2: Intro to Actinides Flashcards
What is the Manhattan project?
WWII project development of Nuclear weapons as well as nuclear power, nuclear medicines and lanthanide based technologies
What are actinides?
Actinides (An) 15 elementsfrom Ac to Lr with either in G3 5f series- second row of the f block First 7 element-compounds similar to early transition states Last 8 element-compounds similar to those of 3+ ion Lanthinide complexes All isotopes of all the actinides are radioactive
What is the brief history of An
Initially proposed in 1944 Th, Pa and U as G 4,5,6
What is trans-Uranium
Z>92, elements beyond uranium, trans-uranium elements are synthetic and are typically formed by neutron capture/heavy ion bombardment Picture!!
Do Actinides naturally occur?
First 4 Ac-U all occur naturally Ac and Pa in trace, U and Th are relatively abundant
Thorium general
-Widely dispersed -3ppm of earth’s crust -Naturally occurs as 100% 232-Th -Occurs in monazite and uranothorite -Obtained as ThO2 from mineral extraction processes
Uranium general
- widely distributed
- found in the faults of old igneous rocks
- Naturally 99.28% 238 and 0.71% 235
What are 5f orbitals
-4f and 5f do not differ in the angular part of their WF and are same shape, 5f orbitals have a radial node!!!! 5f have a greater radial extention w/r to 6s/p orbitals THAN the 4f have w/r to 5 s/p orbitals . (rel effect) -5f orbitals are less shielded from influence of ligands (aka more expopsed) resulting in a larger contribution to covalent bonding earlier in the An series eg UF3-covalent contribution -NdF3 - not
What are “relativistic effects” and why to do with An?
- it is believed that relativistic effects partially determine the chemistry of An’s -Increasing v of e’s orbiting heavy nuclei results in a relativistic contribution to mass 1s in U –> 1.35 me (as v approaches c, m approaches infinity) DROC, direct rel orbital contraction: All s-orbital (and to a lesser extent p-orbitals0 are stabilised IROE, indirect rel orbital expansion: All d and f orbitals are now destabilised- due to the increased shielding of nuclear charge by the now contracted s and p orbitals -the result is an increased radial extension of the 5f orbitals w/r to 6s/6p
What is the electronic configuration of the An’s
-not as easy to confirm Ac - Np: 6d orbitals are lower in energy = 6d filling favourable [Rn]5fn 6d1 7s2 Pu+ 5f stabilized to such an extent =6d filling no longer favourable [Rn]5fn 7s2 Exceptions: Cm/Lr due to half-filled shell and filled shell increasing Zeff stabilises 5f -takes on first five structure 5f orb is more contracted and interacts less with ligand fields -> more lanthanide like
Ionic radii and metallic radii
Ionic radii show a clear actinide contraction for 3+,4+ and partial 5+ ions actinide 3+ and 4+ ions with similar radii to their lanthanide counterparts show similarities in properties that depend on ionic radii Metallic radii show no discernable trend, however the increased radii may reflect the stability of the 3+ OS while early actinides can access high oxidation states???? Picture
Where would you find an Actinide in your home?
Smoke detectors Radiation source w a radiation detector smoke particles get between - disrupt radiation que alarm
The binary halids of actinides
Group valency can be achieved up to U, eg. UF6, PaF5 ThF4 After Uranium the 3+ oxidation state becomes more stable and the halides are mostly MX3 and resemble the lanth trihalides
why is UF6 a very useful act. halide
It is made on a large scale to seperate 235UF6 and 238UF6 and producing 235UF6 enriched 235
Why is enriched uranium important
235-fissile-slower moving neutrons can cause fission –> chain reactions 238-fissionable-fast energetic neutrons only Nuclear fission occurs when a large nucleus is bombarded with a neutron and splits into 2 smaller nuclei, releasing one or more neutrons which can collide with other nuclei For chain reactions, a critical mass of fissile material is required Uranium 235- fissile uranium enriched w 235 from 0.71% to 3-5%
How is UF6 prepared?
Picture
How can U isotopes be seperated?
Gaseous diffusion: UF6 vapour diffuses through barrier
Gas centrifuge: UF6 vapour in a centrifuge 238 on outside
Electromagnetic separation: Ionised UCl4 seperated in cyclotron like system
Laser seperation: selectively ionise U235 w tunable lasler
Most popular modern way to separate U
Gaseous diffusion multiple separations: output of one is input for another achieving v high enrichment levels
How are lanthanides seperated from a mixture of lanthanides?
Mixture of lanthanides produced in the extraction process
1) Fractional crystallisation: relys on 1000s of fractional crystallizations -tedious- not in use anymore
2)
How are actinide metals made?
They are prepared by reduction of AnF3 or AnF4 with vapours of Li, Mg, Ca or Ba at high temperatures 1000C +
Chemistry of An(0)
highly electropositive tarnish in O2 react with water/dilute acid to release H2 react with most non-metals in direct combination
Oxidation states of An
the actinides display a wider range of oxidation states Early An +3 not the most stable OS for most Th is almost exclusively found as 4+ U3 is readily formed from reduction of stable U4 and U6 Later An +3 most stable state- resembling lanthanide chemistry 5f is more radially extended w/r to 7s and 6d as Zeff increases, 5f becomes more core like and more stable - stabilising the trivalent state across the series
How do you produces lanthanide metals
-metallothermic reduction of the anhydrous lanthanide fluorides or chlorides with calcium metal PICTURE Reaction performed urder argon and the product is a Ca/Ln alloy from which the Ca is removed by distillation Divalent Lanthanides (Sm, Eu, Yb) cannot be done this way- only reduced to a 2+ state Ln2O3 reduced with La and then seperated by distillation 2La + M2o3 –> La2O3 +2M
Lanthanides(0) metal chemistry
Metals are soft and silvery white Highly electropositive React with O2 React with H+ or H2O to produce H2 React with N2 to produce nitrides React with H2 to form hydrides Compounds are strongly ionic
What is a sesquioxide
Ln2O3 made by heating the metals in air or heating oxygen containing lanthanide complexes (nitrate or carbonate)
Lanthanide halides chemistry
LnX3 Anhydrous halides known for all elements (2 exceptions (PmX3 and EuI3) They are ionic,crystalline w high mp Apart from LnF3, deliquescent- meaning they absorb water and dissolve in it Early Ln- larger w with F (tysonite structure) adopt a tricapped trigonal prism structure w two longer contacts Late Ln- smaller - lower coordination
Lanthanide halide prep
Prepared by dehydration of their hydrated salt Cl,Br and I also require a dehydrating agent eg SO2Cl2 CeF4 also thermally stable
Perovskites and Lanthanides
Lanthanides are a good source of stable, large cations with a range of ionic radii and can take one of more of the cation postions in complex oxides Perovskites(ABO3) Readily prepared with the lanthanide of the A site e.f. LaFeO3
Superconductors and Lanthanides
La(2-x)BaxCuO4 superconducts at 30K contain a mixture of square pyramidal and square planar copper environments EPR data suggests a mixture of Cu2+ and Cu3+ OS expanded YBa2Cu3O7 etc etc high and higher
Lanthanide O2
LnO2 known for Ce, Pr and Tb and adopt the fluorite structure CeO2 forms substoichiometries with CeO3 and is used in catalytic converters and in self cleaning ovens
