Chemical Properties of the Actinoids

Introduction to Actinoids

The elements in which the last electron enters the f-orbital are known as inner transition elements. They are classified as f block elements in the periodic table and belong to group 3 of the periodic table. Inner transition elements are these f block elements.

Inner transition elements are divided into two groups:

  • Lanthanoid series- The lanthanide series is described as the last electron entering the 4f orbital.

  • Actinoids series is described as a series in which the last electron enters the 5f orbital.

This article will study the chemical properties of actinoids, the oxidation state of actinides, and the similarities between lanthanoids and actinoids.


Chemical Properties of the Actinoids

Following the element Actinium, actinides are elements with atomic numbers ranging from 90 to 103. They include thorium, protactinium, and uranium, as well as eleven transuranic elements generated artificially by nuclear reactions. Both actinides, however, are radioactive.


The word "actinide sequence" comes from actinium, the first element in the series. The symbol An is used to refer to all of the actinide series elements, which have atomic numbers ranging from 89 to 103 in the periodic table.


Both elements in the actinide sequence are radioactive in nature, and radioactive decay releases a significant amount of energy. The most common naturally occurring actinides on Earth are uranium and thorium, while plutonium is synthesised.

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Electronic Configuration of Actinoids

Actinoids are the second sequence of f-block components with the electronic configuration [Rn] 5f1-14 6d 0-17s2. Since the energies of 5f and 6d electrons are similar, electrons enter the 5f orbital.


Physical and Chemical Properties of the Actinoids

Physical Properties of Actinoids:

  1. Metals such as actinoides are common. They're all smooth, silvery in colour (though they tarnish in the air), and have a high density and plasticity. Knives may be used to cut some of them. Their electrical resistance ranges from 15 to 150 Ohm cm. Since thorium's hardness is close to that of soft steel, it can be rolled into sheets and pulled into wire when heated. Thorium is about half the density of uranium and plutonium, but it is also harder.

  2. With the exception of actinium, all actinides are radioactive, paramagnetic, and have several crystalline phases: plutonium has seven, uranium, neptunium, and californium have three. Protactinium, uranium, neptunium, and plutonium have crystal structures that are more similar to those of the 3d-transition metals than those of the lanthanides.

  3. All actinides, particularly finely divided ones, are pyrophoric, meaning they spontaneously ignite when exposed to air at room temperature. The number of f-electrons has no discernible effect on the melting point of actinides. Hybridization of 5f and 6d orbitals, as well as the creation of directional bonds, clarify the unusually low melting points of neptunium and plutonium (640 °C).


Actinoids Contraction: 

Due to increasing nuclear charge and electrons entering the inner (n-2) f orbitals, the atomic size/ ionic radii of tri positive actinides ions decrease gradually from Th to Lw.


Actinide contraction, like lanthanide contraction, is a progressive decrease in size with increasing atomic number. The contraction is larger over time due to the weak shielding provided by 5f electrons.


Formation of Coloured Ions:

Like the d-block elements, actinides like lanthanides ions have electrons in f-orbitals as well as empty orbitals. The f-f electron transition creates a visible colour when a frequency of light is absorbed.


Ionization of Actinides:

Since 5f electrons are more easily protected from nuclear charge than 4f electrons, actinides have lower ionisation enthalpies than lanthanides.


Oxidation State of Actinides:

Since the energy difference between the 5f, 6 d, and 7s orbitals is smaller in actinides, they have variable oxidation states. Because of the strong shielding of f-electrons, other oxidation states are possible, even though 3+ is the most stable.


The highest oxidation state of actinides rises until the middle of the sequence, then falls, i.e. it rises from +4 for Th to +5, +6 and +7 for Pa, V, and Np, but falls in the following elements.


Formation of Complexes:

Because of their smaller size but higher nuclear charge, actinides are stronger complexing agents than lanthanides. They can also form P – complexes.


Similarities Between Actinoids and Lanthanoids

The element is said to belong to the first sequence of transition elements since the last electron is filled into the 4f orbital. After lanthanum, the lanthanoid sequence contains 14 elements. These are known as lanthanides or lanthanoids because they occur directly after lanthanum in the periodic table. While lanthanum does not have any 4f electrons, it is frequently used in lanthanide due to its resemblance to lanthanoids.


The electrons obtained by successively filling 5f orbitals are known as actinides or actinides. They get their name from the fact that they appear in the periodic table right after actinium (Ac).


The sequence of actinides, which includes 14 elements ranging from Th(90) to Lw(103), is also known as the second set of inner transitions. Since actinium (Z=89) has no 5f electrons, it is common to analyse it with actinoids.


Did You Know?

Although actinides have some well-known uses in everyday life, such as smoke detectors (americium) and gas mantles (thorium), they are primarily used in nuclear weapons and as reactor fuel. The last two areas make use of actinides' ability to release massive amounts of energy in nuclear reactions, which can become self-sustaining chain reactions under some circumstances.


Approximately half of the thorium produced is used as a light-emitting material in gas mantles. Thorium is also used in multicomponent magnesium and zinc alloys. As a result, Mg-Th alloys are light and solid, with a high melting point and ductility, and are widely used in the aviation and missile industries. Thorium also has excellent electron emission properties, with a long lifetime and low emission potential barrier. The ratio of thorium and uranium isotopes in different things, including stars, is commonly used to estimate their age (see radiometric dating)


The isotope plutonium-239 was a crucial component of nuclear weapons because of its ease of fission and availability. The critical mass of plutonium-based designs can be reduced to around a third of that of uranium-235.


Thorium is also used in multicomponent magnesium and zinc alloys. As a result, Mg-Th alloys are light and solid, with a high melting point and ductility, and are widely used in the aviation and missile industries. Thorium also has excellent electron emission properties, with a long lifetime and low emission potential barrier.

FAQs (Frequently Asked Questions)

Question: What are Lanthanoids and Actinoids?

Lanthanoids are chemical elements that belong to the lanthanide series of the periodic table's f block. Actinides are chemical elements that belong to the f block of the periodic table's actinide series.

Question: What are Actinides Used for?

The actinides are particularly useful due to their radioactivity. These components can be used as energy sources for a wide range of applications, including cardiac pacemakers and electrical energy generation for lunar instruments. Nuclear bombs and nuclear power plants have both used uranium and plutonium.

Question: Why are All Actinides Radioactive?

The nuclear instability of actinide elements causes their radioactivity. The nucleus of an actinide element undergoes radioactive decay to become more stable, releasing gamma rays, alpha particles, beta particles, or neutrons.