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Actinides – Definition, Properties, Formation, Uses

  • Last Updated : 15 Mar, 2022

The d and f block mainly contains elements that include groups 3-12. The f block has elements in which 4f and 5f are progressively filled. These elements are placed below the periodic table in a separate table. The d and f block elements are majorly known as transition or inner transition elements.


Actinides are elements with atomic numbers ranging from 90 to 103 that follow Actinium. They include naturally occurring thorium, protactinium, and uranium elements, as well as eleven transuranic elements created artificially through nuclear processes. Despite this, all actinides are radioactive. The actinide series gets its name from the first element in the series, actinium. The symbol An is used to refer to any of the actinide series elements, which have atomic numbers ranging from 89 to 103 on the periodic table. All elements in the actinide series are radioactive in nature, releasing a considerable amount of energy during radioactive decay. The most numerous naturally occurring actinides on Earth are uranium and thorium, while plutonium is synthesized.

These elements are found in nuclear reactors as well as nuclear weapons. Uranium and thorium are used in a number of applications, whilst americium is used in current smoke detector ionization chambers. Actinides have the following general electrical configuration: [Rn] 5f1 – 14 6d0 – 1 7s2. Radium is the nearest noble gas, and its electronic configuration is [Rn].

Physical Properties of Actinides 

  1. They are all radioactive. There are no stable isotopes of these elements.
  2. Actinides have a strong electropositivity.
  3. Metals tarnish quickly in the air. These elements are pyrophoric (ignite spontaneously in the air), especially as finely split powders.
  4. Actinides are metals that are extremely dense and have different structures. There are numerous allotropes that can form—plutonium has at least six allotropes. Actinium is an exception, as it contains fewer crystalline phases.
  5. Actinide metals are often soft. Some of them can be sliced with a knife.
  6. These elements have a malleable and ductile nature.
  7. The actinides are all paramagnetic.
  8. They produce hydrogen gas when they react with hot water or dilute acid.
  9. At room temperature and pressure, all of these elements are silver-colored metals that are solid.
  10. Most nonmetals mix immediately with actinides.

Chemical Properties of Actinides 

  1. All actinides, like lanthanides, are extremely reactive with halogens and chalcogens; however, actinides react more readily. Actinides, particularly those with a low number of 5f-electrons, are susceptible to hybridization.
  2. Actinium and lanthanum are chemically related, as evidenced by their comparable ionic radii and electronic structures.
  3. Actinium, like lanthanum, usually invariably has an oxidation state of +3 in compounds, but it is less reactive and has stronger basic characteristics.
  4. Thorium is a chemically active element. Tetravalent thorium compounds are colorless due to a lack of electrons on the 6d and 5f orbitals.

f-block Elements, Actinides, and Lanthanides

With atomic numbers ranging from 57 to 71, actinides are the second series of f-block elements, while lanthanides are the first series of f-block elements. Actinides are radioactive in nature, but lanthanides, with the exception of promethium, are not. Lanthanides are soft metals with a silvery-white appearance. Lanthanides contraction occurs when the atomic and ionic radii of lanthanum drop from lanthanum to lutetium. Lanthanides are good conductors of electricity and heat, with melting values ranging from 1000 K to 1200 K, with the exception of Samarium, which has a melting point of 1623 K. The properties of f block elements are such that electrons are added to the ‘f’ sub-orbitals of the n – 2 level, and they are located in the periodic table between (n – 1) d and ns block elements. Their attributes are identical to those of d-block elements.

In terms of similarities, both Actinides and Lanthanides have a dominant oxidation state of +3. Both contribute to the filling of (n – 2) f-orbitals. Both have a high electropositivity and are very reactive in nature. With a rise in atomic number, there is a decrease in ionic and atomic size. Magnetic characteristics are shared by actinides and lanthanides.

Actinide Contraction

Because of the growing nuclear charge and electrons entering the inner (n – 2) f orbital, the ionic radii or atomic size of tri positive actinide ions tend to decrease continuously from Th to Lw. As a result, this steady decrease in size with increasing atomic number is known as actinide contraction, and it occurs similarly to lanthanide contraction. Because of the inadequate shielding by 5f electrons, contraction may be greater along the period.

Electronic Configuration

Actinides are the second series of f-block elements, with a terminal electronic configuration of [Rn] 5f1-14 6d0-1 7s2. Because the energies of 5f and 6d electrons are near, electrons enter the 5f orbital.

Actinide Contraction

The atomic size/ionic radius of tri positive actinides ions falls progressively from Th to Lw due to increased nuclear charge and electrons entering the inner (n-2) f orbital. Actinide contraction, like lanthanide contraction, refers to a gradual reduction in size as the atomic number increases. Due to the weak shielding provided by 5f electrons, contraction is stronger over the period.

Formation of Coloured Ions

Actinides, like lanthanides, have electrons in f-orbitals as well as empty orbitals, as do d-block elements. The f-f electron transition creates visible color when a frequency of light is absorbed.


Because 5f electrons are more effectively protected from nuclear charge than 4f electrons, actinides have lower ionization enthalpies than lanthanides.

Oxidation State

Because of the narrower energy difference between the 5f, 6d, and 7s orbitals, actinides have varying oxidation states. Although 3+ is the most stable oxidation state, more oxidation states are possible due to the significant shielding of f-electrons. The maximal oxidation state increases up to the middle of the series and then declines; for example, it increases from +4 for Th to +5, +6, and +7 for Pa, V, and Np but falls in the next elements.

Formation of Complexes

Because of their smaller size but higher nuclear charge, actinides are superior complexing agents than lanthanides. In the sequence of appearance, the degree of complexion lowers.

M4+ > MO22+ > M3+ > MO22+

Chemical Reactivity

Actinides are more electropositive and reactive than lanthanides due to their lower ionization energy. They react when exposed to hot water. Form a passive coating by reacting with oxidizing substances. Halides and hydrides are formed. Actinides are extremely effective lowering agents.

Physical Properties

  1. Except for thorium and americium, all actinides have extremely high densities.
  2. Actinides, like lanthanides, have relatively high melting points, but there is no discernible pattern in the melting and boiling temperatures of lanthanides.
  3. Because of the existence of unpaired electrons, all actinides are paramagnetic. Because of the shielding of 5f electrons, the orbital angular moment is quenched, and the observed magnetic moment is smaller than the calculated magnetic moment.

Similarities Between Lanthanides and Actinides

The (n-2) f subshell is employed for filling and characterization of all Lanthanides and Actinides. Lanthanides and Actinides have very similar electrical configurations. The following are some of the significant commonalities between these two,

  1. Lanthanides and actinides have a high Oxidation State of +3.
  2. The filling of these elements involves (n – 2) f orbitals.
  3. Lanthanides and actinides are both reactive and electropositive.
  4. As the atomic number of these elements increases, so do their ionic and atomic sizes.
  5. Lanthanides and actinides both have strong magnetic characteristics.

Differences Between Lanthanides and Actinides

  1. The filling of Lanthanides involves 4f-orbitals, whereas the filling of Actinides involves 5f-orbitals.
  2. The energy that binds this 4F atom is smaller than that of actinides, which is 5F electrons.
  3. The shielding of 5F electrons is also less than that of 4F electrons.
  4. The paramagnetic characteristics of Lanthanides are fairly simple to explain. In the case of Actinides, however, it is difficult to explain all of the paramagnetic features.
  5. Except for Promethium, the majority of Lanthanides are non-radioactive. The elements in the Actinide series are all radioactive.
  6. There are multiple oxocations of the elements in the Actinides class, but none in the Lanthanides.
  7. In contrast to the chemicals found in Lanthanides, the compounds generated by Actinides are highly basic in nature.

Availability of Actinide

The actinide elements thorium and uranium are prevalent in the earth’s crust. Uranium also contains trace amounts of Plutonium and Neptunium. A variety of synthetic elements are found in the actinide series. Because they are not produced naturally, but rather as a result of the decay of a component of a heavier element, these elements are referred to as synthetic elements. When exposed to air, the actinide element tarnishes.

Uses and Applications of Actinides

  1. Americium and other actinides are utilized in smoke detectors.
  2. Thorium is mostly employed in gas mantles.
  3. Actinium is used by scientists and researchers to conduct scientific research or study.
  4. Actinium is also employed as a gamma source, an indicator, and a neutron source.
  5. A significant number of actinides are used in defense activities, nuclear weapons, and energy generation.
  6. Plutonium is used in nuclear reactors as well as nuclear bombs.
  7. Many actinide elements are employed in nuclear power plants as well as in the creation of electronic power.
  8. Every actinide is distinguished by its own atomic number as well as its various features and characteristics. It is critical to investigate the chemical and physical features of actinides in order to predict their reactions.
  9. The actinides lack stable isotopes.

Sample Problems

Question 1: What are actinides?


Following the element Actinium, actinides are elements with atomic numbers ranging from 90 to 103.

Question 2: What are actinides used for?


Nuclear reactors and nuclear weapons both employ these materials. Uranium and thorium are being used in a variety of applications, whereas americium is used in the ionisation chambers of modern smoke detectors.

Question 3: What are the trends observed with the chemical reactivity of actinides?


Actinides are more electropositive and reactive than lanthanides due to their lower ionisation energy. When they come into contact with hot water, they react. Form a passive coating by reacting with oxidising substances. Halides and hydrides are formed. Actinides are powerful reducers.

Question 4: What are the physical properties of actinides?


  1. Except for thorium and americium, all actinides have extremely high densities.
  2. Actinides, like lanthanides, have relatively high melting points, but there is no discernible pattern in lanthanide melting and boiling temperatures.
  3. In nature, all actinides are paramagnetic, which is determined by the existence of unpaired electrons. Because of the shielding of 5f electrons, the orbital angular moment is quenched, and the observed magnetic moment is less than the calculated.

Question 5: What is actinide contraction?


Due to increasing nuclear charge and electrons entering the inner (n-2) f orbital, the atomic size/ ionic radii of tri positive actinides ions decrease progressively from Th to Lw. Actinide contraction, like lanthanide contraction, is a steady decrease in size with rising atomic number. The contraction is larger over the period due to the inadequate shielding provided by 5f electrons.

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