Actinides: History, Occurence, Properties, and Uses

Actinides are a group of fifteen metal elements with atomic numbers ranging from 89 to 103. The name of the series is derived from actinium, the element that appears at the start of the series. Chemically, each member of the series is similar to the others.

Both the Actinide Series and the Lanthanide Series are referred to as Rare Earth Metals. These substances are all radioactive and have a wide range of oxidation numbers. The most prevalent and well-known element is uranium, which is transformed into plutonium through a nuclear reaction and utilized as nuclear fuel.

Actinide – Introduction

Actinides are elements with atomic numbers ranging from 90 to 103, which come after the element Actinium. They include eleven transuranic elements, or those created artificially through nuclear reactions, as well as naturally occurring forms of thorium, protactinium, and uranium. All actinides, however, are radioactive.

The Actinides series consists of the following 15 elements: Actinium (Ac), Thorium (Th), Protactinium (Pa), Uranium (U), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Californium (Cf), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), and Lawrencium (Lr).

History

Actinides are a class of elements with similar properties to lanthanides. Transuranium elements, which appear after uranium in the periodic table, and transplutonium elements, which appear after plutonium, are two overlapping families of actinides.

Enrico Fermi hypothesized the presence of transuranium elements in 1934 based on his experiments. Even though there were four actinides known at the time, it wasn’t yet known that they belonged to the same family as lanthanides.

Uranium and thorium were the first actinides to be found, by Klaproth in 1789 and Berezelius in 1829, respectively. However, the majority of actinides were created by humans in the 20th century. Small amounts of actinium and protactinium can be discovered in nature as breakdown products of 253- and 238-uranium. Plutonium is created in minute quantities naturally by the neutron capture process of uranium. The main source of thorium ore is monazite. It is a phosphate ore with significant Lanthanide content. Due to its appearance as masses of dark, pitch-like material, the primary uranium ore, U3O8, is also known as pitchblende. Beyond uranium, all elements are synthetic. Because many actinides are radioactive, they must be handled carefully.

Occurrence

The most prevalent actinides in nature are uranium and thorium, with mass concentrations of 16 ppm and 4 ppm, respectively.

The majority of uranium is found in the Earth’s crust as a combination of its oxides in the mineral uraninite, often known as pitchblende due to its dark hue.

There are a large number of other uranium minerals, such as

• autunite (Ca (UO₂)₂(PO₄)₂. nH₂O)
• carnotite (KUO₂VO₄. 3H₂O).

 The three isotopes that make up natural uranium are:

• ²³⁸U (relative abundance: 99.2742%)
• ²³⁵U (relative abundance: 0.7204%), and
• ²³⁴U (relative abundance: 0.0054%).

The most abundant thorium minerals are:

• thorianite (ThO₂)
• thorite (ThSiO₄) and 
• monazite, ((Th, Ca, Ce) PO₄).

Only about 5* 10^-15% of the Earth’s crust contains actinium. Although it is found in considerably lesser amounts in other minerals, actinium is mostly found in uranium-containing minerals. Samples of lunar soil did not contain any traces of plutonium. The majority of plutonium is created synthetically due to its rarity in nature.

Actinides: Elements Name and Symbol

Position of Actinide in Periodic Table

The actual position of actinides in the periodic table is group number 3 and period number 7. In the seventh period, the electrons are preferentially filled in the inner 5f subshell. The fourteen elements following actinides (Th to Lr) show similar chemical properties.

Hence, they are grouped and placed at the bottom of the periodic table with a general electronic configuration: [Rn] 5f^2-14 6d^0-2 7s²

Characteristics of Actinides

Actinides have the following characteristics:

• They’re all radioactive. There are no stable isotopes of these elements.
• Actinides have strong electropositive potential.
• In the air, the metals tarnish easily. These substances can spontaneously fire in the air and are particularly pyrophoric as finely separated powders.
• Metals called actinides have peculiar structures and are quite dense. Many different types of allotropes can develop; plutonium has at least six. Actinium is an exception since it has fewer crystalline phases.
• They produce hydrogen gas when they interact with hot water or weak acid.
• The majority of actinide metals are soft. Knives can be used to cut some.
• These substances are ductile and malleable.
• Actinides are all paramagnetic elements
• These substances are all solid at room temperature and pressure and have the color silver.
• Most nonmetals can be combined directly with actinides.
• The 5f sublevel is gradually filled with actinides. Numerous actinide metals exhibit both d-block and f-block characteristics.
• Actinides often exhibit more valence states than lanthanides. Most are open to blending.
• The actinides (An) can be created by reducing AnF₃ or AnF₄ at 1100–1400 degree Celsius with vapors of Li, Mg, Ca, or Ba.

Electronic configurations of actinides

Oxidation states of Actinides

Actinides show variable oxidation states because of the smaller energy gap between 5f, 6d, and 7s orbitals. Other oxidation states are possible despite the fact that 3+ is the most stable oxidation state due to the efficient shielding of f-electrons.

The maximal oxidation state initially rises until about halfway through the series, after which it falls; for example, it rises from +4 for Th to +5, +6, and +7 for Pa, V, and Np but falls for the next elements.

• The most prevalent oxidation state for actinides, like lanthanides, is +3. This condition isn’t usually the most stable, though, in contrast to the first four constituents (Th, Pa, U, and Np).
• U³⁺ is readily oxidized, for instance, in solution in the air. The most stable state for the later components Am –> Lr is the +3 state (except No).
• The most stable oxidation states for the first four elements are Th (+4), Pa (+5), and U (+6). In these high oxidation states, all of the outer electrons, including the f- electrons, are utilized for bonding.
• Np is oxidizing and has an oxidation state of +7, while its most stable state is +5.
• Pu displays all oxidation states, with +4 being the most stable, from +3 to +7.
• Am represents the +2 to +6 range of oxidation states. The Am²⁺ is set up in the f⁷ configuration. However, it only exists as fluoride in solid form and is the equivalent of Eu²⁺. The most stable state, however, for Am and essentially all of the other elements is +3.
• From Th to Bk, all elements exist in the +4 oxidation state.
• Es²⁺, Fm²⁺, and Cf²⁺. Md²⁺ and No²⁺ are present as ions in solutions.

Chemical Reactivity of Actinides

Actinides are more reactive and electropositive than lanthanides due to the lower ionization energy. They react with hot water. When exposed to oxidizing chemicals, it forms a passive coating. create hydrides and halides. Actinides are potent reducers. Because they are less ionized in energy than lanthanides, actinides are thought to be electropositive and more reactive. Actinides begin to react when placed in hot water. Usually, when they come into contact with an oxidizing chemical, they develop a passive coating. Halides and hydrides then follow. The best illustration of efficient lowering agents are actinides.

Only americium and thorium are not thought to be very high-density actinides. One of the actinides, the lanthanides have a far greater melting point than the others but no boiling point.

Formation of Colored Ions

Similar to lanthanides, actinides also feature ions with unfilled orbitals and f-orbital electrons. The f-f electron transition creates a visible color when a specific frequency of light is absorbed. 

Ions with 2 to 6 electrons in 5f-orbitals show color in aqueous solution as well as crystalline structures. The color is provided via the f-f transition. The majority of actinide ions are colored. 

The quantity of electrons in 5f-orbitals affects the color of the ions. The ions with zero 5f-orbital electrons (i.e., 5f⁰) or seven 5f-orbital electrons (i.e., 5f⁷) are colorless.

The colored ions in different charges are shown in the below table:
 

Formation of complexes

Actinides are marginally more likely to produce complex compounds than lanthanides. Their smaller ions and higher charge are what cause this.

The majority of the actinide halides form complex combinations when they interact with alkali metal halides.

Chelates are created when actinides react with organic compounds like oxime and EDTA.

For the ions M⁴⁺, MO₂²⁺, M³⁺,and MO²⁺ the degree of complex formation declines in the order.
M⁴⁺> MO₂²⁺> M³⁺> MO²⁺.

Physical Properties of Actinides

• All actinides are silvery metals.
• All actinides, with the exception of thorium and americium, have extremely high densities.
• Unlike lanthanides, which have obvious trends in their melting and boiling points, actinides have relatively high melting points.
• Actinides all possess paramagnetic properties, which are dependent on the existence of unpaired electrons. The shielding of the 5f electrons causes the orbital angular moment to be quenched, resulting in a difference between the calculated and observed magnetic moments.
• This results in a “actinide contraction,” which is comparable to the lanthanide contraction. Actinides are present in high quantities.
• Actinoids are highly reactive metals, especially when finely separated.

Uses and Applications of Actinides

• Americium and other actinides are utilized in smoke detectors.
• Most thorium is used in gas mantles.
• Scientists and researchers utilize actinium to carry out scientific research or study.
• As a gamma source, an indicator, and a neutron source, actinium is also used.
• Actinides are widely employed in the production of energy, nuclear weapons, and defense products.
• Nuclear reactors and nuclear bombs both require plutonium.
• Both nuclear power plants and the production of electronic electricity need a lot of actinide materials.
• In addition to having a unique atomic number, each actinide also has a variety of unique properties. To forecast actinide reactions, it is essential to look into their physical and chemical characteristics.
• Actinides don’t have stable isotopes.
• The isotope of Plutonium 238 was fueled by the backup generator from the Apollo 12-led lunar mission. It produced less than 1.5 kW of heat, which thermoelectric components converted to electricity. It also was used as a source of power for orbiting satellites and missions to photograph Jupiter. Additionally, ²³⁸Pu is the cardiac pacemakers’ power source. The device can be used for a longer period of time thanks to nuclear energy.

References

• J. D. Lee, Concise Inorganic Chemistry, 5th Edition, John Wiley and Sons. Inc. 2007.
• F. A. Cotton, G. Wilkinson & C. Gaus, Basic Inorganic Chemistry, 3 rd Edition, John Wiley & Sons (Asia), Pvt., Ltd., 2007.
• D. F. Shriver & P. W. Atkins, Inorganic Chemistry, 5th Edition, Oxford University Press, 2010.
• https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/4_fBlock_Elements/The_Actinides/1General_Properties_and_Reactions_of_The_Actinides#:~:text=The%20Actinide%20series%20contains%20elements,body%20of%20the%20periodic%20table.
• https://en.wikipedia.org/wiki/Actinide
• https://byjus.com/jee/actinides/
• https://www.aakash.ac.in/important-concepts/chemistry/actinides