Actinides

Question 1

Why is working with the actinides so problematic?

Answer 1

• The heavier actinides have only been made with only a few atoms at a time, and these have very short half-lives
• Known chemistry of Curium onwards is mostly limited to simple halides and oxides as they cannot be made and isolated in the usual manner.
• Extreme safety precautions must be taken.

Question 2

How are Oxidation states of the actinides determined?

Answer 2

• Oxidation states are established through coprecipitation with a nonradioactive metal.

Question 3

How was An²⁺ established?

Answer 3

• An2+ established by adding Ba(NO3)2 in solution and precipitating with SO₄^2-
• An²⁺ ions will be included in BaSO₄ precipitate while An³⁺ remains in solution
• Can then use the radioactivity of An to determine if it is solid or solution.
• Using repeated experiments with different reducing agents of different strengths enables stability of different An ox. states to be accurately determined.

Question 4

How are the ionic radii of the An determined?

Answer 4

• Ionic radii of an can be determined accurately from the rates they elute from ion-exchange columns when complexed with different ligands.
• Smaller cations generally complex first.
• Radioactivity of the An is used to detect when it is eluted without the need to isolate it.

Question 5

What is the electron configuration of the actinides?

Answer 5

• The 5f subshell, like the 4f orbitals, is filled “two shells late” and the electronic configuration of Actinide can be considered to be [Rn]7s2 6d1 5f_n
• Actinides are electropositive with similar electronegativities to the lanthanides
• Chemistry is divided into the early actinides Ac-Am and the late actinides, Cm-Lr whose oxidation states resemble the TM and Ln respectively.

Question 6

What is the chemistry of the early actinides?

Answer 6

• The 5f orbitals are not contracted into the [Rn] core for the early actinides
• 7s, 6d and 5f orbitals Ac-Am all have similar energies, with differences in the range of chemical binding energies.
• These electrons can all be ionised for Ac-Np making high maximum oxidation state accessible along with a lower range of lower ox states, similar to early d-block elements.

Question 7

What is the chemistry of the late actinides?

Answer 7

5f-orbitals are contracted into the [Rn] core for late An, Cm-Lr

5f orbitals are therefore held tightly to the nucleus and are no longer readily ionisable.

As for Ln, An³⁺ is the most common oxidation state except for No²⁺ which is stabilised as it leads to a f14 configuration.

Question 8

Why are Uranium complexes so colourful?

Answer 8

• Many uranium complexes are strongly coloured due to charge transfer transitions
• Yellow-green colour of UO₂ (f0) is due to ligand-metal transfer

Question 9

What are the electronic properties of Uranium?

Answer 9

• Chemistry of uranium typical of early actinides.
• Resembles that of W, Tungsten.
• Stable from 3+ to 6+ oxidation states so has more diverse chemistry than Th4+
• As f-orbitals are more diffuse and higher in energy than in Ln, they are used in covalent bonding in some complexes.