What are Inner Transition Elements?
Inner transition elements are the elements in which the last electron enters in the f-orbital. They generally belong to group 3 in the periodic table but are mentioned separately as the f block elements. These f block elements are known as inner transition elements.
There are two series of inner transition elements-
Lanthanoid series- If the last electron enters in 4f orbital, it is said to be the lanthanoid series.
Actinoid series- If the last electron enters in 5f orbital, it is said to be actinoid series.
In this article, we will study the main characteristics of inner transition elements. Also, the difference and similarities between lanthanoids and actinoids.
Main Characteristics of Inner Transition Elements
Lanthanoids: (Atomic number: 58 to 71)
Atomic radii: There is a fairly decrease in the atomic radii of lanthanoids along with the series. This is due to the lanthanoid contraction. Lanthanoid contraction can be defined as the poor shielding effect of 4f orbital due to which positive nuclear charge has more effect on the outermost electron thus decreasing the atomic radii along with the series.
Oxidation states: The most common oxidation followed by lanthanoids is +3. Sometimes also show +2 and +4 oxidation states. This variation is due to the extra stability of empty, partially filled, or fully filled f -orbital.
They are generally metals and thus are good conductors of heat and electricity. The hardness of metals increases with an increase in atomic number.
They form coloured ions due to the presence of electrons in f- orbital (narrow absorption bands).
Actinoids: (Atomic number: 90 to 103)
Atomic radii: There is a fairly decrease in the atomic radii of actinoids along with the series. This is due to the actinoid contraction. Actinoid contraction can be defined as the poor shielding effect of 5f orbital due to which positive nuclear charge has more effect on the outermost electron thus decreasing the atomic radii along with the series.
Oxidation state: The most common oxidation state shown by actinoids is +3(but not stable necessarily).
Actinoids are highly reactive metals.
Actinoids are mostly radioactive in nature.
They are not naturally found in the earth’s crust, they are synthesized.
Electronic Configuration:
Lanthanoids: 4f1-145p65d0-16s2 {from cerium(Z=53) to Lutetium(Z=71)}
Actinoids: 5f1-146s26p66d0-17s2 {from thorium (Z = 90) to lawrencium (Z = 103)}.
Similarities Between Lanthanoids and Actinoids
The filling of the last electron to 4f orbital, the element said to belong to the first series of transition elements. 14 elements are there in the lanthanoid series after the lanthanum. As they occur immediately after lanthanum in the periodic table, these are called lanthanides or lanthanoids. Although lanthanum does not have any 4f electrons, since lanthanum closely resembles lanthanoids, it is commonly included in lanthanide.
Actinides or actinides are the electrons obtained upon successive filling of 5f orbitals. They are named so because they appear in the periodic table immediately after actinium (Ac). Fourteen elements from Th(90) to Lw(103) form the sequence of actinides and are also known as the second series of inner transitions. Since actinium (Z=89) has no 5f electrons, the analysis of actinium with actinoids is customary.
Distinguish Between Lanthanoids and Actinoids
Some difference between lanthanoids and actinoids are listed below-
Applications of Inner Transition Elements:
They are used in making nuclear weapons, for example, uranium. Uranium is highly reactive as its naturally occurring isotopes are unstable. Also, plutonium is widely used in making explosives.
They are used in generating nuclear power plants.
Lanthanoids are used to produce lasers.
They are used in determining the age of the fossils and rocks. Widely used elements are Samarium and lutetium.
Lathanoids are used to make strong magnets.
They are widely used in making sunglasses.
Medicinal uses: they are used in destroying the particular targeted cells in the body. Example cancerous cells.
Uranium is also used as a protective shield against radiations
They are also used as a tracker in the body.
Do You Know?
According to Lenntech, thorium is almost as plentiful as lead and at least three times as plentiful as uranium. According to Chemicool, the concentration of thorium in the Earth's crust is 6 parts per million by mass. Thorium is the 41st most common element in the Earth's crust, according to the Periodic Table.
FAQs on Inner Transition Elements
1. What are inner transition elements?
Inner transition elements are the chemical elements in which the last differentiating electron enters the f-orbital of the antepenultimate shell (the second shell from the outside). These elements are placed in two separate rows at the bottom of the periodic table and are also known as the f-block elements. They comprise two series: the lanthanoids and the actinoids.
2. Where are the inner transition elements located in the periodic table?
The inner transition elements are formally part of periods 6 and 7 and belong to Group 3 of the periodic table. To maintain a practical structure, they are displayed in two horizontal rows below the main body of the table. The first row is the lanthanoid series (period 6), and the second row is the actinoid series (period 7).
3. What is the general electronic configuration for the inner transition elements?
The general valence shell electronic configuration for f-block elements is (n-2)f¹⁻¹⁴ (n-1)d⁰⁻¹ ns².
- For Lanthanoids (4f-series), where n=6, the configuration is [Xe] 4f¹⁻¹⁴ 5d⁰⁻¹ 6s².
- For Actinoids (5f-series), where n=7, the configuration is [Rn] 5f¹⁻¹⁴ 6d⁰⁻¹ 7s².
4. What is lanthanoid contraction and what is its main consequence?
Lanthanoid contraction is the steady, gradual decrease in atomic and ionic radii of the lanthanoid elements with increasing atomic number. This occurs because the electrons entering the 4f subshell have a very poor shielding effect, which fails to counteract the increasing nuclear charge. The primary consequence is that the atomic radii of elements following the lanthanoids in the d-block are almost identical to the elements directly above them (e.g., Zirconium (Zr) and Hafnium (Hf) have very similar sizes and properties).
5. Why do lanthanoids primarily show a +3 oxidation state, while actinoids exhibit a wider range?
Lanthanoids predominantly show a +3 oxidation state because there is a large energy gap between the 4f, 5d, and 6s orbitals. After losing the two 6s and one 5d/f electron, the remaining 4f electrons are held very tightly. In contrast, actinoids show a wider range of oxidation states (e.g., +3, +4, +5, +6) because the 5f, 6d, and 7s orbitals have very similar energy levels, allowing more electrons to participate in chemical bonding.
6. What is the key difference between transition elements and inner transition elements?
The key difference lies in which orbital the last electron enters.
- In transition elements (d-block), the differentiating electron enters the d-orbital of the penultimate (second to last) energy shell.
- In inner transition elements (f-block), the differentiating electron enters the f-orbital of the antepenultimate (third to last) energy shell.
7. Why are most actinoid elements radioactive, while lanthanoids are generally stable?
Most actinoids are radioactive because their nuclei are extremely large and have a high proton-to-neutron ratio, making them inherently unstable. These heavy nuclei tend to undergo radioactive decay to achieve a more stable nuclear configuration. Lanthanoids, being lighter elements, possess more stable nuclear arrangements and are therefore mostly non-radioactive (with Promethium being a notable exception).
8. Why do compounds of inner transition elements often form coloured ions?
Compounds of inner transition elements are frequently coloured due to the presence of unpaired electrons in their partially filled f-orbitals. These electrons can absorb energy from visible light to get excited from a lower energy f-orbital to a higher energy f-orbital, a process called f-f transition. The colour of the compound corresponds to the complementary colour of the light absorbed during this transition.
9. What are some important real-world applications of inner transition elements?
Inner transition elements have significant applications in technology and industry:
- Lanthanoids: They are crucial in making powerful magnets (e.g., Neodymium magnets), as catalysts in the petroleum industry, for producing special-purpose glass, and in laser technology.
- Actinoids: Their radioactivity is vital for nuclear applications. Uranium and Plutonium are used as fuel in nuclear reactors and in atomic weapons. Thorium is also being developed as a safer nuclear fuel.
10. How do lanthanoids and actinoids compare in terms of their key properties?
Similarities:
- Both series involve the progressive filling of f-orbitals (4f for lanthanoids, 5f for actinoids).
- Both show a dominant +3 oxidation state.
- Both exhibit a decrease in atomic size across their series (lanthanoid and actinoid contraction).
- Reactivity: Actinoids are generally more reactive metals than lanthanoids.
- Radioactivity: All actinoids are radioactive, while lanthanoids are mostly not.
- Complex Formation: Actinoids have a much stronger tendency to form complexes compared to lanthanoids.
