Comparison Between Lanthanides and Actinides - Discovery of Actinides, Uses and Characteristics

While reading the periodic table have you noticed there are two rows present at the bottom of the periodic table. There are many questions which may come to your mind, why these are placed separately and placed below the periodic table.

The two separate horizontal rows that are separated and positioned at the bottom of the Periodic Table are where the f-Block elements are typically found.

Starting with the element Lanthanum, the Lanthanides are a group of elements having atomic numbers 57 to 71. Another row with the element Actinium, the Actinides are a group of elements having atomic numbers 89 to 103.

You must ask your teacher and parents about one of the most famous incidents in our nation. In 1998, under Atal Bihari Vajpayee's tenure as prime minister, India conducted nuclear tests at the same site for the second time since 1974 known as the Pokhran-II tests. Five nuclear explosions were conducted as part of the tests in May 1998 at Pokhran with the assistance of the Department of Atomic Energy (DAE) chief 

R Chidambaram, Dr K Santhanam, director of test site preparation, and chief scientist Dr APJ Abdul Kalam, who was also the secretary of the Defence Research Development Organization (DRDO).

Every year on May 11 to commemorate the anniversary of the Pokhran-II tests, National Technology Day is held. Five nuclear explosions were there of which one was a Thorium/U-233 device and four others were weapons-grade plutonium devices.

In this topic, we will be reading more about the Actinides series and will discuss some major properties of them.

TABLE OF CONTENTS

• Discovery of Actinides
• General characteristics of Actinides
• Uses of Actinides
• Comparison between Lanthanides and Actinides
• Practice Problems
• Frequently Asked questions-FAQs

Discovery of Actinides:

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 mineral of thorium 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.

General characteristics of Actinoids:

Electrons obtained by sequentially filling 5f orbitals are known as actinides or actinoids. They get their name from the fact that they are the next element after actinium (Ac) in the periodic table. The actinide series is also known as the second inner transition series, as it contains 14 elements spanning from Th(90) to Lw(103). Despite the fact that actinium (Z = 89) contains no 5f electrons, It is studied under f-block elements and actinides are commonly used to study it due to its resemblance in properties with other elements.

Let's look at some key tendencies of actinoids in the periodic table:

Electronic configuration: 

The electrical configuration of 7s² is thought to be shared by all actinoids, with varied occupancy of the 5f and 6d subshells. The fourteen electrons are formally added to 5f, though not in thorium (Z = 90) but from Pa onwards the 5f orbitals are completed at element 103. The stabilities of the f⁰, f⁷, and f¹⁴ occupancies of the 5f orbitals are related to the abnormalities in the electronic configurations of actinoids, as they were also in lanthanoids. Am and Cm configuration are [Rn]5f⁷7s² and [Rn]5f⁷6d¹7s² respectively. Although the 5f orbitals are similar to the 4f orbitals in terms of their angular component of the wave function, they are not as buried, and so 5f electrons can engage in bonding to a far higher amount.

Electronic configurations of actinoids are given below:

Oxidation states of Actinoids: 

• The actinides, like the lanthanides, have the most frequent oxidation state of +3. However, unlike the first four elements (Th,Pa,U,Np) this state is not always the most stable.
• In the air and in solution, for example, U³⁺ is easily oxidized. For the later elements Am --> Lr, the +3 state is the most stable (except No).
•  Th(+4), Pa(+5), and U(+6) are the most stable oxidation states for the first four elements. All of the outer electrons, including the f- electrons, are used for bonding in these high oxidation states.
• Np has an oxidation state of +7, however, it is oxidizing, and its most stable state is +5. 
• Pu shows all oxidation states from +3 to +7, with +4 being the most stable. 
• Am represents oxidation states ranging from +2 to +6. The configuration of the Am²⁺ is f⁷. It's the equivalent of Eu²⁺, however, it only exists as fluoride in solid form. However, the +3 state is the most stable for Am and practically all of the remaining elements.
• All elements from Th to Bk occur in the +4 oxidation state. Cf²⁺, Es²⁺, Fm²⁺. In solution, Md²⁺ and No²⁺ exist as ions.

Physical properties of Actinides:

• All actinides are silvery metals.

• Although the melting points are high but somehow lower than those of transition elements.

•  Because the increased charge on the nucleus it is inadequately screened by the f electrons, the size of the ions gradually diminishes along the series.

•  An "actinide contraction" is produced as a result, similar to the lanthanide contraction. High densities are found in actinides.

• Actinoids, especially when finely separated, are highly reactive metals.

• When boiling water is applied to them, for example, a mixture of oxide and hydride is formed.

The majority of non-metals combine with them at moderate temperatures.

• Below is a table listing some of the properties of actinides up to berkelium. Heavy actinides are a subject about which little is known.

Colour of the ions :

The majority of actinide ions are coloured. 

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

 Both in a crystalline structure and in an aqueous solution, ions with 2 to 6 electrons in 5f-orbitals exhibit colour. The f-f transition is what gives the colour.

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

Actinide contraction:

• From Thorium (Th) to Lawrencium (Lr), tri-positive actinides' atomic sizes are seen to steadily shrink. The electrons entering the inner orbital (n-2)f and the rising nuclear charge are the causes of this. Actinide contraction describes the shrinkage of an actinides' element caused by an increase in atomic number.

 Magnetic behavior :

• The majority of the actinide ions are paramagnetic because they have unpaired electrons.
  Th⁴⁺(5f0), Pa⁴⁺(5f¹), U³⁺(5f³), Np⁵⁺(5f²), Pu⁴⁺(5f⁴), Am⁵⁺(5f4) etc. are showing paramagnetic character.

•  Actinide cations that only have paired electrons are diamagnetic.
  Few examples are Th⁴⁺(5f⁰), U⁶⁺(5f⁰), Lr³⁺(5f¹⁴) .
   

Formation of complexes : 

• Compared to lanthanides, actinides have a slightly higher propensity to form complex compounds. This is brought on by their smaller ions and higher charge. 

• The majority of the actinide halides combine with alkali metal halides to generate complicated combinations. 

• Actinides combine with organic substances like oxine and EDTA to generate chelates.

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

 Radioactivity: 

• The radioactivity of all actinide elements is inherent. 

• Actinides are heavy elements that contribute to the radioactivity of spent fuel in addition to the fission products present in nuclear reactor cores. Because many actinides are radioactive and/or unstable, they need to be handled carefully. The chemistry and arrangement of the particles in crystals are significantly influenced by the radiation in actinides.

• They are able to experience nuclear reactions.

• The most prevalent isotope of the actinide plutonium, which is by far the most frequently generated, is plutonium (239). 'Minor' actinides like neptunium (238) and americium (241) and (243), as well as curium 244 and 245, are also produced in lower amounts by nuclear reactors.

Uses of Actinides:

Some major uses of a Few actinides are discussed below:

Uses of thorium: 

• When burnt in a gas flame, thorium dioxide with 1% CeO₂ generates a dazzling white light.
• It is utilised to create incandescent gas mantles as a result.

• A mixture of 99 % thorium nitrate and 1% cerium nitrate is used to treat the mantle made of silk fibre. A network of thoria (ThO₂) and ceria (CeO₂) is left behind after this mantle is mounted within the lamp and ignited.

• Th-232 makes up almost all naturally occurring thorium. This isotope is transformed into the fissionable U-233, which is not itself fissionable.

In order to create the fissionable material required for atomic reactors, thorium is employed.

Uses of Actinium:

• For usage in targeted therapy (TAT), a treatment method that uses particle emissions to eradicate malignancies, certain prostate, brain, and neuroendocrine tumours are treated using tailored alpha therapy. The radionuclide actinium-225 has nuclear properties that are very promising.

Uses of Uranium:

• The glass industry uses uranium salts to give the glass a green tint, and is also used in the textile, ceramic, and pharmaceutical industries.

• Atomic reactors and bombs both utilise the U-235 isotope as nuclear fuel.

 Uses of plutonium: 

• Nuclear fuel made of Pu-239 is utilised in nuclear reactors. It comes from the U-238.

Comparison between Lanthanide and Actinides:

Both lanthanides and actinides are f-block elements, meaning that their atoms contain anti-penultimate shells which contain a f-subshell. They have the following general electronic configuration:

where n for actinides is 7, and n for lanthanides is 6. They exhibit commonalities in features due to comparable electronic configurations, but they also exhibit differences in certain of their characteristics.

Similarities:

• Three of the outermost shells in the atoms of the elements in each series are only partially filled, whereas the remaining inner shells are fully filled.

• The outermost energy shell contains two electrons. The penultimate shell has 18 or 19 electrons, and the shell immediately after it has 18 to 32 electrons. 

• The f-subshell is entirely filled in two elements, lutetium and lawrencium. These are the final elements in these two series respectively.

• Both series of elements primarily have a +3 oxidation state.

• Both series' constituents have an electropositive character. They function as potent reducing agents and reactive metals.

• As the atomic number rises, there are a contraction in atomic and ionic (M³⁺ ions) sizes in both of the series. In other words, actinide contraction is similar to lanthanide contraction. The weak shielding between the electrons dwelling in (n-2)f orbitals is the cause of these contractions.

• Cations in both series with unpaired electrons are paramagnetic.

• Actinides and lanthanides both exhibit ion-exchange behaviour.

Dissimilarities:

The table compares some of the features of lanthanides with actinides.

Related link: D and F block

Practice Problems:

Q1. Which actinide characteristic cannot be explained?

A. Oxidation
B. Radioactivity
C. Acidic
D. Magnetic

Answer: D)

Solution: Since they are more complicated, their magnetic properties are difficult to describe. A wide range of magnetic events may manifest themselves as a result of the band creation.

Q2. Which actinide oxidation state is the most stable?

A. +4
B. +2
C. +3
D. +5

Answer: C)

Solution: Because of the lesser energy difference between the 5f, 6d, and 7s orbitals, actinoids exhibit varying oxidation states. Other oxidation states are possible despite the fact that +3 is the most stable oxidation state for the actinides due to the efficient shielding of f-electrons.

Q3.Which acid primarily attacks actinoids?

A. Nitric acid
B. Boric acid
C. Sulphuric acid
D. Hydrochloric acid

Answer: D)

Solution: Metals called actinides are very reactive, especially when they are finely split. Hydrochloric acid attacks all of these metals, but the impact of nitric acid is relatively minimal since a protective oxide coating has formed on their surfaces.

Q4. What type of plutonium is contained in nuclear weapons?

A. Pu-238
B. Pu-239
C. Pu-240
D. Pu-241

Answer: B)

Solution: The most frequent form of plutonium in a normal nuclear reactor is fissile Pu-239, which is created when a neutron is captured from U-238 and then undergoes beta decay. Fissioning Pu-239 produces roughly the same amount of energy as fissioning U-235.

Frequently asked questions- FAQ

Question 1. What practical applications do actinides have?
Answer. Actinides are widely employed in energy production, nuclear weapons development, and defence operations. Nuclear reactors and nuclear weapons both require plutonium. Many of the actinide elements are employed both for the creation of electrical power and in nuclear power plants.

Question 2. What fuel does a nuclear reactor use?
Answer.  Uranium is used as nuclear fuel in reactors. The uranium is transformed into tiny ceramic pellets and piled into fuel rods, which are sealed metal tubes. Usually, a fuel assembly is made up of more than 200 of these rods.

Question 3. Where can you find the source of actinides?
Answer. Thorium and uranium are the only actinides that are found in significant quantities in the earth's crust, however, uranium ores have occasionally contained trace amounts of neptunium and plutonium. Actinium and protactinium are substances that are created during the disintegration of various thorium and uranium isotopes.

Question 4. What procedures are used to prepare actinides?
Answer.  Vacuum evaporation, vacuum reduction-distillation, arc melting, levitation melting, rolling, electroplating, and loading precisely measured amounts of actinide materials into finely machined capsules are some of the methods utilised to manufacture such targeted actinides.

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