The d and f - block Elements

The Actinoids

How many elements can you name? You know just nitrogen, oxygen, carbon? That’s it? What about actinoids? Do you know what these are? If not, don’t panic, we are here to clear your doubts regarding these new set of elements. Let’s learn about not-so-known elements.

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Actinoids

Actinoids or actinides are 15 consecutive chemical elements in the periodic table from actinium to lawrencium (atomic numbers 89­103). As a group, they are significantly large because of their radioactivity. Although several members of the group, including uranium (the most familiar), occur naturally, most are man-made.

Both uranium and plutonium have been used in atomic weapons for their explosive power and currently are being employed in nuclear plants for the production of electrical power.

Actinoids

These elements are also called the actinide elements. However, the International Union of Pure and Applied Chemistry [IUPAC], the International body in-charge of chemical nomenclature, prefers the term actinoid, since the ‘-ide’ ending is usually reserved for negatively charged ions.

Discovery

The first actinoids discovered were uranium in pitchblende ore by Klaproth in 1789 and Thorium by Berzelius in 1829. Actinium was discovered 1899 by André-Louis Debierne, an assistant of Marie Curie.

Location in the Periodic Table

Actinoids form the bottommost row of the periodic table. They include fifteen elements starting from Actinium (Ac) and further include Thorium (Th), Protactinium (Pa), Uranium (U), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), and Lawrencium (Lr).

Are all the actinoids found naturally?

Actinium, thorium, protactinium, and uranium are the only actinoid elements found in nature to a significant extent. The remaining actinoid elements, commonly called the transuranic elements, are all man-made by bombarding naturally occurring actinoids with neutrons in reactors or with heavy ions in particle accelerators.

Thorium and uranium are the most abundant actinoids in nature with respective mass concentrations of 16ppm and 4 ppm. Uranium mostly occurs in the earth’s crust as a mixture of oxides in the minerals uraninite also known as pitchblende because of its black colour. The abundance of actinium in the earth’s crust is about 5 x 10−15%. Actinium is mostly present in uranium-containing, but also in other minerals, though in much smaller quantity.

Characteristics

  • Actinoids are typical metals. All of them are soft and have a silvery colour which tarnishes in air.
  • They have relatively high density and plasticity. Some of them can be cut with a knife.
  • All actinoids are radioactive, paramagnetic with the exception of actinium, have several crystalline phases.
  • All actinoids are pyrophoric, especially when finely divided, that is, they spontaneously ignite upon reaction with air.
  • Together with radium and transuranic elements, actinium is one of the most dangerous radioactive poisons. The real danger with the actinoid elements lies in the radioactive properties of these elements. They are emitters of tissue-destroying and cancer producing rays (alpha, beta, or gamma radioactivity). Actinium can accumulate and remain in the surface layer of skeletons. Less than one-millionth of a gram of some actinoid isotope can be fatal.

Valency

All actinoids, unlike lanthanides, are highly reactive with halogens. Thorium is rather active chemically owing to lack of electrons on 6d and 5f orbitals. Protactinium exhibits two valence states; the +5 is stable and the +4 state easily oxidizes to protactinium (V). Uranium has a valency from 3 to 6, the last being most stable.

Neptunian has valence states from 3 to 7, which can be simultaneously observed in solutions. Plutonium also exhibits valence states between 3 and 7 inclusive and thus is chemically similar to Neptunian and uranium. It is highly reactive and quickly forms an oxide film in air.

Americium shows the diversity from other actinoids by showing valency between 2 and 6. Valency 3 is dominant in all subsequent elements up to lawrencium with the exception of nobelium.

Applications

  • Actinide is mostly used in nuclear weapons and as a fuel in nuclear reactors. The most important isotope for nuclear power is uranium-235. This isotope strongly absorbs thermal neutrons releasing much energy. One fission act of 1 gram of uranium-235 converts into about 1 MW power.
  • About half of the produced thorium is used as the light emitting material of gas mantles.

There is a challenge to develop stable and durable actinoid-bearing materials which provide safe storage, use, and final disposal.

Solved Examples for You

Question: How are the actinoids extracted?

Answer: Owing to their low abundance, the extraction of actinoids is a complex and a multistep process. Fluorides of actinoids, which are reduced with calcium, magnesium or barium, are usually used because they are insoluble in water and can be easily separated by redox reactions. Among the actinoids, uranium and thorium are the easiest to extract.

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