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The term actinoids is taken from actinium, the first element in the series. The actinoid series is denoted by the symbol ‘An’. Following the element Actinium, this series consists of a family of 14 elements ranging from 90 to 103 with Thorium (Th) being the first actinoid and Lawrencium (Lr) being the last. The series also includes Protactinium and Uranium and eleven other transuranic elements created artificially by nuclear reactions.
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Key terms: Actinoids, Position of actinoids, f-block elements, Actinoid contraction, Plutonium, Neptunium, Thorium, Protactinium, Uranium, radioactive decay, energy
Introduction to Actinoids
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Actinoids are the f-block elements or inner transition elements possessing radioactive properties. Few actinoids are found in nature whereas the rest are man-made. Natural actinoid elements include Plutonium, Neptunium, Thorium, Protactinium, and Uranium. These elements were also developed as a result of scientific studies and discoveries.
All elements in the actinoid series are radioactive in nature which means that they release a considerable amount of energy during radioactive decay. The most abundant naturally occurring actinoids on Earth are Uranium and Thorium while Plutonium is synthesized.
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Position of Actinoids in Periodic Table
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The Actinide series includes components with atomic numbers 89 to 103 and is located in the 6th period and 3rd group of the periodic table. The series is the row underneath the Lanthanide series, which is found beneath the main structure of the periodic table.
Electronic Configuration
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Actinoids are the second series of f-block elements, with a terminal electronic configuration of [Rn] 5f1-14 6d 0-17s2, where Rn represents the electronic configuration of nearest noble gas, i.e, Radium. Since the energies of 5f and 6d electrons are near, electrons reach the 5f orbital. Except Thorium, the 14 electrons of other elements are formally added to the 5f orbital. These irregularities in electronic configuration of actinoids arise due to stabilities of the f0, f7 and f14 occupancies of the 5f orbitals. The electrons of 5f orbital also possess the ability of bonding to a greater extent.
Actinoid Contraction
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Due to increased nuclear charge and electrons entering the inner (n-2) f orbital, the atomic size/ionic radius of tri positive actinides ions decreases progressively from Th to Lw. Actinoid contraction, hence, refers to the steady decrease in size of atoms or M3+ ions with rising atomic numbers. Contraction is greater along the period due to the relatively poor shielding by 5f electrons.
Oxidation State
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Due to the general smaller energy difference between the 5f, 6d and 7s orbitals, actinoids have varying oxidation states. Though 3+ is the most stable oxidation state, due to the strong shielding of f-electrons, additional oxidation states are feasible. The maximal oxidation state rises until the middle of the series and then falls, i.e. it rises from +4 for Th to +5, +6, and +7 for Pa, V, and Np but falls in the middle.
Chemical Reactivity
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Actinoids are more electropositive and reactive than lanthanides because of their lower ionisation energy. With hot water, they react to form a mixture of oxide and hydride. These elements also combine with nonmetals at moderate temperatures. Actinides are very strong reducing agents as well.
Formation of Complexes
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Due to their smaller size but higher nuclear charge, actinides are superior complexing agents than lanthanides. P – complexes can also be formed by them.
In the order, M4+ > MO22+ > M3+ > MO22+, the degree of complexion diminishes.
Properties of Actinoids
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Some of the properties of actinoids are:
- Actinides have huge atomic masses ranging from 227g/mol to 262g/mol, hence they are heavy.
- The earlier members of actinoid series have relatively long half-lives and the latter ones have half-life values ranging from a day to 3 minutes for lawrencium.
- Since 5f electrons are more effectively protected from nuclear charge than 4f electrons, actinides have lower ionisation enthalpies than lanthanides.
- Actinides are radioactive in nature.
- Metals that are found naturally are mostly smooth, lustrous and silvery in colour with a good density and flexibility. Thorium has a hardness equivalent to mild steel and may be rolled into sheets or pulled into a wire when heated.
- Elements in the actinide series have the same characteristics as those in the d-block.
- Actinoids can lose several electrons and create various ions as a result. Actinium is a crystalline metal that is neither radioactive nor paramagnetic.
- The actinoids are also pyrophoric, meaning that when exposed to air, they spontaneously fire. The melting point of actinoids does not depend on the numbers of f-electrons.
- Because of halogens and chalcogens, actinoids are particularly reactive. For hybridization, actinoids with less than 5 f-electrons are used.
- Actinoids also have a wide range of valence states.
- Actinide elements are both malleable and ductile.
- These elements can be combined positively with the non-metals.
Differences between Actinoids and Lanthanoids
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| Actinoids | Lanthanoids |
|---|---|
| For the filling of Actinoids, 5F-orbitals are used. | For the filling of Lanthanoids, 4F-orbitals are used. |
| The paramagnetic properties of Actinoids are difficult to explain | The paramagnetic characteristics of Lanthanoids are simple to explain. |
| All elements in Actinoid series are radioactive. | With the exception of Promethium, most Lanthanides are non-radioactive. |
| The Actinoid series has multiple oxo cations | The Lanthanides series contains no oxo cation |
| Actinoid compounds are highly basic in nature | Lanthanoid compounds are less basic in nature |
Uses of Actinoids
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Actinoides are used for a variety of purposes such as:
- Americium is basically used in smoke detectors. Thorium is commonly found in gas mantles.
- Actinium is used for scientific research and study by scientists and researchers. Actinium is known as a gamma source, a neutron source and an indicator.
- Actinoids are also used in defence, such as, in making of nuclear weapons and energy production.
- Plutonium is utilised in nuclear reactors as well as nuclear weapons. Many actinide elements are used in nuclear power plants and electronic power generation.
Things to Remember
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- Actinoids possess radioactive properties and large amounts of energy.
- Actinoids have lower ionisation enthalpies and high chemical reactivity.
- Actinoids are believed to have 7s2electronic configuration with variability in 5f and 6f subshells.
- The atomic and ionic size of the elements decreases as we go from Th to Lr.
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Sample Questions
Ques. What are the similarities between Lanthanides and Actinides? (3 marks)
Ans. The following are some of the significant similarities between these two:
- Lanthanides and actinides have a high Oxidation State of +3.
- These elements are filled with (n-2) f orbitals.
- Lanthanides and actinides are both reactive and electropositive.
- As the atomic number of these elements increases, so does their ionic and atomic size.
- Lanthanides and actinides both have strong magnetic properties.
Ques. What is the availability of actinide? (2 marks)
Ans. Thorium and Uranium are the two actinide elements that are abundant in the earth’s crust. Small amounts of Plutonium and Neptunium can also be found in Uranium. The actinide series contains a number of synthetic elements. These elements are referred to as synthetic elements since they are not generated naturally, but rather as a result of the decay of a portion of a heavier element. When exposed to air, the actinide element tarnishes.
Ques. Which actinoids are effective in cancer treatment? (2 marks)
Ans. Thorium (atomic number 90, symbol-Th) is utilised in atom reactors and in cancer treatment. Its salts are utilised in the production of incandescent gas mantles. Thorium is a radioactive isotope of uranium that is used to treat tumours.
Ques. What is meant by lanthanide contraction? Compare it with actinide contraction. (2 marks)
Ans. Lanthanide contraction is the reduction in atom size with increasing atomic number in the lanthanide series, whereas actinide contraction is the reduction in atom size with increasing atomic number in the actinide series.
Ques. What are the effects of lanthanoid and actinoid contraction? (2 marks)
Ans. The effect of lanthanide contraction is caused by 4f electrons' poor shielding of nuclear charge (via the attractive nuclear force on electrons); 6s electrons can be attracted nearer the nucleus, resulting in a decreased atomic radius. Actinoid contraction is caused due to the poor shielding by 5f electrons.
Ques. Where are lanthanoids and actinoid in the periodic table? (2 marks)
Ans. Lanthanoids and actinoids are found at the bottom of the periodic table due to their characteristics and the block in which electrons fill up. Lanthanides consist of elements 58 to 71 (which complete the 4f subshell) and actinides consist of elements 89 to 103. (fill out the 5f subshell).
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