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Actinides: Definition, Properties, Uses, and Types

Xomety X
By Team Xometry
May 2, 2024
 16 min read
Mark Osterman, VP of Technical Sales and Pre-Sales Engineering
June 7, 2024
 3 min read

The actinides, or actinoids, are a series of metallic elements that run from actinium (Ac) through lawrencium (Lr) on the periodic table. These elements are known for their radioactive properties and typically fill the 5f electron shell, with some variation due to interelectronic repulsion. The actinides include both naturally occurring elements such as uranium and thorium, and synthetic elements such as americium and curium. 

The actinides are distinguished by their radioactivity and huge atomic and ionic radii, and they display a wide range of oxidation states. Actinides play a critical role in nuclear chemistry, as they find use in the production of smoke detectors, nuclear reactors, and weapons.

This article will discuss actinides: their definition, properties, uses, and types.

What Are Actinides?

Actinides are a group of elements known for their high levels of radioactivity, making them crucial in applications of nuclear energy. They constitute the f block of elements in the periodic table. These elements are all metals, ranging from actinium, with atomic number 89, up to lawrencium, with atomic number 103. The properties of actinides include: high density, metallic appearance, and a variety of possible oxidation states. 

What Are the Different Properties of Actinides?

Some properties of actinides include:

1. Density

Actinides have high densities due to actinide contraction, a trend where atomic size decreases as atomic numbers rise. Poor shielding by 5f electrons leads to a stronger nuclear pull on 6d and 7s electrons, causing a reduction in atomic radius. This allows atoms to pack more densely, increasing the series' density, particularly in later actinides, influencing their chemical properties.

2. Radioactivity

All actinides are radioactive, which makes them useful in nuclear reactors and nuclear medicine. However, it also means that they can pose serious health and environmental risks if not handled properly.

3. Transuranic Elements

Transuranic elements (atomic numbers 93-103) are synthetic actinides, including neptunium and lawrencium, produced in nuclear settings. Emitting alpha particles, their isotopes have varying half-lives and energies, affecting their health hazard levels. Additionally, not all of the actinides are transuranic. Elements like berkelium and einsteinium are used for research, while neptunium and plutonium are more abundant and pose significant radioactive waste concerns due to their long-lived isotopes. Californium serves as a neutron source, highlighting the diverse applications and risks of these elements.

4. Oxidation States

The oxidation states of actinides are a notable property due to their variety and complexity. Actinides can exhibit multiple oxidation states, typically ranging from +3 to +6. The most common oxidation state for actinides is +3, and they often form oxides in this state, such as M2O3, where (M) represents an actinide element. Examples of uranium oxidation states include +3, +4, +5, and +6, with +6 being prevalent in compounds such as uranium dioxide UO2 and uranyl UO2^{2+}. The complicated chemistry of actinides and their capacity to create a wide range of compounds are facilitated by their variety of oxidation states.

5. Malleability and Ductility

Actinides exhibit a silvery hue and possess a soft texture. Known for their malleability and ductility, they can be easily shaped or stretched. Thorium, one of the actinides, shares a comparable hardness with soft steel. When heated, thorium becomes pliable enough to be fashioned into thin sheets or drawn into wires. Thorium's density is about half that of uranium, making it significantly lighter yet still robust.

6. Magnetic Properties

The actinides exhibit interesting magnetic properties. Like the lanthanides, actinide elements are strongly paramagnetic. This means they are attracted to magnetic fields. The magnetic susceptibility of actinides increases with the number of unpaired electrons, similar to lanthanides, but the values are generally higher for activists.

7. Chemical Reactivity

Actinides are known for having a high degree of chemical reactivity. They are more electropositive than lanthanides because of their low ionization energies. They readily react with hot water, and oxidizing agents, and form passive coatings. Actinides are strong reducing agents and their reactivity is further influenced by their radioactive nature, which affects their electron arrangements and chemical bonds.

8. Melting and Boiling Points

The melting and boiling temperatures of actinides are relatively high. These characteristics show that these elements have strong metallic bonds. For instance, Actinium, the first element in the series, has a melting point of approximately 1050 °C and a boiling point reaching 3200 °C. Moving further along the series, Neptunium, another actinide, exhibits a melting point of 637 °C and an impressive boiling point of 4174 °C. The significant melting and boiling points of actinides reflect the considerable amount of energy required to break their metallic bonds. This, coupled with their inherent radioactivity, renders the actinides a group of great interest and importance in the field of nuclear chemistry.

9. Nuclear Properties

Actinides, spanning atomic numbers 89 to 103, are all radioactive and lack stable isotopes. Highly electropositive, they react with air and some are pyrophoric. These elements are pivotal in nuclear reactions, fueling reactors and weapons. Handling them requires caution due to their radioactivity and potential hazards. Notably, Uranium powers reactors, while Americium is used in smoke detectors, showcasing their diverse applications despite the inherent risks.

10. Complex Formation

Actinides are known for their ability to form complex compounds, a trait stemming from their small ionic sizes and high charges. These elements readily bond with ligands, creating stable complexes. The propensity for complex formation is influenced by the ion's charge and size, with a higher charge and smaller size leading to stronger and more stable bonds. This characteristic is exemplified in the order of complex stability: M4+ > MO2^2+ > M3+ > MO2+, where M represents an actinide metal. Such complexity is crucial in both nature and technology.

Are Actinides Metals?

Yes, actinides are metals. This is a group of fifteen metallic elements ranging from actinium (Ac) to lawrencium (Lr) on the periodic table. The radioactivity of actinide elements is one of its characteristics. One characteristic that distinguishes this group is the progressive filling of the 5f electron shell with an increase in atomic number.

What Are Examples of Actinides?

Actinides are a group of elements that appear on the periodic table between actinium (Ac, atomic number 89) and lawrencium (Lr, atomic number 103). Among these, the first four — actinium (Ac), thorium (Th), protactinium (Pa), and uranium (U) — occur naturally. Synthetic actinides which include Americinium (Am), plutonium, and Neptunium have been created in laboratories through nuclear reactions. Uranium-235, an isotope of uranium, is widely used in nuclear reactors for electricity generation, while plutonium-239 is utilized in nuclear weapons and some nuclear reactors.

What Are the Uses of Actinides?

Actinides, a group of radioactive elements, are primarily used in the nuclear industry as fuel for nuclear reactors and in the production of nuclear weapons due to their radioactive properties. They are used in the medical field as part of radiation therapy for cancer treatment. Actinides, such as thorium, are also used in the glass and ceramics sector, especially in the production of high-grade glass and as a catalyst in industrial operations. For example, glass-ceramics based on zirconolite are thought to be useful in waste management because they can immobilize actinides. management.

What Industries Use Actinides?

Industries that use actinides include: the nuclear power industry for electricity generation; the defense industry for nuclear weapons; the medical industry for cancer treatment; and the glass and ceramics industries for coloration and material properties.

What Are the Different Types of Actinides?

Few actinides occur naturally. The majority are synthetic. All are radioactive. The following list briefly summarizes the characteristics of each of the fifteen actinides:

1. Plutonium

Plutonium (Pu), with atomic number 94, is a radioactive chemical element. It is silvery-gray, but when exposed to air, it tarnishes and forms a dull oxide coating. Plutonium is used as a fuel in nuclear reactors and to make nuclear weapons. Its most important isotope is plutonium-239, which is a key fissile component in nuclear reactors and nuclear weapons.

2. Uranium

Uranium, with atomic number 92 and symbol U, is a heavy, silvery-white, weakly radioactive metal. Uranium, the heaviest naturally occurring element, is notable for its radioactive properties and its isotopes' ability to undergo nuclear fission. Uranium-235 is sought after for nuclear reactors and atomic weaponry due to its fissile nature. Uranium-238, the most prevalent isotope, serves as a decay series progenitor, while Uranium-234, found in trace amounts, also contributes to natural decay chains.

3. Thorium

Thorium, with the symbol Th and atomic number 90, is a naturally occurring radioactive element. It surpasses uranium in abundance in the Earth's crust. Thorium has several significant applications. First of all, it is used as a nuclear fuel, especially in reactors with high temperatures. Its attraction over uranium-based fuels stems from its ability to breed into fissile uranium-233. Second, substances containing thorium are used in optics. As a result of their refractive qualities, they are used to make high-quality lenses for scientific instruments and cameras.

4. Einsteinium

Einsteinium, with the atomic number 99 and the symbol Es, is a synthetic element named in honor of Albert Einstein. It was discovered as a component of the debris from the first hydrogen bomb explosion in 1952. Although einsteinium has no significant economic uses due to its extreme radioactivity, it plays a crucial role in research. Notably, einsteinium-253, its most common isotope, is produced artificially from the decay of californium-253. This isotope is used for physicochemical studies and has a half-life of 20.47 days. Additionally, einsteinium has contributed to the creation of other heavier elements in scientific investigations.

5. Californium

Californium, symbolized as Cf and with atomic number 98, is a synthetic radioactive element. It's primarily used in devices known as neutron moisture gauges to detect water and oil in wells and as a neutron source in nuclear reactors for initiating reactions. Due to its ability to produce neutrons for use in neutron therapy, the isotope Californium-252 is noteworthy for its use in the treatment of cancer. It is also useful in industrial radiography and explosive identification due to its high neutron emission rate.

6. Americium

Americium, with the symbol Am and atomic number 95, is a synthetic element notable for its radioactivity. It's primarily used in smoke detectors, where it acts as a source of alpha particles to detect smoke. Americium-241 is a particularly helpful isotope for both medical and smoke detection purposes. It offers gamma rays as an alternative to beta particle radiation, which is more frequently utilized in cancer treatments.

7. Neptunium

The radioactive element neptunium has the atomic number 93 and the symbol Np. Its main application is as a precursor to produce plutonium-238, which powers spaceship electrical generators using radioisotope thermoelectric generators.

8. Actinium

Actinium is a soft, silvery-white radioactive metal with the symbol Ac and atomic number 89. It is used as a neutron source and in thermoelectric power generation. Actinium-225 is being studied for use in targeted alpha therapy, a type of radiation therapy for cancer.

9. Curium

Curium is a synthetic radioactive element with the symbol Cm and atomic number 96. It is used in alpha-particle X-ray spectrometers aboard lunar and Mars rovers to analyze the surface composition of these celestial bodies. Curium isotopes are also considered for use in radioisotope thermoelectric generators for space missions.

10. Lawrencium

Lawrencium is a synthetic radioactive element with the symbol Lr and atomic number 103. It is the heaviest element in the actinide series and is primarily used for scientific research purposes due to its short half-life and instability.

11. Fermium

The synthetic element fermium (Fm) has an atomic number of 100. Since it is scarce, it is highly radioactive, and has a short half-life, it has no major commercial applications. The main applications of fermium are in science, specifically in the study of nuclear processes and elemental synthesis research.

12. Nobelium

Nobelium (No), with atomic number 102, is a transuranic radioactive element that has been instrumental in nuclear research. It has aided in the study of atomic reactions and the synthesis of heavier elements. The isotope Nobelium-259, known for being the most stable, has a half-life of approximately 58 minutes.

13. Protactinium

Protactinium is a radioactive chemical element with the symbol Pa and atomic number 91. Its melting and boiling points are respectively 1568 °C and 4027 °C. Its metallic luster is bright and silvery, and its density is 15.37 g/cm³. Known for its intense radioactivity, protonium is less common than radium.

14. Mendelevium

Mendelevium is a synthetic chemical element with the symbol Md and atomic number 10. Discovered in 1955 at the Lawrence Berkeley National Laboratory, Mendelevium does not occur naturally and can only be produced in particle accelerators by bombarding lighter elements with charged particles. The most stable isotope of Mendelevium is 258 Md, which has a half-life of 51.5 days. However, 256 Md is more commonly used in chemistry despite its shorter half-life of 1.17 hours. Predicted to be solid at room temperature, Mendelevium has a melting point of 1100 K (800 °C) and a density of 10.3 g/cm³.

15. Berkelium

Berkelium, with the symbol Bk and atomic number 97, is another synthetic element used primarily for scientific research. It has been used to synthesize heavier transuranic elements and trans-actinides.

How To Choose Which Actinide To Use?

Here's a list to guide you in choosing the right type of actinide for your needs:

  1. Application: Match the actinide to the intended use, such as nuclear power or medical imaging.
  2. Radioactivity: Consider the level of radioactivity and the ability to manage it safely.
  3. Availability: Natural actinides like uranium and thorium are more accessible than synthetic ones.
  4. Cost: Factor in the cost, especially for synthetic actinides which are more expensive to produce.
  5. Safety Protocols: Ensure proper safety measures are in place for handling radioactive materials.
  6. Regulatory Compliance: Check that the use of the actinide complies with all relevant regulations.
  7. Environmental Impact: Actinides, like uranium and plutonium, are radioactive and pose significant environmental risks. Proper waste management is crucial to minimize contamination and long-term ecological damage. Strategies include containment, immobilization, and monitored storage to mitigate the impact of their long half-lives and potential bioaccumulation.

Which Actinides Are Used for Medical Purposes?

In medicine, actinium (Ac), more especially the isotope actinium-225, is employed. It releases alpha particles, which are useful in targeted alpha therapy (TAT), especially in the treatment of cancer. When actinium-225 is bonded to compounds that target cancer, the alpha particles can kill cancer cells. Actinides like americium and curium are used in particular nuclear medicine diagnostic and therapeutic roles, helping to identify and treat a variety of medical conditions.

Which Actinides Are Used for Nuclear Applications?

Actinides like uranium and plutonium are almost always used for nuclear applications. Uranium is widely used as a fuel in nuclear reactors, and plutonium is used in both nuclear reactors and nuclear weapons. Thorium is another actinide that has been recognized for its potential in nuclear energy applications. It is being explored as an alternative nuclear fuel in reactors, particularly in the development of thorium-based nuclear power systems. These actinides are essential for the production of energy, but they also pose significant challenges in terms of waste management and environmental impact due to their long half-lives and potential for bioaccumulation.

What Is the Advantage of Using Actinides?

Actinides, with their potent nuclear reactions, offer a significant energy source, particularly as nuclear fuel. The radioactive nature of actinides extends their utility beyond energy production to diverse fields such as medical devices, including cardiac pacemakers, and deep-space exploration missions. Nuclear power has the advantage of having a clean energy profile because it produces no greenhouse gases. RTGs, or radioisotope thermoelectric generators, use actinides to generate continuous power for spaceship systems. Furthermore, nuclear medicine highlights the adaptability and life-improving advantages of actinides by using their special qualities to diagnose and treat medical diseases that would otherwise be incurable.

What Is the Disadvantage of Using Actinides?

The main disadvantage of using actinides is their radiotoxicity and the potential environmental impact if they are not properly contained or disposed of. Their radioactive nature requires careful handling and long-term storage solutions.

Are Actinides As Toxic as Heavy Metals?

Due to their radiotoxicity, actinides are typically more hazardous than heavy metals. In contrast to heavy metals, which mainly induce chemical toxicity, they present both radiation and chemical dangers.

How Do Actinides Contribute to the Production of Nuclear Energy?

Actinides, such as uranium, thorium, and plutonium, serve as vital energy sources in nuclear reactors. These elements undergo fission reactions, which release substantial amounts of energy. After that, this energy is captured and used to generate nuclear electricity. These actinides serve as the primary fuel for nuclear energy production. This fuel runs out over time and needs to be replaced to keep the reactor producing energy.

Are Actinides Rare Earth Metals?

No, actinides are not typically classified as rare earth metals. Actinides are different from lanthanides in that they are generally radioactive and contain artificially created elements, even though they are commonly referred to as rare earth metals together because of their similar characteristics.

What Is the Difference Between Actinides and Lanthanides?

Actinides and lanthanides, two distinct series of elements, exhibit differences in several key areas. Actinides typically have a darker appearance and exhibit a wider range of oxidation states compared to lanthanides metals. They also differ in their binding energies, stability, radioactivity levels, and electron configurations. While both series have their own set of common oxidation states and atomic numbers, these properties vary between the two groups. In terms of radioactivity, elements like promethium among the lanthanides are known to be radioactive, but this trait is common to all actinides. The periodic table's period 7 houses the actinides, where the 5f orbital is progressively filled as one moves from elements with lower atomic numbers to those with higher ones. Conversely, period 6 contains the lanthanides, characterized by the gradual filling of the  4f orbital.


This article presented actinides, explained them, and discussed their properties and various applications. To learn more about actinides, contact a Xometry representative.

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.