Nuclear Chemistry Chapter 15. Types of Radiation There are four main types of ionizing radiation:...

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Transcript of Nuclear Chemistry Chapter 15. Types of Radiation There are four main types of ionizing radiation:...

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Nuclear Chemistry Chapter 15 Slide 2 Types of Radiation There are four main types of ionizing radiation: 1.alpha rays: Helium nuclei - 2 protons + 2 neutrons 2.positron rays: Positrons - the antimatter counterparts to electrons; same mass but charge is +1 3.beta rays: Electrons 4.gamma rays: High energy photons - 10 20 Hz - higher energy than x- rays radiationsymbolcharge mass (per particle) penetrating power alpha+26.65 10 -24 glowest positron+19.11 10 -28 gmedium beta9.11 10 -28 gmedium gamma00highest Slide 3 Nuclear Reactions o Chemical reactions never alter the identity of the atoms involved oIn nuclear reactions, the total numbers of nucleons protons, neutrons and electrons remains constant, but chemical identity can change when the particles interchange oNuclear reactions permit the transmutation of elements Balancing Nuclear Reactions (1) The sum of the mass numbers of reacting nuclei must equal the sum of the mass numbers of the nuclei produced (conservation of mass). (2) To maintain charge balance, the sum of the atomic numbers of the products must equal the sum of the reactants Example of Balancing a Nuclear Reaction Mass number (protons+neutrons)226 4 +222 Atomic number (protons only)88 2 +86 Slide 4 7 Classes of Nuclear Reaction: 1 - 2 1.Alpha emission In this spontaneous reaction an alpha particle is emitted. 2.Beta emission In this spontaneous reaction an beta particle (electron) is emitted. This implies that the nucleons are transformed during the process, where one of the neutrons is converted into a proton and a high-energy electron is ejected: Note that beta emission increases the atomic number by one. Slide 5 7 Classes of Nuclear Reaction: 3 - 4 3.Positron emission This is the opposite of a beta emission where a new particle, called the positron is emitted. Essentially, a proton decomposes to a neutron and a positron which is emitted by the nucleus. This transformation, at the nucleon level, implies the reaction occurs within the nucleus: Positrons and electrons are antiparticles, meaning that If the two were to meet, they annihilate each other releasing two high-energy gamma ray photons. This reaction decreases the atomic number by 1. 4.Electron (K) capture An electron from the atom is incorporated into the nucleus. The electron is from the 1 s level also known by the older name of K -level. This spontaneous process is the opposite of beta emission, where a proton takes on an electron to form a neutron This reaction gives same net product as positron emission, decreasing the atomic number by 1. Slide 6 7 Classes of Nuclear Reaction: 5 5.Nuclear fission Nuclear fission is always an induced process, where a large nucleus splits into several smaller nuclei. The actual fission process below is the decay of uranium-236. 236 U is made by neutron bombardment of 235 U. : Fission is accompanied by the release of a lot of energy, and this is the source of the power both of the atomic bomb and nuclear power Slide 7 7 Classes of Nuclear Reaction: 6 - 7 6.Nuclear fusion Fusion is the combining of two light elements into a heavier nucleus. A lot of energy is released. ( hydrogen bomb - where the hydrogen refers to the heavier hydrogen isotopes deuterium and tritium). 7.Nuclear transmutation The combination of heavier nuclei to produce artificial elements. All the elements beyond uranium are artificial, in the sense that none have stable isotopes. This classification includes all the remaining artificial, i.e. induced, nuclear reactions Slide 8 Practice Question 1 oComplete or balance the following nuclear reaction equations by identifying the missing components. Classify each reaction according to one of the 7 classes of reaction introduced. a) b) c) d) e) f) g) emission Nuclear fusion Positron emission Nuclear fission Electron capture Nuclear transmutation Induced emission, nuclear transmutation Example Problem Slide 9 Gamma rays (and neutrinos) oGamma rays accompany most nuclear reactions. oWhen a nucleus emits an alpha particle, the resulting nucleus is usually in an unstable, higher energy form - Excited nuclear state oIt quickly decays to the nuclear ground state by releasing the excess energy as a gamma ray. oThe most dangerous part of a nuclear process, as gamma rays have high penetrating power. oThe energy of the gamma rays varies greatly, but is always characteristic of the type of nuclear transformation. oG amma spectroscopy analyzes the wavelengths of the emitted gamma rays, which can be used to identify the source of the nuclear reaction. oHence it is possible to detect the kind of radioactive process occurring when radioactivity is detected Slide 10 Why are some nuclei stable, others not? oBlack dots stable nuclides oRed dots - unstable nuclides oPink region - all combinations of N (number of neutrons) and Z (number of protons) that cannot exist oZone of black dots - belt of stability - neutrons are needed to stabilize protons (+ve repulsion) oAt low Z, N Z; at high Z, N > Z oUnstable nuclides surround the belt of stability oZ > N - Nuclides with more protons allowed for stability, attain it, by changing the ratio of N/Z by: Heavy nuclei - alpha () emission Light nuclei- positron emission or electron capture oN > Z -Nuclides with more neutrons than protons convert neutrons into protons This is achieved by beta ( ) emission Slide 11 Binding energy Region of greatest stability Fusion zone Susceptible to fission reactions Binding energy is the energy change that occurs if a nucleus were formed directly from its component protons and neutrons. The source of the energy is a loss of mass All nuclei strive to maximize stability via nuclear reactions Slide 12 Slide 13 Calculating binding energy oWe use the tabulated masses of free protons and neutrons, and compare them to that of any given nuclide oThe difference in mass is converted to energy using the Einstein equation from special relativity The mass of one mole of free protons is 1.0072765 g/mol The mass of one mole of free neutrons is 1.0086649 g/mol The mass of one mole of free electrons is 0.0005486 g/mol Calculate the binding energy of the 16 O isotope with mass of 15.994916 g/mol, and the binding energy per nucleon. Note that the isotope mass includes the electrons protonneutronisotopeelectron Slide 14 Range of binding energies oIn the sample calculation on the previous slide, we calculated the binding energy of the 16 O nuclide and also the binding energy per nucleon oA nucleon is either a proton or a neutron oIt is found that the binding energies of all the known nuclides falls in the range from: ~ 0 9.0 10 8 kJ/mol of nucleon = 0 900 GJ/mol of nucleon oThe most stable nuclide is 56 Fe oSometime values are given in MeV, which is a measurement per individual nucleon oChemists most often work out energies in units of kJ for mole quantities of matter, so that we will always work out total and per-nucleon binding energies in kJ/mol oHowever, the MeV unit is widely used in the nuclear industry. The conversion factor is: 1 eV = 96.485342 kJ/mol1 MeV = 9.6485342 10 7 kJ/mol 10 8 kJ/mol = 100 GJ/mol oThe range of binding energies per nucleon in MeV is therefore: ~ 0 9.0 MeV Slide 15 Nuclear decay is measured as the number of nuclei that disintegrate in a given period of time. This is termed the activity (A) of a sample and is directly proportional to the number of radioactive atoms (N) in the sample: where k is the rate constant (decay constant). By integrating this equation, we get a first-order rate equation: where A 0 and N 0 are the initial activity and number of radioactive atoms while A and N are the activity and number of radioactive atoms after time t. At t 1/2, N = N 0 and A = A o. A relationship between k and t 1/2 can be found as: Rates of Nuclear Decay Slide 16 Example 1 Radioactive Decay Slide 17 Example 2 Radiocarbon Dating Slide 18 Nuclear Decay The Uranium Series All isotopes of uranium are unstable, but 238 U decays extremely slowly. This is the 238 U decay series, showing the steps in the decay of this nucleus to eventually produce stable 206 Pb In this region there are several pathways, but all lead to the same species, 206 Pb Slide 19 Nucleosynthesis by transmutation oAll the elements beyond 92 U are synthetic. They are made either as a by product of operating a nuclear reaction (neutron source) or by deliberate transmutation oLatest to be recognized: It was made by a nucleosynthesis reaction: oRoentgenium is the most recent of the transuranium elements which start with 93 Np, complete the actinides and most of the 6 d series of the PT oEarly efforts at transmutation date back all the way to Rutherford. For example, he performed the reaction: oAlpha-particles: high repulsive forces experienced when the 2+ alpha nucleus comes near a strongly positive charged nucleus oNeutron: uncharged particle reaches target much easier, and efforts at transmutation switched to neutron bombardment oThe discovery of fission was a result of such studies: neutron bombardment of Slide 20 Periodic Table with Element Names (using the 1-18 group nomenclature) A new element is recognized (IUPAC Bureau decision) Slide 21 oSuccessful neutron bombardment: an example is the manufacture of plutonium, which is a two-stage process: oThis is the process that is used in breeder reactors to convert unusable 238 U into fissionable 239 Pu oThis method works well up to element 101, Mendelevium, but beyond this larger bullets must be used, as for example in: oSuch reactions suffer from very strong nuclei-nuclei repulsion, and require great effort and very specialized apparatus oMany transuranium elements from the Actinides have been made in sufficient quantity to have some of their basic chemical properties investigated oThe super-heavy elements of the 6 d block are usually only made a few atoms, or events at a time and nothing is known of their chemistry. Roentgenium is he