Emslie Group - THORIUM AND URANIUM

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General Properties of the Actinides

The lanthanide and actinide series are shown below (version of the periodic table in which lutetium and lawrencium are the first elements in the 3rd and 4th row transition metals - for more discussion, see Jensen, W. B. J. Chem. Ed. 1982, 59, 634).

People tend to think of thorium and uranium as extremely radioactive and rare elements. However, this is in fact not the case; Thorium is approximately as abundant in the earth's crust as boron, and uranium is more abundant than Sb, Hg, Cd, Bi, Ag, Sn or Au. To put this in perspective, the soil removed to build a typical family home (~100 tonnes) contains ~1kg of uranium and ~3kg of thorium.

A table with selected properties of the actinide elements is shown below:

Z 89 90 91 92 93 94 95
Element Ac Th Pa U Np Pu Am
Natural Abundance wt.% 6 x 10-14 1 x 10-3 9 x 10-11 2 x 10-4 4 x 10-17 2 x 10-19  0
Method of Preparation 226Ra  + n - b --> 227Ac Extraction Extraction Extraction 238U + n -->  237Pu &  239Np Nuclear Reactors 241Pu & 243Pu
decay
Amount of Pure Material in Existence 5-10g Huge Amounts A few kg Huge Amounts Tonnes Thousands of Tonnes Many kg
Toxicity of the most available elements (not due to radiation) --- Similar to lead ???? Similar to lead Highly Toxic Highly Toxic
????
Most Stable Isotope / half life 227Ac
22 y		 232Th
14 x 109 y		 231Pa
33 x 103 y		 238U
4.5 x 109 y		 237Np
2.1 x 106 y		 244Pu
83 x 106 y		 243Am
7.4 x 103 y
Typical Oxidation States
3
4
4
5

3
4
5
6
3
4
5
6
7
3
4
5
6
7
3
4
5
6
7

Z 96 97 98 99 100 101 102
Element Cm Bk Cf Es Fm Md No
Natural Abundance wt.%  0  0  0  0  0  0  0
Method of Preparation 241Am and 243Am decay 239Pu + neutrons 239Pu + neutrons 239Pu + neutrons 239Pu + neutrons 253Es + 4He --> 256Md 249Cf + 12C
--> 255No
Amount of Pure Material in Existence Many kg mg Grams mg mg Millions of atoms Thousands
of atoms
Toxicity of the most available elements (not due to radiation) ???? --- --- --- --- --- ---
Most Stable Isotope / half life 247Cm
16 x 106 y		 247Bk
1.4 x 103 y		 251Cf
898
years		 252Es
472 days		 257Fm
100
days		 258Md
56
days		 259No
58
min 
Typical Oxidation States
3
4
3
4
2
3
4
2
3
2
3
2
3
2
3

This table quite clearly shows that Thorium and Uranium are the only actinides with significant natural abundance, long half lives (ie. low radioactivity), and only moderate toxicity; features which make Th and U the heaviest elements that can be studied in a regular synthetic chemistry laboratory. The only other elements which are produced in mult-tonne quantities and have isotopes with half lives greater than 1 million years are Np and Pu. However, these elements are typically only accessible to researchers in government facilities.

An additional important feature of the table above is the interesting abuncance of oxidation states accessible for U, Np, Pu and Am. The early actinides are quite distinct from lanthanides in that the f-orbitals are not core orbitals (this is generally attributed to more pronounced secondary relativistic effects than observed for the lighter lanthanides). As a result, a variety of oxidation states are accessible (although the earliest actinides tend to prefer their group oxidation state: AcIII, ThIV, PaV). In addition, bonding is often significantly covalent, and 5f-orbitals, in addition to 6d- and 7s-orbitals, may play a significant role. However, beyond Am (Cm --> No), f-orbital contraction as a result of ineffective shielding leads to a switch to more lanthanide-like behaviour: largely ionic bonding and the predominance of a single oxidation state.

Uses of the Actinides

Thorium Oxide (ThO2)
Catalyst - for conversion of NH3 to HNO3, petroleum cracking and H2SO4 production
Welsbach Ceramic Mantles (99% ThO2 / 1% CeO2) for portable gas lights - yttrium now often replaces thorium.
Highly Refractory Crucible Material for very pure metals
Thorium oxide containing glass is often used in high quality lenses for use in cameras, telescopes and scientific instruments

Thorium Metal
Coating for W filaments in electrical devices (e.g. TV sets)
A component in some welding rods
Th/Mg Alloys for construction (e.g. in heating coils in electric ovens and in nuclear reactors).

Uranium Metal
Depleted uranium has a variety of milatary uses.

Uranium Oxides
As a pigment in glass (typically yellow or green) and pottery (may different colours, but bright orange fiestaware is the best known).

Americium (241Am a-emitter; half life of 433 years)
Smoke Alarms - 241Am is used as a source of a-particles for an ionization chamber. Smoke disrupts the migration of ions, and the drop in the number of ions is registered as a drop in the current flowing through the ionization chamber.
Neutron Sources - in combination with beryllium
Mono-energetic a-particles for Determining the Density and Thickness of Materials

233U, 235U and 239Pu
Nuclear Fuels and Weapons

238Pu (half life of 88 years)
Heart Pacemakers, Deep-Sea Diving Suits, Space Satelites - In combination with thermoelectric Pb/Te elements, 238Pu is used as a compact and reliable energy source.

252Cf
High neutron fluxes for uses in tumor therapy and ingition of nuclear fuel elements.
Californium Plasma Desorption Mass Spectrometers - as a source of energetic nuclear fragments (generated by Cf fission) to volatilize high molecular mass compounds.

Naturally Abundant Isotopes and Radioactive Decay of Thorium and Uranium

The table below shows information on the abundance and decay of naturally occurring Th and U isotopes. It can be seen that both Th and U are predominantly a-emitters (although b-radiation is produced by other members of the decay chain - see below) and that Unat is approximately 6 times more radioactive than Thnat. Interestingly, half of this radiation comes from 234U which is present in much smaller amounts than 238U or 235U but has a much shorter t1/2.

Isotope
Natural Abundance
Decay mode / energy
Half Life (billions of years)
Specific Activity
Activity per g of Thnat or Unat
232Th
100.00 %
a / 4.0 MeV
14.05
4.07 kBq/g
4.07 kBq/g
238U
99.27 %
 a / 4.2 MeV
4.468
12.445 kBq/g
12.356 kBq/g
235U
0.72 %

 a / 4.4 MeV
g / 0.21 MeV
0.7038
80.011 kBq/g
0.568 kBq/g
234U
0.0055 %
 a / 4.8 MeV
0.0002455
231.3 MBq/g
12.356 kBq/g
Unat
100 %
 a
n/a
n/a
25.280 kBq/g

Decay series for 232Th and 238U are shown below:

Thorium series
Uranium series

a b b
a a a a
b (b) (a)
 ---



a b b
a a a a a
b b a
b b a

Atomic and Ionic Radii of the Actinides

Graph of Atomic Radius (empirical) from WebElements:


----------------------
Revised Effective Ionic Radii (C.N. 6) of Shannon and Prewitt
(Shannon, R. D. Acta Cryst. 1976, A32, 751):

Oxidation State 3+ 4+ 5+ 6+
Ionic Radii (A) U = 1.03
La = 1.03
Lu = 0.86		 Th= 0.94
U = 0.89
Ce = 0.87
Hf = 0.71
Ti = 0.61		 U = 0.76
Ta = 0.64
V = 0.54		 U = 0.73
W = 0.60
Cr = 0.44

Thorium and Uranium Minerals

A wide range of thorium and uranium minerals exist, with the common minerals varying from place to place.
A few of my favorites are shown below:

Department of Chemistry, McMaster University

 emslied@mcmaster.ca october2006; djhe