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Nuclear Unit. Nuclear Chemistry Unit for Honors Chemistry By E. Garman. Review of the Atom . Atoms are composed of three major subatomic particles. Review of the Atom . Atomic number ; Z – represents the number of protons in an atom’s (element’s) nucleus - identifies the element
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Nuclear Unit Nuclear Chemistry Unit for Honors Chemistry By E. Garman
Review of the Atom Atoms are composed of three major subatomic particles
Review of the Atom Atomic number; Z – represents the number of protons in an atom’s (element’s) nucleus - identifies the element Mass number; A – represents the sum of protons and neutrons in the nucleus - indicates the number of major nucleons Nucleons -
Discovery of x-rays Wilhelm Roentgen discovered x-rays in 1895 - observed the bone structure and wedding ring of his wife’s hand when working with rays emanating from vacuum tubes
Radioactivity Henri Becquerel discovered radioactivity in 1896 - an accidental discovery while working with fluorescence and phosphorescence of potassium uranyl sulfate Fluorescence – Phosphorescence - - radioactivity – the spontaneous emission of particles and energy from unstable atomic nuclei
Radioactive elements Marie and Pierre Curie discover the elements polonium and radium as being radioactive - these elements were extracted from uranium ore Thus begins the study of nuclear chemistry or nuclear physics
Nuclear vs. Chemical Nuclear chemistry pertains to the properties, energies, and behaviors of the nucleus Chemistry pertains to the study of matter, its properties and behaviors - these are established by the electron; its energies, locations, and behaviors within the electron cloud
Isotopes Isotopes are atoms of the same element that differ in mass - these mass differences are due to differing numbers of neutrons - radioactive isotopes are referred to as radioisotopes
Radiations Radiation may be particulate (composed of particles) or all energy (electromagnetic radiation) Particulate radiations: Alpha - α – helium nuclei (helium atom w/o electrons) - has + 2 charge – 42He (equation symbol) - poor penetrating radiation - strong ionizing radiation
Radiations Beta - β- - high speed electron - charge of negative 0ne (- 1) - 0-1e (equation symbol) - fair penetrating radiation - fair ionizing radiation
Radiations Non-particulate radiations X-ray – χ–ray - all energy radiation - part of electromagnetic spectrum - good penetrating radiation - fair ionizing radiation Gamma rays - γ – all energy radiation - similar to x-rays but carrying greater energy - strong penetrating radiation - variable ionizing ability (pending upon energy)
Radiation Energies eV – electron volt – unit of energy - defined as energy required by an electron to move through a potential difference of one volt in a vacuum Volt – unit of potential difference in the “mks” system equal to the potential difference between two points for which one coulomb of electricity will do one joule of work - or, electromotive force (emf) or “push” needed to force one coulomb of charge to do one joule of work
Coulomb – unit of electrical charge equivalent to 6.241 5 x 1018 charges (p+’s or e-’s which are “elementary” charges) - equivalent to an ampere-second (A-s) 1 eV = 1.602 192 x 10-19 J
Radiation Energies Alpha – usually in 3 – 9 MeV range Beta – usually 0 – 3 MeV range Neutrons – 0 – 10 MeV range X-rays – range of 1 eV to 100 keV Gamma rays – range of 10keV to 10MeV
Additional Particles Neutron – 10n - possess no charge - result from fission reactions - strong penetrating radiation - poor ionizing radiation
Additional Particles Positron – β+ - positive electron - antiparticle of the electron (same properties; opposite charge) - short-lived – combines with electron to form positronium which decomposes forming two gamma rays - called annhiliation - 0+1e (equation symbol)
Nuclear Stability Elements are composed of isotopes Some isotopes are naturally occurring and some are man-made Atoms of specific isotopes are referred to as nuclides These nuclides have a specific mass number
Nuclear Stability Stability may be defined as the tendency of an object or system to return to its equilibrium position after it has been moved/shifted by an external force It is related to enthalpy and entropy Enthalpy basically refers to the energy content of a system Entropy refers to the state of disorder within a system
Nuclear Stability Stability is inversely related to energy and directly related to disorder Meaning, both low energy and higher disorder favor stability The converse is true
Nuclear Stability Systems and objects in the universe are driven to attain the maximum stability possible for them Unstable isotopes will be radioactive, thus they change in an attempt to become more stable
Predicting stability of radioisotopes All nuclei with 84 protons are unstable The quantity of positive charge in the nucleus is too great causing tremendous repulsive forces which result in high energies
Predicting stability of radioisotopes Isotopes having a certain number of protons or of neutrons tend to be more stable These numbers are sometimes referred to as the “magic numbers” The magic numbers are • proton: 2, 8, 20, 28, 50, 82, 114 • neutron: 2, 8, 20, 28, 50, 82, 126, 184 We find a similar situation in regarding to chemical activity and number of electrons
Predicting stability of radioisotopes Nuclei with an even number of nucleons tend to be more stable than those with an odd number
Predicting stability of radioisotopes The neutron to proton ratio is also indicative of isotope stability Note: All nuclei with two or more protons have neutrons For lighter nuclei (Z ≤ 20), a one to one ratio usually provides stability For moderately heavy nuclei, a 1.25 to 1 ratio tends to provide stability
Predicting stability of radioisotopes For heavy nuclei, a ratio of 1.5 to 1 tends to provide the needed stability These stable isotopes may be found in what is called the “belt of stability”
Nuclear Change Isotopes outside the belt of stability will change in an attempt to move towards the belt These changes often result in alteration of the element’s identity Referred to as nuclear transmutation Occurs because the number of protons in the nucleus is changed
Neutron Rich Nuclides These nuclides, being above the belt, have too many neutrons and will often undergo beta emission This results in a neutron decomposing into a proton and an electron 0n ----------> p+ + e-
Proton rich (neutron deficient) nuclides These nuclides, being below the belt, have too few neutrons and will often undergo positron emission or K – electron capture Lighter nuclei prefer positron emission while heavier nuclei prefer K-electron capture p+ ---------> 0n + e+ p+ + e- ----------> 0n
Nuclides with Z greater than 83 These nuclides will usually attempt to decrease their nuclear size in the most expedient manner These nuclides often emit alpha particle(s) in attempting to stabilize
Examples of Radioactive Decay Recognize, nuclear changes are represented by nuclear equations Nuclear equations must conserve mass and charge!!!! Beta emission 141Ce ------> 0-1e + 141Pr
Examples of Radioactive Decay Positron emission 155Dy --------> 155Tb + 0+1e K- electron capture 178W + 0-1e --------> 178Ta Alpha emission 152Ho ---------> 42He + 148Tb
Natural Decay Series There are several natural decay series that result because a radioisotope cannot achieve stability in one or two decays Some of the series are: Uranium series – 238U goes to 206Pb Actinide series – 235Ac goes to 207Pb Thorium series – 232Th goes to 208Pb
Formation of new Isotopes Isotopes may be formed by driving “bullet particles into nuclei The nuclei impacted are referred to as “targets” Bullet particles having a charge require more energy in order to overcome Coulombic repulsion (e.g. 11p (+1), 42He (+2), etc.) Energies are determined from bullet particle speed
Cyclotrons/Synchocyclotrons Cyclotrons are composed of hollow “D” shaped electrodes called “dees” The projectile particle enters the vacuum chamber and is accelerated by alternating the polarity on the dees Magnets above and below the dees keep the particle moving in a spiral path
Formation of new Isotopes Accelerators are used in order to increase bullet projectile speeds Many accelerators utilize electrical or magnetic fields Some newer accelerators use lasers
Cyclotrons/Synchocyclotrons Once the projectile obtains sufficient energy it is sent from the accelerator towards the target nucleus
Neutrons, although requiring special means for acceleration (i.e.proton packaging), make good bullet particles Often neutrons are used without any acceleration by merely selecting different sources Alpha, beta, protons, etc. while good for accelerating, are often difficult to use
