CHE 141 1st Edition Lecture 36Belt of Stability - Neutron-richo Above the belt of stability (n/p=too high)o Mass number A> atomic weight (periodic table)o Tend to convert neutrons to protons via beta decay- Neutron-pooro Below the belt of stability (n/p=too low)o Mass number A< atomic weight (periodic table)o Tend to convert protons to neutrons via positron emission or electron capture- Bismuth is the heaviest element with stable (nonradioactive) isotopes- Atoms with Z>83 are all radioactive and tend to undergo alpha emission which decreases both the number of neutrons and the number of protons by two, movingthe nucleus diagonally toward the belt of stability- These decays occur in one or more steps involving primarily alpha and beta decayDetecting Radioactivity - Common products of radioactivity have enough energy to tear chemical bonds apart, producing odd electron free radicals or free electrons and cationsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Ionizing radiation high energy products of radioactive decay that can ionize molecules- Ionizing radiation is undetectable by the human senses- Devices are needed to detect and measure radioactivity, so we can avoid exposure above safe levels- A radiation detector detects particles through their interactions with atoms or molecules- Geiger Muller counter a portable device which determines nuclear radiation levelsby measuring how much the radiation ionizes the gas in a sealed detector- Radioactive decay particle passes through window into Ar (g) filled chamber- Particle collides with Ar(g) particle which ionizes to yield Ar+(g) and free electrons- Electrical potential difference applied between positively charged cylinder shell and negatively charged central electrode- The SI unit for radioactivity is Becquerel (Bq) which is one decay per second- Curie (Ci) is also sometimes used to measure radioactivity- Both these form of measurement depend on the rate at which a radioactive substance decays and on how much of it is in the sampleKinetics of Decay- All radioactive nuclei decay via first order kinetics - Rate of decay is directly proportional to the number of nuclei present Rate=k/N- K=rate constant N=number of radioactive nuclei- Some nuclides decay quickly and others decay slowly- Every radioactive decay process has a characteristic half life- Number of half lives n can be calculated using Nt/N0=0.5^n- Half life: the time interval during which the quantity of radioactive particles decreases by one-halfRadiometric Dating- Radiometric dating is a method for determining the age of an object based on the quantity of a radioactive nuclide the object contains- Radiocarbon dating uses the activity of carbon 14 remaining in the objectNuclear Fission- Nuclear fission: a nuclear reaction in which the nucleus of an element splits into two lighter nuclei. The process is usually accompanied by the release of one or more neutrons and energy splitting the atom- Chain reaction: a self-sustaining series of fission reactions in which the neutrons released when the nuclei split apart initiate additional fission events and sustain the reaction- Critical mass the minimum quantity of fissionable material needed to assure that every fission event produces another, thus sustaining a chain reaction- Nuclear fission produces large amounts of energy E=mc^2Mass Defect and Nuclear Binding Energy- A stable nucleus has less mass than the sum of the masses of its separate components- Mass defect the difference in mass between a stable nucleus and the individual nucleons it is made up of- Nuclear binding energy the amount of energy required to break apart the nucleus into its component nucleons BE=delta mc^2- Binding energy per nucleon=BE divided by number of nucleons in
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