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:
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.
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.
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 on Chemical Properties of the Actinoids
1. What are actinoids and where are they located in the periodic table?
The actinoids are a series of 14 metallic chemical elements with atomic numbers from 90 (Thorium) to 103 (Lawrencium). They are named after the first element in the series, Actinium (Ac). In the periodic table, they are placed at the bottom, forming the second row of the f-block elements, also known as the second inner transition series. Their general electronic configuration is [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s².
2. What are the main chemical properties of the actinoids?
Actinoids are highly reactive metals with several key chemical properties:
- Reactivity: They are highly electropositive and react with boiling water or dilute acids, liberating hydrogen gas.
- Pyrophoric Nature: When finely divided, many actinoids are pyrophoric, meaning they ignite spontaneously in the air.
- Reaction with Non-metals: They react with most non-metals upon heating.
- Complex Formation: Actinoids have a stronger tendency to form complexes than lanthanoids due to their higher charge density.
- Basicity: Actinoid hydroxides are basic in nature.
3. Why do actinoids exhibit a wider range of oxidation states than lanthanoids?
Actinoids show a wider range of oxidation states (from +3 to +7) because the energy difference between the 5f, 6d, and 7s orbitals is very small. This allows electrons from all three subshells to participate in chemical bonding. In contrast, the 4f orbitals in lanthanoids are more deeply buried and have a larger energy gap to the outer orbitals, making it energetically difficult to remove more than three electrons. Therefore, lanthanoids are mostly restricted to the +3 oxidation state.
4. What is actinoid contraction and how does it compare to lanthanide contraction?
Actinoid contraction is the steady decrease in the atomic and ionic radii of the elements across the actinoid series. This occurs because the incoming electrons enter the 5f subshell, where they provide very poor shielding of the nucleus. As the atomic number increases, the effective nuclear charge on the outermost electrons increases, pulling them closer. The contraction is actually more pronounced in actinoids than the lanthanide contraction because 5f electrons have a more diffused shape and are even less effective at shielding than 4f electrons.
5. Why are the chemical properties of actinoids less studied and understood than those of lanthanoids?
The study of actinoid chemistry is challenging for two main reasons:
- Intense Radioactivity: All actinoids are radioactive. Their instability and the hazardous nature of their radiation require specialised, expensive shielded laboratories (hot labs) for safe handling, which limits extensive research.
- Availability: Most actinoids beyond uranium are synthetic and produced in only minute, often milligram or microgram, quantities through nuclear reactions. This scarcity makes it difficult to perform comprehensive chemical experiments.
6. Explain the magnetic properties of actinoids.
The magnetic properties of actinoids are generally more complex than those of lanthanoids. Most actinoid ions are paramagnetic. However, their magnetic moments are not as accurately predicted by simple models as those for lanthanoids. This is because the 5f electrons are less effectively shielded and can interact with their surroundings and participate in bonding, a phenomenon known as orbital quenching, which complicates their magnetic behaviour.
7. What are some important real-world uses and applications of actinoids?
Despite their radioactivity, actinoids have several critical applications:
- Nuclear Fuel: Isotopes like Uranium-235 and Plutonium-239 are used as fuel in nuclear reactors to generate electricity.
- Nuclear Weapons: Plutonium-239 is a key fissile component in the construction of nuclear weapons.
- Smoke Detectors: A tiny amount of Americium-241 is used in many household smoke detectors, where its alpha particle emissions ionise the air to create a current.
- Power Sources: The heat generated from the radioactive decay of elements like Plutonium-238 is used in radioisotope thermoelectric generators (RTGs) to power space probes and rovers.
8. How does the electronic configuration of actinoids directly influence their chemical behaviour?
The electronic configuration [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s² is fundamental to understanding actinoid chemistry. The very small energy gap between the 5f, 6d, and 7s orbitals has direct consequences:
- It allows for variable oxidation states because electrons can be lost or shared from all three subshells, not just the valence shell.
- It makes the 5f electrons more available for bonding than the 4f electrons in lanthanoids, leading to greater reactivity.
- The availability of these orbitals facilitates the formation of a wide variety of coordination complexes.
9. Why are actinoids generally considered more reactive than lanthanoids?
Actinoids are more reactive primarily because their 5f orbitals extend further into space relative to the nucleus compared to the more contracted and deeply buried 4f orbitals of lanthanoids. This makes the 5f electrons more accessible and more likely to participate in chemical bonding. As highly electropositive metals, they react readily with hot water and acids to release hydrogen gas and tarnish quickly in the air, forming an oxide layer.
10. Do actinoids form chemical complexes? Explain why.
Yes, actinoids have a very strong tendency to form chemical complexes. This ability stems from their atomic properties: they possess a high charge density due to their small ionic size and high nuclear charge. Furthermore, their 5f orbitals are available to accept electron pairs from electron-donating ligands. This allows them to form stable complexes with ligands like EDTA and oxalates, and even π-complexes with organic molecules like cyclooctatetraene.
