What are Actinides?

The elements in which the extra electron tends to enter the 5f-orbitals of (n–2)th main shell are considered 5f-block elements. They are also named actinides. Actinides include 15 elements, from atomic number 89 to 103.

Which elements are in the Actinide Series?

The elements of the actinide series are 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).

The term “actinide” is obtained from the first element of this series, actinium. The symbol used to represent the actinide series is “An.” Lanthanides and actinides are depicted as two separate rows placed below the periodic table.

Ionic Size of Actinides

The general trends in actinides are similar to those in lanthanides. Across the actinide series, the atomic as well as ionic sizes of elements gradually decrease. It is referred to as actinide contraction.

Actinide Contraction

Actinide contraction occurs due to the imperfect shielding of 5f-electrons. From Ac to Lw, there is a gradual decrease in the atomic size/radius and ionic radius of tri positive ions. It occurs because of an increase in the number of electrons in the n-2 f-orbital. Thus, the effective nuclear charge of 5f-electrons increases with the atomic number, which results in the contraction (shrinkage) of atomic and ionic radii from actinium (Ac) to lawrencium (Lw). This successive contraction is collectively known as actinide contraction. The size of actinide ions decreases due to actinide contraction along with an increase in the atomic number. Actinide contraction is greater than lanthanide contraction due to the poorer shielding effect of 5f-electrons.

Oxidation States of Actinides

Actinides show several oxidation states. The most common oxidation state of actinides is +3. In the first half of the actinide series, the elements exhibit high oxidation states. For example, the oxidation states of Th, Pa, U, Np are +4, +5, +6 and +7. The oxidation state decreases in succeeding elements. Only americium (Am) exists in a stable +2 state. Neptunium and plutonium exhibit the +7 oxidation state.

The oxidation states of actinides are given in the table below.

The oxidation state of Actinides

General Characteristics of Actinides

The actinides are radioactive elements. Their general characteristics are discussed below.

  • All actinides are radioactive elements that release a large amount of energy on decay.
  • All the elements of the actinide series do not have stable isotopes.
  • The earlier elements possess relatively long half-lives. And the later elements have half-lives between one day to three minutes (for lawrencium).
  • Uranium and thorium are the most abundant naturally occurring actinides, and plutonium is synthetically prepared.
  • Uranium has composed of three major isotopes—uranium-238 (U-238), uranium-235 (U-235), and uranium (U-234).
  • Naturally occurring thorium mainly exists as thorium-232 (Th-232), thorium-230 (Th-230), and thorium-228 (Th-228).
  • The radioactivity of chemical elements increases with an increase in their atomic numbers.
  • The ionization enthalpies of actinides are lower than lanthanides due to the poorer shielding of 5f electrons than 4f electrons.

Physical Properties of Actinides

Following are the physical characteristics of actinides.

  • The properties of the first half of the actinides are different from the lanthanides, whereas those of the second half of the actinides are similar to lanthanide (Ln).
  • All actinide metals are typical metals. They are silvery and soft, and they tarnish in the presence of air.
  • Actinides show a variety of structures due to the irregularities in their metallic radii.
  • They possess high density and plasticity. All the actinides possess very high density, except thorium and americium.
  • Many actinides have characteristics similar to d-block and f-block elements.
  • Actinide elements are highly electropositive.
  • All actinides, except actinium, are paramagnetic in nature.

Color and Electronic Spectra

The electronic spectra in actinide ions are f-f transition, f-d (5f-6d) transition, and charge transfer.

The absorption bands of actinide ions are ten times more intense and two times broader than those of lanthanide ions. The absorption bands in the electronic spectra of heavier elements resembles corresponding lanthanides. Also, the spectra of lighter actinides resemble transition elements.

Chemical Properties of Actinides

Actinides are highly reactive metals. They have a greater tendency to form complexes than lanthanides because they possess more electrons for bonding.

  • Actinides react with boiling water and produces hydride as hydrogen gas and oxides.
  • At moderate temperatures, actinides can be combined with most of the non-metals.
  • Actinides are more affected by hydrochloric acid than nitric acid.
  • Actinides function as strong reducing agents.
  • The metal-ligand bonds (M-L) in actinides resembling the properties of transition metals (in the first half) are covalent.
  • The metal-ligand bonds (M-L) in actinides resembling the properties of lanthanides (in the second half) are ionic.

Complex Formation Tendency

The most stable oxidation state of uranium is +6. The coordination number of complexes formed by Ur is 8. The structure of one of the complex is:

An image shows the structure of uranyl triacetate in which uranium exists as the central metal atom. It is bonded with three acetate and two oxygen ligands. The coordination number of the complex is 8.
Structure of uranyl nitrate
The image shows the structure of uranyl triacetate in which uranium exists as the central metal atom, bonded with three acetate and two oxygen ligands. The coordination number of the complex is 8.
Structure of uranyl triacetate

Thorium also forms complexes such as [ThNO362-]2- and [ThNO33H2O3] . The common coordination numbers of complexes of thorium are 8, 9, 10, and 12.

Separation of Actinide Elements

Various methods are used for the separation of actinide elements. The most common and the best method used to separate the actinide ions is the ion-exchange method. The separation of the second half of the actinides by ion-exchange methods follows a similar order to lanthanides. Due to greater hydration, actinide ions having a smaller size have a larger hydrated radius in an aqueous medium. Thus, they have the largest size in the aqueous solution.

Due to the larger size, actinide ions have the least tendency to exchange with hydrogen ions of resin, and thus, they remain at the bottom of the column. The eluent (complexing agent) used in the process is a buffer solution of citric acid and ammonium citrate. On the addition of the agent, the largest actinide ion will leave the column first, followed by the lighter elements. 

An image shows the separation of actinide ions by using a cation exchanger resin with hydrogen ion-containing eluent and a mixture of actinide tripositive ions.
Separation of actinide three positive ions by using the ion-exchange method

Applications of Actinides

The applications of actinides are discussed below:

  • Most of the actinide elements are used as nuclear weapons and as nuclear reactor fuel.
  • Americium is used in smoke detectors.
  • Thorium is used in atomic reactors and in the treatment of cancer.
  • Various actinides are used in nuclear power plants and in the production of electronic power.
  • Actinium is used as a gamma source, indicator, as well as a neutron source.

Common Mistakes

Students may confuse actinides with transition elements or lanthanides. They may interpret the trends in properties of actinides and transition elements to be identical, which is incorrect. Only early actinides resemble the properties of transition metals.

  • Actinide elements show variable occupancy of f-orbitals that causes irregularities in their electronic configurations. However, most of the transition metals have regular electronic configurations.
  • Only the second half of the actinide series resembles the properties of lanthanides.
  • The magnetic moments of lanthanides and actinides are not similar; the magnetic moment of actinides is lower than that of lanthanides.

Context and Applications

This topic is applicable in the professional exams for both undergraduate and graduate courses, especially for:

Bachelors in Chemistry

Masters in Chemistry

Masters in Pure and Applied Chemistry


d-block elements



Practice Problems

Q1: Which actinide/s is/are found in appreciable amount/s on the earth?

  1. Uranium
  2. Protactinium
  3. Plutonium
  4. Uranium and thorium

Correct option: (d)

Explanation: Uranium and thorium occur naturally on the earth. They are also found in plants, rocks, animals, and salts in a small amount.

Q2: What is the total number of actinide elements present in the 5f-block of the periodic table?

  1. 15
  2. 10
  3. 5
  4. 20

Correct option: (a)

Explanation: The total number of actinides is 15. They are named actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium.

Q3: Where are the actinide elements placed in the periodic table?

  1. Period 3 and group 7
  2. Period 7 and group 3
  3. Period 8 and group 4
  4. Period 6 and group 3

Correct option: (b)

Explanation: Actinide elements have an atomic number ranging from 89 to 103. In the periodic table, these elements belong to the sixth period and third group.

Q4: What is the application of americium?

  1. Gamma source
  2. Cancer treatment
  3. Smoke detectors
  4. Atomic reactors

Correct option: (c)

Explanation: The common application of americium is smoke detection because it serves as an origin for alpha particles and these alpha particles ionize the molecules in the air.

Q5: What is the nature of actinide elements?

  1. Electropositive
  2. Electronegative
  3. Neutral
  4. None of the above

Correct option: (a)

Explanation: Actinide elements are electropositive and highly reactive in nature because of their low value of ionization energy (I).

Want more help with your chemistry homework?

We've got you covered with step-by-step solutions to millions of textbook problems, subject matter experts on standby 24/7 when you're stumped, and more.
Check out a sample chemistry Q&A solution here!

*Response times may vary by subject and question complexity. Median response time is 34 minutes for paid subscribers and may be longer for promotional offers.

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in

Inorganic Chemistry

f-Block Elements


Actinides Homework Questions from Fellow Students

Browse our recently answered Actinides homework questions.

Search. Solve. Succeed!

Study smarter access to millions of step-by step textbook solutions, our Q&A library, and AI powered Math Solver. Plus, you get 30 questions to ask an expert each month.

Tagged in

Inorganic Chemistry

f-Block Elements