Nuclear Chemistry

Some Definitions Isotope atoms with the same atomic number but different atomic masses Atoms with an equal number of protons but an unequal number of neutrons Nucleon a particle found in the nucleus of an atom A proton or a neutron Nuclide the nucleus of a specific isotope of an element Each nuclide has a specific number of both protons and neutrons. Any change in the quantity of either changes the identity of the nuclide.

Chemical vs. Nuclear Reactions Chemical Reactions Nuclear Reactions Reactions involve the interaction of valence electrons Regardless of what reaction they undergo, the atomic species NEVER change Rates of reaction are affected by chemical and physical change The energy released or absorbed is orders of magnitude less than that of nuclear reactions Reactions involve the nucleons and inner shell electrons Regardless of what reaction they undergo, the identities of the nuclides ALWAYS change. Rates of reaction are almost completely independent of chemical and physical change The energy released or absorbed is orders of magnitude greater than that of chemical reactions

Atomic Notation Atomic Mass Atomic Species   Atomic Mass Atomic Species Atomic Number: Number of protons in the nucleus Atomic Mass: Number of nucleons in the nucleus and their approximate mass. Atomic Species: Name of the element, dependent on atomic number Atomic Number

Review of the Nucleus The nucleus is made of tightly bound nucleons Protons carry a positive charge and a mass ≈1.0073amu Neutrons carry no charge and a mass ≈1.0087amu Positively charged protons are bound together by the Strong Nuclear Force Strong nuclear force is very dependent on neutrons

Stable Nuclides Belt of Stability Protons repel each other because of like charges. This is known as the Coulomb force. The greater the number of protons in the nucleus, the greater the proportional repulsive force Result: The greater the number of protons, the greater the proportion of neutrons needed becomes Data points represent stable isotopes with natural abundance greater than 1%

Radioactive Nuclides Nuclides exist which have unstable proton to neutron ratios These unstable nuclides are called radionuclides Radionuclides will under go a nuclear reaction or nuclear decay in which they change identity to a more stable nuclide Many methods exist for a radionuclide to stabilize and each emits a characteristic particle or particles accompanied by massive amounts of energy

Gamma Ray Radiation In a nuclear reaction, the product nuclides can be emitted in a ground state or in one or more excited states A product nuclide in an excited state will transition to its ground state (or another excited state of lower energy) by emitting gamma rays Gamma rays are high energy photons represented as

U Th He U Th α Alpha Particle Decay + + A type of radioactivity in which a radionuclide emits a helium nucleus Most common natural mode of decay for heavy, neutron deficient radionuclides U 238 92  Th 234 90 He 4 2 + OR U 238 92  Th 234 90 α 4 2 +

Alpha Particle Decay http://www.cyberphysics.pwp.blueyonder.co.uk

Negatron Beta Particle Decay A type of radioactivity in which a neutron turns into a proton through the emission of a negatron (an electron) and an anti-neutrino Most common natural mode of decay for a neutron rich radionuclide I 131 53 Xe 54  + e −1 OR I 131 53 Xe 54  +  −1

Negatron Beta Particle Decay http://outreach.atnf.csiro.au

Positron Beta Particle Decay A type of radioactivity in which a proton turns into a neutron through the emission of a positron and a neutrino A positron is an atomic particle with equal mass but opposite charge of an electron C 11 6  B 5 + e 1 OR  C 11 6  B 5 + 1

Electron Capture A type of radioactivity in which a proton absorbs one of the inner shell electrons and turns into a neutron Most common natural mode of decay for light, neutron deficient radionuclides Has a very similar effect as positron decay, but is more common because it is more energetically advantageous C 11 6  B 5 + e -1

Radioactivity and Matter http://hyperphysics.phy-astr.gsu.edu http://www.arpansa.gov.au

Ionizing Radiation Defined as radiation of high enough energy to ionize water (E > 1216 kJ/mol) Alpha, beta, and gamma rays possess energies greater than 1216 kJ/mol The ionized water molecules will react with surrounding water molecules The product OH is a free radical, possessing an unpaired electron.

Free Radicals in the Body Living tissue, in general, is at least 70% water The rest of the tissue is made up of biomolecules responsible for the structure, proper function and reproduction of cells Biomolecules carry out these functions through incredibly complex chemical reactions Living tissue is essentially a bunch of chemical reactions being carried out in solution Radiation of high enough energy can ionize the water in living tissue, creating the free radical OH One free radical in solution with other molecules (living tissue) can quickly create more free radicals Free radicals ionize biomolecules, affecting their ability to function properly

Radiation, DNA, and Cancer Tissue is made of cells Cells reproduce themselves using the “instructions” embedded in the sequence of nucleotides, or molecular structure, of DNA The free radicals created by radiation affect DNA like any other molecule, changing its molecular structure Altered DNA can result in abnormal cell growth or cancer Tissues made of cells which reproduce more quickly, like those involved in blood cell production, are more susceptible to abnormal cell growth and cancer

Radioactivity as a Reaction Rate Levels of radioactivity are often expressed as the rate at which radionuclides undergo decay Activity=λN N is the number of radionuclides available to decay λ is the decay constant or the probability that a given radionuclide will decay in a given time interval

Decay Constant The decay constant is based on the probability that a given radionuclide will decay in a given way The decay constant is analogous to the rate constant in chemical kinetics The decay constant differs in that it is virtually unaffected by changes in physical and chemical properties Physical and chemical properties of matter are determined by the interaction of valence electrons No interaction of valence electrons can change the make up of a nucleus The make up of a nucleus determines the energetic advantage of a particular mode of decay, thus determining the probability that the nuclide will decay in that manner

U X α U Th α Calculating Activity + + Determine level of alpha radioactivity released by 2 moles of uranium-235 (λ= ) U 238 92  X A Z α 4 2 + U 238 92  Th 234 90 α 4 2 + Activity=2( )( ) Activity= disintegrations/year

Decay Constants and Half-Lives When discussing and comparing the radioactivity of nuclides, it is often helpful to use half-lives instead of the decay constant By integrating time into the rate equation, we obtain From this equation, we can determine the relationship between the decay constant and the half-life

K X  K Ca  Calculating Activity + + Determine level of negatron beta radioactivity released by 3.75 moles of (half-life= ) K 40 19 X A Z  +  −1 K 40 19 Ca 20  +  −1 Activity= 3.75 ( )( ) Activity= disintegrations per year

Binding Energy and Mass Defect Mass defect is the difference in the mass of a nuclide and that of the total mass of its nucleons An alpha particle is 0.03188amu lighter than the mass of two neutrons and two protons Binding energy is the energy required to separate a nucleus into its individual nucleons Because energy is added, the mass must increase. This is given by the equation

Enthalpy in Nuclear Reactions The enthalpy of a nuclear reaction is determined by the change in nuclear mass from reactants to products c is the speed of light in a vacuum (c ) If ∆m is given in kilograms and the speed of light is used, ∆E will be in joules If ∆m is given in atomic mass units and is replaced with the conversion factor 931.501 MeV/amu, then ∆E will be in MeV. 1MeV=

Atomic Mass vs. Nuclear Mass The nuclear mass of any isotope is equal to its atomic mass minus the total mass of its electrons Calculate the nuclear mass of given that its atomic mass is 231.03630amu and the mass of one electron is amu.

Calculating Enthalpy Calculate ∆E for the alpha decay of 1 mole = 234.99345 g/mol = 230.98693 g/mol = 4.00150 g/mol

Binding Energy and Enthalpy Fission Fusion http://en.wikipedia.org/wiki/File:Binding_energy_curve_-_common_isotopes.svg

Fission The decay of a larger nucleus into two or more smaller nuclei Usually initiated by a slow moving neutron Method of decay for nuclides of mass greater than 100amu Releases more energy than other decay modes for large nuclei Can happen spontaneously, but is uncommon Even though fission is more exothermic than other decay modes, it is less common because of the high energy barrier

Calculating Enthalpy - Fission Calculate the energy released by 1 mole when it fissions via the following reaction = 234.99345 g/mol = 96.88901 g/mol = 136.89677 g/mol = 1.0866 g/mol

Fusion The nuclear collision and rearrangement of light nuclei into heavier nuclei Releases more energy than even fission because of the relative difference in mass (binding energy) between its products and reactants For the reaction ∆m= -0.01889g/mol which means Fusion reactions release more energy gram for gram than any other known fuel source Responsible for the heat produced in the sun, which is a self- sustaining fusion reaction Has an even higher energy barrier than fission

Fission Bomb “Nuclear Weapon” Chain Reaction – A reaction which is initiated by the products of a previous reaction. Recall Each fission of a U-235 nucleus produces two neutrons which can then fission two more nuclides Critical Mass – The mass of fission material required to maintain a constant rate of fission Supercritical mass – Any mass greater than critical. Causes fission reaction rate to increase exponentially, which can cause an explosion http://en.wikipedia.org/wiki/ File:Fission_bomb_assembly_methods.svg

Fusion Bomb “Thermonuclear Weapon” http://en.wikipedia.org/wiki/ File:Teller-Ulam_device_3D.svg

Nuclear Power - Fission Fission fuel is allowed to react in the core Control rods are neutron acceptors that keep the reaction at the desired rate Heat is transferred first to a liquid coolant and then to water which produces high pressure steam From this point, power generation is the same as with other fuels McGraw-Hill www.accessscience.com Drawbacks Possible leakage of radioactive fuel or products Long half-lives of products cause disposal/storage challenges

Nuclear Power - Fusion The Advantages Light nuclei are available in a virtually inexhaustible supply Product nuclei in fusion reactions are rarely radioactive The Limitations Lack the technology to heat fusion fuel to a high enough temperature (40,000,000 K) Lack the material technology to contain a fusion reaction McGraw-Hill www.accessscience.com