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DESCRIPTIONNuclear Chemistry. Mr. Montero Chemistry Dr. Michael M. Krop High School. Atomic Symbols. Mass Number ( A ) (p + + n 0 ). Atomic Symbol. Atomic number ( Z ) (number of p + ). Isotopes of Uranium. Uranium - 238. Uranium - 235. Uranium - 233. n 0 = 146. n 0 = 143. n 0 = 141. - PowerPoint PPT Presentation
Nuclear ChemistryMr. MonteroChemistryDr. Michael M. Krop High School
Atomic SymbolsAtomic SymbolAtomic number (Z)(number of p+)Mass Number (A)(p+ + n0)
Isotopes of UraniumWhat is the difference between these three isotopes?The number of neutrons!n0 = 146n0 = 143n0 = 141Uranium - 238Uranium - 235Uranium - 233
The spontaneous disintegration of an unstable atomic nucleus with accompanying emission of radiation.
RadioactivityOnly unstable nuclei decay. The majority of atoms in nature do not decay
Radioactive nuclei are called Radionuclei. Atoms containing these nuclei are called Radioisotopes
The stability of the nucleus depends on number of protons and neutrons
Radioactive Emissions , , Alpha particles () are the nucleus of Helium (2 protons, 2 neutrons) +2 charge
Beta particles () are high-speed electrons. -1 charge.
Gamma Rays () are high energy radiation, not particles. Zero Charge
Separation of , , emissionsMOVIE
Penetrating ability of , , radiation
Alpha () EmissionsHelium Nucleus (2 p+ and 2 n0)Notation: or
Uranium-238 Alpha decayAfter alpha decay Uranium changed to Thorium
Alpha () EmissionsWhat product is formed when radium-226 undergoes alpha decay?2 protons are lost and 2 neutrons so mass number goes down by 4 and atomic number goes down by 2
Beta () EmissionsHigh Speed electrons (1 e-)Notation: or Iodine-131 Beta decayAfter beta decay Iodine changed to Xenon
Beta () EmissionsWhat product is formed when Thorium-231 undergoes beta decay?No protons are lost and 1 neutron turns into a proton. Atomic number goes up by one while mass number stays constant
Beta () EmissionsIf there are no electrons in the nucleus, where do beta particles come from?
Why does the atomic number of an atom increases by one when there is beta decay?
Beta () EmissionsThe reason is unknown. Possible theories involve quarks or particles which supposedly compose protons and neutronsHowever, it is known that a neutron disappears and somehow turns into a proton
Positron EmissionsSame mass as electrons but opposite chargeNotation: Carbon-11 Positron decayAfter positron decay carbon changed to boron
Positron EmissionsWhat product is formed when oxygen-15 undergoes positron emission?No neutrons are lost and 1 proton turns into a neutron. Atomic number goes down by one while mass number stays constant
Gamma Radiation ()High energy photons
Very short wavelength
Always accompanies other radioactive emission
Represents the energy lost when the remaining nucleons reorganize
Why does radioactivity happen?If protons repel each other, how can they stick together in the nucleus?What forces are present in the nucleus?STRONG NUCLEAR FORCES
Strong nuclear forcesWhen nucleons get close enough a very strong force binds them together overcoming electrostatic repulsion
Neutrons are not repelled so they help to cement protons together
As soon as protons are slightly apart repulsion overcomes strong nuclear force.
Strong Nuclear ForcesAll nucleons attract one another by Strong Nuclear forces.Only protons repel one another
Patterns of Nuclear Stability As Z increases the number of neutrons must increase to guarantee stability
All nuclei with 84 or more protons are radioactive and tend to undergo alpha decay
Radioactive Series of UraniumUranium-238 does not become stable with only one decay
This nuclei undergo a combination of alpha and beta decay and finally reaches stability as lead (Pb)
Nuclear TransmutationNuclear reactions which are induced (not spontaneous.)
Induced by a high speed nucleon striking the nucleus
Example: An alpha particle strikes a nitrogen nucleus to synthesize Oxygen-17
Particle AcceleratorsCharged particles have to be accelerated in order to overcome the repulsive forces in the nucleus.Particle accelerator at Fermi National Lab.The circumference is 6.3 km.
Using NeutronsNeutrons are not charged, therefore they do not need to be accelerated.
Cobalt-60 is an important isotope used in radiation therapy for cancer. It is produced from iron-58 which is bombarded by neutrons
Transuranium ElementsMan-made elements heavier than Uranium.
Many of these only exist for brief moments and then decay rapidly
In 1940 elements 93 (Neptunium) and 94 (Plutonium) were discovered by bombarding Uranium with neutrons
Rates of Radioactive DecayDifferent nuclei undergo radioactive decay at different rates
Some decay very rapidly (seconds) others decay very slowly (years)
Radioactive decay is a first-order process
First Order ProcessThe rate of decay is proportional to the number of radioactive nuclei (N)
Rate = kN
Where k is the decay constant The more nuclei you have the faster it decays and vice versa
Half-lifeTime it takes for half of the nuclei to decay.
Half-life is always constant
First Order process and half-lifeMOVIE
Half-life problemThe half-life of cobalt-60 is 5.3 yr. How much of a 1000-mg sample of cobalt-60 is left after a 15.9-yr period?
Every half life the initial amount is reduced by halfHow many half-lives is 15.9 yrs?
Half-life problem contAfter three half-lives how much cobalt-60 willthere be if we started with 1000 mg?So after 3 half-lives there will be 125 mg of 60CoThe previous series is equivalent to:
Half-Life CalculationsGeneral formula for Half-Life problems:mf = final massmi = initial massn = Number of Half-Lives
Another Half-Life ProblemThe half-life of 131I is 0.022 yr. On July 12th there are 52.3 mg of this radioisotope. How much 131I was there on June 22nd? We must find the initial mass mi. Therefore:
Nuclear Binding Energy (Mass Defect)Recall: The mass of the carbon-12 nucleus is exactly 12 amu. The mass of 6 protons and 6 neutrons is 12.096 amu
How can the mass of 12 subatomic particles separate be more than their mass when they combine?
Nuclear Binding Energy (Mass Defect)The mass and energy of an object are proportional. If a system loses mass it loses energy; if it gains mass it gains energy
E = mc2E = Energy
m = mass
c = Speed of light
Nuclear Binding Energy (Mass Defect)In every process when energy is released, mass also changes.
In chemical reactions the change of mass is too small to be detected.
In nuclear reactions the mass change is very significant.
Because c is so large, even small changes of mass release huge amounts of energy
Nuclear Binding Energy (Mass Defect)Nuclear Binding Energy:Energy required to separate a nucleus into its individual nucleons.
Mass Defect:Mass difference between a nucleus and its constituent nucleons
Finding Mass defect and binding energy for 12CThe mass of one 12C atom is EXACTLY 12 amu.
12C has 6 protons and 6 neutrons.
6 x (1.0073) + 6 x (1.0087) = 12.096 u
Mass Defect (m) = 12.096 12 = 0.096 u
Finding Mass defect and binding energy for 12CTo find the nuclear binding energy in Joules we must convert the mass defect to kilograms. Recall that 1 Joule = 1 kg.m2/s2
Converting to kilograms:Finding the Binding Energy E
Nuclear Binding Energy of Isotopes
Nuclear FissionThe splitting of a larger nucleus into smaller ones.
The splitting of the nucleus produces a mass change; therefore, energy is released.
The fission of a heavy atom like Uranium is a highly exothermic process
Nuclear FissionThe splitting of Uranium produces 2 smaller nuclei and 3 neutrons. These neutrons can collide against other atoms of uranium and make them split as well.
Chain ReactionThe original fission causes 2 or 3 neutrons. These neutrons can cause 2 or 3 more fissions and produce 6 or 9 more neutrons. The process escalates quickly and violently if not controlled.
Critical MassA chain reaction will not occur unless a certain critical mass of fuel is present. The critical mass of 235U is about 1 kg.If there is less than the critical mass some of the neutrons may escape and miss the nuclei of uranium and the reaction fizzles out.
Fission reactorsCadmium or boron control rods regulate the flux of neutrons
Nuclear FusionThe joining of two lighter nuclei to form a more massive one.
When two light nuclei fuse into a heavier one a great amount of energy is released
Fusion reactions are responsible for energy produced by the Sun.
Electrostatic repulsion is the largest obstacle to overcome when trying to fuse two light atoms
Nuclear FusionTo overcome repulsion an extreme high temperature environment is necessary.The fusion reaction that requires the least temperature is the fusion of tritium (3H) and deuterium (2H)The temperature necessary for this reaction is 40,000,000 K
Nuclear FusionSuch high temperatures have been achieved using atomic bombs.
Could we control fusion and harness its energy?
Plasma Fusion Movie
Uses of Nuclear EnergyNuclear Reactors (Power Plants)Nuclear Weapons (Fission bomb, H-Bomb)Nuclear Medicine (Drugs, Radiotherapy, Scanning)Food irradiation