Actinides are elements with atomic numbers from 90 to 103, following the element Actinium. They include naturally occurring elements of thorium, protactinium and uranium and eleven transuranic, i.e., artificially produced by nuclear reactions. Nevertheless, all actinides are radioactive.
What Are Actinides?
The term ‘actinide series’ has been derived from the first element of the series, actinium. The symbol An is used to refer to any of the actinide series elements, which range in the periodic table from atomic numbers 89 to 103.
All actinide series elements are radioactive in nature, they release a large amount of energy on radioactive decay. Uranium and thorium are the most abundant naturally occurring actinides on earth, whereas plutonium is synthetically obtained.
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These elements are used in nuclear reactors and nuclear weapons. Uranium and thorium have diverse current uses, whereas americium is used in ionization chambers of modern smoke detectors.
In the modern periodic table, lanthanides and actinides are shown as two separate rows below the main periodic table.
Electronic Configuration of Actinides
Actinides are the second series of elements of the f-block having a terminal electronic configuration of [Rn] 5f1-14 6d 0-17s2. The energy of 5f and 6d electrons are close to each other, and so electrons enter into the 5f orbital.
The atomic size/ionic radii of tri positive actinides ions decrease steadily from Th to Lw due to increasing nuclear charge and electrons entering the inner (n-2) f orbital.
This gradual decrease in size with an increasing atomic number is called actinide contraction, like lanthanide contraction. Because of the very poor shielding by 5f electrons, contraction is larger along the period.
Formation of Coloured Ions
Actinides like lanthanide ions have electrons in the f-orbital and also empty orbitals like the d-block elements. When a frequency of light is absorbed, the f-f electron transition produces a visible colour.
Ionization of Actinides
The actinides have lower ionization enthalpies than lanthanides because 5f electrons are more effectively shielded from nuclear charge than 4f.
Oxidation State of Actinides
Actinides show variable oxidation states because of the smaller energy gap between 5f, 6 d and 7s orbitals. Though 3+ is the most stable oxidation state, other oxidation states are possible because of the good shielding of f-electrons.
The maximum oxidation state first increases up to the middle of the series and then decreases, i.e. it increases from +4 for Th to +5, +6 and +7 for Pa, V and Np, but decreases in the succeeding elements.
Formation of Complexes
Actinides are better complexing agents than lanthanides due to their smaller size but higher nuclear charge. They can form Pπ – complexes as well.
The degree of complexion decreases in the order M4+ > MO22+ > M3+ > MO22+.
Chemical Reactivity of Actinides
Because of the lower ionization energy, actinides are more electropositive than lanthanides and most reactive. They react with hot water and react with oxidizing agents and form a passive coating, forming halides and hydrides. Actinides are strong reducing agents.
Physical Properties of Actinides
Density of Actinides: All actinides except thorium and americium have very high densities.
Melting and Boiling Points of Actinides: Actinides have fairly high melting points, like lanthanides, but there is no definite trend in the melting and boiling point of lanthanides.
Magnetic Properties of Actinides: All actinides are paramagnetic in nature, which depends on the presence of unpaired electrons. The orbital angular moment is quenched because of the shielding of 5f electrons so that the observed magnetic moment is less than the calculated.