per unit of time in seconds: dps)  SI unit of radioactivity = Becquerel (Bq)  1 Bq = 1 dps  Older Curie (Ci) still widely used  1 Ci = 3.7 x dps

Radioactive Dating Naturally occurring radioisotopes can be used to determine how old a sample is. Carbon-14 is used to determine ages of organic compounds. We assume the ratio of 12 C to 14 C has been constant over time. Object must be less than 50,000 years old.  Shroud of Turin dated A.D.

Examples Packet Top Page 9  The half-life of cobalt-60 is 5.3 years. How much of a mg sample of cobalt-60 is left after a 15.9 year period?  Half-life (t ½ ): time required for the number of nuclides to reach half the original value.

Packet Example Middle Page 9  Carbon-11, used in medical imaging, has a half-life of 20.4 min. The carbon-11 nuclides are formed, then incorporated into a desired compound. The resulting sample is injected into a patient and the medical image is obtained. The entire process takes five half-lives. What percentage of the original carbon-11 remains at this time? Assume you start with 1.00 g carbon-11.

Packet Example 1 Page 11  A rock contains mg of lead-206 for every milligram of uranium-238. The half-life for the decay of uranium- 238 to lead-206 is 4.5 x 10 9 yr. How old is the rock?  Remember: t ½ = 0.693/k  Remember: t = -1/k ln N/N 0

Packet Example 2 Page 11  A wooden object from an archeological site is subjected to carbon dating. The activity of the sample due to carbon-14 is measured to be 11.6 dps. The activity of a carbon sample of equal mass from fresh wood is 15.2 dps. The half-life of carbon-14 is 5715 yr. What is the age of the archeological sample?  Remember: t ½ = 0.693/k

Packet Example 3 Page 11  A sample to be used for medical imaging is labeled with fluorine-18, which has a half-life of 110 minutes. What percentage of the original sample remains after 300 minutes?

Detection of Radiation Matter is ionized by radiation. Geiger-Muller counter (Geiger counter) determines the amount of ionization of Ar by radioactive particles. Scintillation counter: measures flashes of light given off as substance is struck by radiation.

Geiger-Muller Counter

Medical Applications  Radiotracers: radioactive nuclides that can be put into organisms and whose pathways can be traced.  Sensitive and noninvasive  Iodine-131 diagnoses and treats illnesses of thyroid  Thallium-201 and technetium-99m assess damage to heart

Energy Changes in Nuclear Reactions Einstein showed that mass and energy are proportional: If a system loses mass it loses energy; if a system gains mass it gains energy. Since c 2 is a large number (8.99  m 2 /s 2 ) small changes in mass cause large changes in energy. Mass and energy changed in nuclear reactions are much greater than chemical reactions.

Energy as Matter  Energy is a form of matter!  Consider U  Th He  For 1 mol the masses are g  g g.  Change in mass during reaction is g g g = g  The process is exothermic because the system has lost mass.

Mass Defect Mass of a nucleus is less than the mass of its nucleons! Mass defect: difference in mass between nucleus and component nucleons. Binding energy: energy needed to separate a nucleus into its nucleons.  Energy change =  E = ∆mc 2

Example Packet Top Page 14  How much energy is lost or gained when a mole of cobalt-60 undergoes beta decay to form nickel-60? The mass of cobalt-60 is amu and that of nickel-60 is amu.  Use  E = ∆mc 2

Example Packet Bottom Page 14  Positron emission from carbon-11 to form boron-11 occurs with the release of 2.87 x joule per mol of carbon-11. What is the mass change per mole of carbon-11 in this nuclear reaction?  Use  E = ∆mc 2

Binding Energy Larger binding energy = more stable nucleus. Average binding energy per nucleon increases to a maximum at mass number , and decreases afterwards.  Most stable nucleus is iron-56.

Fission and Fusion  Energy is released when a process goes from less stable to more stable state.  Fusion: combining two light nuclei to form a heavier, more stable one.  Fission: splitting a heavy nucleus into two nuclei with smaller mass numbers.  Both processes energy changes millions of times larger than those from chemical reactions!

Nuclear Fission  Discovered in late 1930’s.  Fission of uranium-235 gives 26 million times more energy than combustion of CH 4.

Nuclear Chain Reactions For every 235 U fission 2.4 neutrons are produced. Each neutron produced can cause the fission of another 235 U nucleus. Number of fissions and energy increase rapidly. Eventually, a chain reaction forms. Without controls, an explosion results.

Critical Mass Each neutron can cause another fission. Minimum mass of fissionable material is required for a chain reaction (or neutrons escape before they cause another fission). When enough material is present for a chain reaction, we have critical mass. Subcritical mass: neutrons escape and no chain reaction occurs. Supercritical mass: any mass over critical

Nuclear Bomb Two subcritical wedges of 235 U separated by a gun barrel. Explosives used to bring two subcritical masses together to form one supercritical mass = nuclear explosion. Manhattan Project during WWII: Hiroshima and Nagasaki in 1945.

Nuclear Reactors  Controlled fission used to produce electricity. Use a subcritical mass of 235 U (enrich 238 U with about 3% 235 U). Enriched 235 UO 2 pellets are encased in Zr or stainless steel rods to absorb neutrons. Heat produced in the reactor core is removed cooling fluid to a steam generator. Steam drives an electric generator.

Future of Nuclear Reactors? Cons of Nuclear Reactors Pros of Nuclear Reactors  Storage of used fuel rods  Cost to build and maintain new reactors  Safety: Three-Mile Island and Chernobyl  Produces no greenhouse gases  Lower volume of waste products than fossil fuels  Almost unlimited supply of fuel  Excellent safety record

Nuclear Fusion Light nuclei can fuse to form heavier nuclei. Most reactions in the Sun are fusion. Fusion products are not usually radioactive, so fusion is a good energy source. Hydrogen needed for reaction can easily be supplied by seawater. However, high energies are needed to overcome repulsion between nuclei before reaction can occur.

Effects of Radiation  Energy transferred to cells can break chemical bonds and wreak havoc with sell systems.  Radiation damage can be subtle-feel effects years later.  Somatic damage: damage to organism = sickness of death  Genetic damage: produces malfunction in offspring

Biological Effects Depend on…  Energy of the radiation: dose measured in rads (radiation absorbed dose)  Penetrating ability of radiation:  <  <   Ionizing ability of radiation: extraction of electrons from biomolecules very damaging  Chemical property of radiation source: affects residence time in organism