Lecture 2 Flashcards
What is the chemistry of actinides similar to?
For early actenides - similar to transition metals
For later actinides the 3+ ions dominate and they are similar to transition metals
Which actinides are naturally occurring?
All artificial except thorium and uranium.
- Actinium and protactinium are naturally occurring but only in trace amounts
- neptunium and plutonium occur in minute amounts in uranium ores
Actinides and radioactivity
- all radioactive
- most of the longer lived isotopes decay by alpha emission
Where are uranium and thorium found?
Uranium - In old igneous rock faults, in pitchblende and uronothorite. 99% is U 238.
Thorium- widely dispersed, accounting for > 3% of the earths crust. Also found in monazite and uranothorite as ThO2. 100% Th 232
All actinide reactions must:
Occur under negative pressure
Describe the 5f orbitals
- Same as the 4f except they have a radial node
- more extended in comparison to the 6s and 6p orbitals
Bonding of actinides
Larger covelant contribution for early actinides but contraction means later in the series bonding is lanthanide like/ ionic
What is the relativistic effect?
Increasing velocity of electrons orbiting heavy nuclei results in a relativistic contribution to mass, that results in a direct relativistic orbital contraction of s and p orbitals and expansion of d and f orbitals due to increased shielding by the new s and p orbitals
- overall there is an extension of the 5f orbitals compared to the 6s and 6p
Electronic configuration of actinides
- hard to predict
- From Ac to Np the 6d electrons are included
- After Np the actinide contraction means the 5f is stabilised and it is no longer favourable to fill the 6d orbitals (except Curium and Lawrencium)
Ionic radii trends
Clear contraction with similar properties to lanthanides with the same oxidation state
Metallic Radii trends
No discernable trends, but decreased later on reflecting 3+ stability
Oxidation State
Varies early on - then becomes 3+ later
Which elements can achieve group valency?
All up to uranium
Describe UF6
Prepared from UO2 and HF followed by oxidation by F2
used to separate uranium isotopes
U235 is enriched and U238 is depleted
Methods for uranium isotope separation
- gaseous diffusion
- gaseous centrifugation
- electromagnetic separation
- laser separation
Lanthanide extraction from monazite and xenotime
- treated under alkali conditions
- acidification to produce LnCl3, ThO2 is removed by precipitation
Lanthanide extraction from bastnaesite
- oxidizing roast
- acid conditions with H2SO4 to generate a mixture of Ln2(SO4)3 from which CeO2 can be removed.
Separation techniques for lanthanide ions
- Fractional crystallisation
- Chemical separation by oxidation of Ce3+ to Ce4+ or reduction of Eu3+ to Eu2+
- Ion-exchange chromatography – produces small amounts but high purity can be obtained
- Solvent extraction
Production of Lanthanide Metal
- metallothermic reduction of anhydrous lanthanide chlorides or fluorides with calcium.
- Ca/Ln alloy forms
- calcium can be separated by distillation.
Production metal from Ln 2+ ions
metal sesquioxides Ln2O3 (Ln = Eu, Sm, Yb) are reduced with La metal and then separated by distillation
Reactions of lanthanides
- highly electropositive metals
- react withhalogens, O2, H2O or H+
- react with N2 to form ionic nitrides H2 to form lanthanide hydrides.
- Mischmetal is a mixture of Ce, La and other light lanthanides
- used in steel as an additive or in conjunction with iron to form lighter flints.
Lanthanide halides
- known for all elements
- Apart from triflourides from liquids
- small lanthanides and big halogens have low coordination numbers
- larger lanthanides have a tricapped trigonal prismatic structure with two extra long contacts
Sesquioxides (Ln2O3) synthesis
- made by the reaction of air and Ln - or by heating of oxygen containing molecules such as carbonates and nitrates
Sesquioxide use
- in superconducting ceramics
- contain a mixture of Cu(II) and Cu(III) oxidation states in sq planar and sq pyramidal arrangements
- Cooper pairs for electron transport
- Pairs of electrons that travel in concert, the first deforming the lattice and the second following
Ln2O2 and Ln2O3
- first is formed by Ce, Pr, and Tb under oxidising conditions
- then reduced to form the second with H2
- CeO2 is colourless when pure and forms sub stochiometries between Ce2O3 and CeO2 and is used in catalytic converters and self cleaning ovens
