ACTINIDES
Deepshikha
Department of chemistry
Introduction-This group of elements also known as heavier elements or inner transition series elements following actinium element contains metal and are also known as actinide series i.e. Ac to Lr (Z=89-103).
The electron enters in the 5f subshell one at each step .Actinides series elements of which uranium (U, Atomic no. 92) and plutonium (Pu, Atomic no. 94) are best known are built up by completion of the 5f orbitals.
They are found at the final row of the periodic table followed by filling of the 6p orbital. Actinides series belongs to a series of elements followed by actinium in the seventh period of periodic table
. According to IUPAC these elements are called actinoids with the ending - oid rather than being called actinides or actinones. Because the ending -ide represent the more precisely means negative ion such as oxide or chloride but all the actinides are clearly metallic in character
Occurrence and Preparation of The Elements
With the exception of Th and U which is natural, the other actinides are manmade, synthesized artificially by bombardment with slow moving alpha particles, neutron, atoms etc. in cyclotrons. After Uranium i.e. Z=92 none of the elements occur in nature. They are all radioactive in nature. The natural ores of thorium and uranium are thorite and carnotite , pitch blend
General Properties of actinides
4.They are reactive metals like the lanthanum.
Electronic configuration
The electronic distribution of these elements starts from the filling of 6d and 7s orbital remains same in each element. But differentiation occurs in 5f sub shell, in which electron enters from 1-14 from Th to Lr. The general electronic configuration f actinides in the ground state may be written a
The electronic configuration of actinide elements are given the following table
Physical Properties: The important physical properties of actinide elements are
(i) It is found that the melting point and boiling of these elements high but lower than d-block elements
(ii) Density of these elements are high, especially near uranium and neptunium are very high.
(iii) A close comparison of M2+ionic radii with lanthanide indicates that actinide ions are very similar in size. The size of the ions decreases regularly along the actinide series. This steady decrease in ionic radii with increase in atomic number is called actinide contraction. The reasons are same as lanthanide
cpntraction
Oxidation state
These elements show a greater range of variable valency due to the almost same energy of 5f, 6d and 7s orbitals.
The most stable oxidation state shown by these elements is +3. Thus stability increases with atomic number of these elements.
They also show +2, +4, +5, +6 and +7 oxidation number in their compounds.
The+6 oxidation state is most important from uranium to these elements.
The +6 oxidation, state is most important from (92 U) to americium (95Am).
In AMO2 +and UO2 2+ , PuO2 2+ etc. shown +6 oxidation number. Neptunium and Plutonium show +7
Magnetic Properties
The magnetic properties shown by actinides are similar to lanthanides in terms of magnetic moments. The presence of unpaired electrons in f- orbital resemble to the paramagnetic behavior for the actinides, which is much more than 4f electron in lanthanides. Because of the existence of unpaired electrons, all actinides are paramagnetic. Because of the shielding of 5f electrons, the orbital angular moment is quenched, and the observed magnetic moment is smaller than the calculated magnetic moment.
Formation of complexes :
Compared to lanthanides, actinides have a slightly higher propensity to form complex compounds. This is brought on by their smaller ions and higher charge.
The majority of the actinide halides combine with alkali metal halides to generate complicated combinations.
Actinides combine with organic substances like oxine and EDTA to generate chelates.
For the ions M4+, MO22+, M3+ and, MO2+ the degree of complex formation declines in the order.�M4+>MO22+>M3+>MO2+
Similarities Between Lanthanides and Actinides
The (n-2) f subshell is employed for filling and characterization of all Lanthanides and Actinides. Lanthanides and Actinides have very similar electrical configurations. The following are some of the significant commonalities between these two,
Lanthanides and actinides have a high Oxidation State of +3.
The filling of these elements involves (n – 2) f orbitals.
Lanthanides and actinides are both reactive and electropositive.
As the atomic number of these elements increases, so do their ionic and atomic sizes.
Lanthanides and actinides both have strong magnetic characteristics.
Differences Between Lanthanides and Actinides
The filling of Lanthanides involves 4f-orbitals, whereas the filling of Actinides involves 5f-orbitals.
The shielding of 5f electrons is also less than that of 4Ffelectrons.
The paramagnetic characteristics of Lanthanides are fairly simple to explain. In the case of Actinides, however, it is difficult to explain all of the paramagnetic features.
Except for Promethium, the majority of Lanthanides are non-radioactive. The elements in the Actinide series are all radioactive.
There are multiple oxocations of the elements in the Actinides class, but none in the Lanthanides.
In contrast to the chemicals found in Lanthanides, the compounds generated by Actinides are highly basic in nature.
Colour and Electronic Spectra
The ions of actinides are colored in nature due to presence of number of unpaired electrons in 5f-orbital and their transition (f-f transition) in visible region of spectrum. It is also remarkable to show that the 5f electron in case of actinides, accommodate more place than 4f electron in lanthanides. Thus, 5f electrons are strongly interacted by the ligand field, produced by ligand. It has been experimentally observed that absorption spectra of actinides are stronger and broader than the lanthanides ions. Due to higher oxidation state actinide ions show charge transfer band in visible region. Because of the fact that by the high oxidation ions, easily oxidizes the ligands. Thus, lanthanide do not exhibit charge transfer spectrum because they do not display oxidationstates higher than four.
Isolation of Actinides
The elements of actinide series apart from actinium are radioactive in nature. They are derived from uranium, which was discovered first by klaproth in 1789 from the mineral pitchblende and its radioactive property was first of all investigated by Hennery Becquerel in 1985.
uranite (pitch blende) which has the empirical formula U3O8.The chief minerals are:
3. Now the roasted ore is treated with dil. Sulphuric acid in presence of an oxidizing agent like NaClO3 or MnO2
4. When above solution of uranyl sulphate is is treated with ammonium carbonate, ammonium uranyl carbonate is precipitate
Whicn when ignited gives U3O8 as
5. In this step the mixed uranium oxide, U3O8 is reduced toUO2 by heating either with charcoal or with hydrogen
6. Elemental uranium can be obtained in this step from uranium dioxide by the treatement with HF or Mg metal and Cl element in the presence of charcoal through following reactions:
Chemistry of neptunium, plutonium and americium from uranium
Neptunium, plutonium and americium are produced from the uranium in nuclear reactors. It is interesting to note that the name of these two elements were introduced on the name after two planets, Neptune and Pluto in 1940 by the team of scientist by the bombardment of uranium with neutrons in U S A. Naturally occurring uranium have two isotopes:
Neptunium and plutonium are obtained by the bombardment of the two isotopes of uranium by the following nuclear reactions
Americium and curium can be prepared from the isotopes of plutonium as
Curium and americium can be separated from plutonium because of their different chemical nature, whereas neptunium and americium can be obtained by the use of appropriate oxidation-reduction cycles coupled with solvent extraction.
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