Oxidation States
The common or dominant oxidation state of the actinides is +III. Some of them form higher oxidation states. The oxidation state in parenthesis denotes the principal oxidation state for each An.
Oxidation states of the actinide elements
| Ac | Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf |
| +III | +III | - | +III | +III | +III | (+III) | (+III) | (+III) | +III |
| (+IV) | +IV | +IV | +IV | (+IV) | +IV | +IV | +IV | +III | |
| (+V) | +V | (+V) | +V | +V | |||||
| (+VI) | +VI | +VI | +VI | ||||||
| +VII | +VII |
Oxidation State (+V)
In aqueous solutions, AnO2+ (An = Pa to Am) may be formed in the absence of strongly coordinating ligands. These ions are linear and less stable than AnO22+.
Oxidation State (+IV)
Th and U form stable (+IV) oxidation states. For the other actinides, this state is not stable. Bk (+IV) is moderately stable due to its f electron structure. An(+IV) oxides, carbonates and nitrates are stable compounds.
Oxidation State (+III)
All the actinides except Th and Pa exhibit this oxidation state. From elements U onwards, resistance to oxidation in an aqueous solution increases progressively with an increase in atomic number for this state. It becomes the most stable oxidation state for Am and the subsequent actinides (except for No, for which the +II state is the most stable because of its f14 configuration). U3+ can be obtained by the reduction of UO22+ or U6+ either electrolytically or with Zn amalgam. U3+ is thermodynamically unstable to oxidation by O₂ or aqueous acid and even by pure water.
Oxidation State (+II)
The +II state is found in six actinides, Cf to No. For No, it is the most stable oxidation state. The +II actinides are more stable than the +II lanthanides, especially at the end of the actinide series; this is because of the greater energy separation between the 5f and 6d than between the 4f and 5d orbitals at the end of the two series.
Occurrence
Only Th and U occur in nature in significant quantities. Ac and Pa occur in traces as products of natural radioactivity. The other elements beyond U (Np to Lw) do not occur in nature; they are prepared artificially. These synthetic elements beyond U are called transuranium elements. Thorium occurs in substantial quantities in India, Malaysia, Brazil, Australia and South Africa.
Uses
23290Th, a natural isotope of Th, is used in nuclear industries. It is called a fertile isotope as it can be converted to a fissile isotope, the latter used as fuel in nuclear reactors.
ThO₂ containing 1% cerium is used in incandescent gas mantles (the lights carried on heads through streets during festivals and processions in India).
235U is a nuclear reactor fuel. 238U is a fertile fuel and is converted to 239Pu fuel for use in reactors and also for making nuclear bombs.
ThO₂ is an industrial catalyst used in making nitric acid, sulphuric acid, and cracking petroleum.
ThO2 is used for making high-temperature oratory crucibles.
Important Notes for Quick Recap
The terms lanthanides and lanthanoids are used interchangeably.
The lanthanides are denoted by the common symbol Ln and the actinides by the common symbol An.
Names of artificial actinides: –
| Element | Origin of name |
| Neptunium | The planet Neptune |
| Plutonium | The planet Pluto |
| Americium | America |
| Curium | P and M Curie |
| Berkelium | Berkeley, California |
| Californium | California |
| Einsteinium | Albert Einstein |
| Fermium | Enrico Fermi |
| Mendelevium | Dimitri Mendeleev |
| Nobelium | Alfred Nobel |
| Lawrencium | Ernest Lawrence |
The chemistry of the transuranium elements is yet to be studied fully as these do not occur in nature and synthetic manufacture of these elements is extremely expensive.
The trivial name ‘rare earths’ used to denote the lanthanides is not correct as they are neither rare in abundance nor are they earths. IUPAC recommends the name ‘lanthanoids’ rather than lanthanides as the suffix ‘ide’ generally indicates negative ions, whereas the suffix ‘oid’ indicates similarity to one of the members of the family of elements. Similarly, ‘actinoids’ is the name accepted for actinides.
An organometallic compound of a lanthanoid is called an organolanthanide, and that of an actinoid is called an organoactinide.
The concept of the actinide series was proposed by Glenn T Seaborg, an American nuclear a chemist who pioneered the syntheses of most actinoids.
In 1961, Antoni Pazybylski (Polish-Australian astronomer) discovered a star that contains unusually high amounts of actinoids. This star is named after him.
Monazite sand occurs on the western and eastern coasts of India. Residents of these areas, especially Kerala and Tamil Nadu, are exposed to natural radioactivity ten times greater than the world average.
Environmental Pu, a deadly material, originates from atomic batteries (used in space vehicles and pacemakers), bomb detonations, bomb safety trials, nuclear accidents, nuclear crime and the nuclear fuel cycle.
