Actinides are elements with atomic numbers from 90 to 103, beginning with Actinium. They include naturally occurring elements such as Thorium, Protactinium and Uranium, as well as eleven transuranic elements, which are artificially produced through nuclear reactions. All actinides are radioactive.

Actinides Guide

Electronic Configuration of Actinides

Actinide Contraction

Ionization of Actinides

Oxidation State of Actinides

Physical Properties of Actinides

Actinides are a series of elements in the periodic table, located in the f-block, that have atomic numbers from 89 to 103.

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 are located in the periodic table between atomic numbers 89 and 103.

All actinide series elements are radioactive in nature, releasing a large amount of energy on radioactive decay. Uranium and thorium are the most abundant naturally occurring actinides on earth, while plutonium is synthetically obtained.

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Lanthanides

F Block Elements

Uranium and thorium have multiple uses, including in nuclear reactors and nuclear weapons, while americium is mainly used in the ionization chambers of modern smoke detectors.

In the modern periodic table, the lanthanides and actinides are displayed in two distinct rows below the primary periodic table.

The general electronic configuration of actinides is [Rn] 5f^1-14 6d^0-1 7s². Here, [Rn] is the electronic configuration of the nearest noble gas, Radium.

Electronic Configuration of Actinides

The energy of the 5f and 6d electrons of Actinides, the second series of elements of the f-block, is close to each other, resulting in electrons entering into the 5f orbital with a terminal electronic configuration of [Rn] 5f^1-14 6d^0-17s².

Actinide Contraction

The atomic size/ ionic radii of tri positive actinides ions decreases steadily from Th to Lw due to increasing nuclear charge and electrons entering the inner (n-2) f orbital.

This gradual decrease in the size with an increasing atomic number is called Actinide Contraction, similar to Lanthanide Contraction. This is because of the very poor shielding by 5f electrons, resulting in a larger contraction along the period.

Formation of Colored Ions

Actinides, like lanthanides, have electrons in f-orbitals and empty orbitals like 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 due to the fact that 5f electrons are more effectively shielded from the nuclear charge compared to 4f electrons.

Oxidation States of Actinides

Actinides show variable oxidation states due to the small energy gap between the 5f, 6d, and 7s orbitals. Although the 3+ oxidation state is the most stable, other oxidation states are possible due to the strong shielding of f-electrons.

The maximum oxidation state increases from +4 for Th up to +7 for Np, but decreases in the succeeding elements.

Formation of Complexes

Actinides have an advantage over lanthanides as complexing agents due to their smaller size but higher nuclear charge. This allows them to form Pπ– complexes, which lanthanides cannot.

M4+ > MO22+ > M3+ > MO22+

Chemical Reactivity of Actinides

Actinides have a lower ionization energy than lanthanides, making them more electropositive and reactive. They react with hot water, oxidizing agents, and form a passive coating. Additionally, they form halides and hydrides, and are strong reducing agents.

Physical Properties of Actinides

Density of Actinides: Except for thorium and americium, all actinides have very high densities.

Melting and Boiling Points of Actinides: There is no definite trend in the melting and boiling points of actinides, although they typically have fairly high melting points like lanthanides.

The Magnetic Properties of Actinides: All actinides exhibit paramagnetism, which is determined by the presence of unpaired electrons. The orbital angular momentum is inhibited by the shielding of 5f electrons, resulting in an observed magnetic moment that is less than the calculated value.