Lecture 2

Question 1

What is the chemistry of actinides similar to?

Answer 1

For early actenides - similar to transition metals

For later actinides the 3+ ions dominate and they are similar to transition metals

Question 2

Which actinides are naturally occurring?

Answer 2

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

Question 3

Actinides and radioactivity

Answer 3

• all radioactive

- most of the longer lived isotopes decay by alpha emission

Question 4

Where are uranium and thorium found?

Answer 4

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

Question 5

All actinide reactions must:

Answer 5

Occur under negative pressure

Question 6

Describe the 5f orbitals

Answer 6

• Same as the 4f except they have a radial node

- more extended in comparison to the 6s and 6p orbitals

Question 7

Bonding of actinides

Answer 7

Larger covelant contribution for early actinides but contraction means later in the series bonding is lanthanide like/ ionic

Question 8

What is the relativistic effect?

Answer 8

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

Question 9

Electronic configuration of actinides

Answer 9

• 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)

Question 10

Ionic radii trends

Answer 10

Clear contraction with similar properties to lanthanides with the same oxidation state

Question 11

Metallic Radii trends

Answer 11

No discernable trends, but decreased later on reflecting 3+ stability

Question 12

Oxidation State

Answer 12

Varies early on - then becomes 3+ later

Question 13

Which elements can achieve group valency?

Answer 13

All up to uranium

Question 14

Describe UF6

Answer 14

Prepared from UO2 and HF followed by oxidation by F2

used to separate uranium isotopes

U235 is enriched and U238 is depleted

Question 15

Methods for uranium isotope separation

Answer 15

• gaseous diffusion
• gaseous centrifugation
• electromagnetic separation
• laser separation

Question 16

Lanthanide extraction from monazite and xenotime

Answer 16

• treated under alkali conditions

- acidification to produce LnCl3, ThO2 is removed by precipitation

Question 17

Lanthanide extraction from bastnaesite

Answer 17

• oxidizing roast

- acid conditions with H2SO4 to generate a mixture of Ln2(SO4)3 from which CeO2 can be removed.

Question 18

Separation techniques for lanthanide ions

Answer 18

• 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

Question 19

Production of Lanthanide Metal

Answer 19

• metallothermic reduction of anhydrous lanthanide chlorides or fluorides with calcium.
• Ca/Ln alloy forms
• calcium can be separated by distillation.

Question 20

Production metal from Ln 2+ ions

Answer 20

metal sesquioxides Ln2O3 (Ln = Eu, Sm, Yb) are reduced with La metal and then separated by distillation

Question 21

Reactions of lanthanides

Answer 21

• 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.

Question 22

Lanthanide halides

Answer 22

• 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

Question 23

Sesquioxides (Ln2O3) synthesis

Answer 23

• made by the reaction of air and Ln - or by heating of oxygen containing molecules such as carbonates and nitrates

Question 24

Sesquioxide use

Answer 24

• 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

Question 25

Ln2O2 and Ln2O3

Answer 25

• 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