F Block Elements - Properties, Oxidation States and Examples

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:55 PM IST

F-block elements are called inner transition elements as they provide a transition in the 6th and 7th rows of the periodic table. They generally have high melting and boiling points. They consist of two series: Lanthanides and actinides. In the periodic table, the element in which the last electrons enter in (n - 2)f orbitals are called f- block elements. They come in the third period of the periodic table with two series 4f and 5f. The 4f series elements are called Lanthanides and the 5f series elements are actinides.

This Story also Contains

Properties of F-Block Elements
Some Solved Examples
Conclusion

F Block Elements - Properties, Oxidation States and Examples

The f-block elements on the periodic table are shown green in the periodic table and contain elements 57-71 called lanthanides, and elements 89-103 being called actinides. Then can be seen in two different chemical series, such as lanthanides under the 4f, and actinides under the 5f block and deep-seated 4f with an increase in atomic number and how the electronic configuration of the f-block element was done. The 5f elements of the actinides constitute the second inner transition metal series, and the lanthanides constitute the first inner transition series in chemistry.

Properties of F-Block Elements

Electronic Configurations

It may be noted that atoms of these elements have an electronic configuration with 6s² common but with variable occupancy of 4f level. However, the electronic configurations of all the tripositive ions (the most stable oxidation state of all the lanthanoids) are of the form 4fⁿ (n = 1 to 14 with increasing atomic number).

Atomic and Ionic Sizes

The overall decrease in atomic and ionic radii from lanthanum to lutetium (the lanthanoid contraction) is a unique feature in the chemistry of lanthanoids. It has far-reaching consequences in the chemistry of the third transition series of the elements. The decrease in atomic radii (derived from the structures of metals) is not quite as regular as it is in M³⁺ ions. This contraction is, of course, similar to that observed in an ordinary transition series and is attributed to the same cause, the imperfect shielding of one electron by another in the same sub-shell. However, the shielding of one 4f electron by another is less than one d electron by another with the increase in nuclear charge along the series. There is a fairly regular decrease in the sizes with increasing atomic number.

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Oxidation States

In the lanthanoids, La(II) and Ln(III) compounds are predominant species. However, occasionally +2 and +4 ions in solution or solid compounds are also obtained. This irregularity (as in ionization enthalpies) arises mainly from the extra stability of empty, half-filled, or filled f subshell. Thus, the formation of Ce(IV) is favored by its noble gas configuration, but it is a strong oxidant reverting to the common +3 state. The E^o value for Ce⁴⁺/ Ce³⁺ is + 1.74 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce(IV) is a good analytical reagent. Pr, Nd, Tb, and Dy also exhibit +4 state but only in oxides, MO₂. Eu²⁺ is formed by losing the two s electrons and its f⁷ configuration accounts for the formation of this ion. However, Eu²⁺ is a strong reducing agent changing to the common +3 state. Similarly, Yb²⁺ which has f¹⁴ configuration is a reductant. Tb^IV has half-filled f-orbitals and is an oxidant. The behavior of samarium is very much like europium, exhibiting both +2 and +3 oxidation states.

Some Solved Examples

Q.1 In the context of the lanthanoids, which of the following statements is not correct?

1)There is a gradual decrease in the radii of the members with increasing atomic number in the series.

2) All the members exhibit a +3 oxidation state

3) Because of similar properties the separation of lanthanoids is not easy.

4) Availability of 4f electrons results in the formation of compounds in the +4 state for all the members of the series.

Solution:

As we learned,

Generally, all Lanthanides form compounds in the +3 oxidation state. Due to Lanthanide contraction, the elements show a gradual decrease in their atomic as well as Ionic radii. These elements are often found together in nature, and as a result of these elements having similar properties, their separation also becomes difficult.

Hence, the answer is the option (4).

Q.2 The outer electronic configuration of Gd (Atomic No: 64) is

1) 4f³5d⁵6s²

2) 4f⁸5d⁰6s²

3) 4f⁴5d⁴6s²

4) 4f⁷5d¹6s²

Solution:

As we learned

The electronic configuration of Gd is given as

Gd⇒1s²,2s²,2p⁶,3s²,3p⁶,4s²,3d¹⁰,4p⁶,5s²,4d¹⁰,5p⁶,6s²,4f⁷,5d¹

It is to be noted that the electronic configuration of Gd accommodates one electron in the 5d orbital as it has a stable, half-filled f⁷ configuration

Hence, the answer is the option (4).

Conclusion

F-block elements play a crucial role in modern science and technology, offering unique properties and applications, albeit with challenges related to their handling and environmental impact. Neodymium, samarium, and others are used to make strong permanent magnets, essential in electronics, wind turbines, and electric vehicles. Lanthanides like europium and terbium are used in color television screens, LED lights, and fluorescent lamps.

Frequently Asked Questions (FAQs)

1. What are f-block elements and where are they located in the periodic table?

F-block elements are a group of elements in the periodic table where the f orbitals are progressively filled. They are located at the bottom of the periodic table, separated into two rows: the lanthanides (elements 57-71) and the actinides (elements 89-103). These elements are often placed below the main body of the periodic table to keep it more compact.

2. Why are f-block elements also called inner transition elements?

F-block elements are called inner transition elements because they represent the inner transition series in the periodic table. The f orbitals being filled are not in the outermost shell, but rather in an inner shell. This filling occurs between the s-block and d-block elements, hence the term "inner transition."

3. What is the general electronic configuration of f-block elements?

The general electronic configuration of f-block elements is [Noble gas](n-2)f1-14(n-1)d0-2ns2, where n is the outermost principal quantum number. This means that electrons are being added to the f sublevel of the third-to-last energy level while the outermost s orbital is fully occupied.

4. How do the atomic radii of f-block elements change across a period?

Contrary to the general trend in the periodic table, the atomic radii of f-block elements decrease slightly across a period. This phenomenon is known as the lanthanide contraction. It occurs because the added f electrons poorly shield the outer electrons from the increasing nuclear charge, causing a stronger pull on the outer electrons and a slight decrease in atomic size.

5. What is the lanthanide contraction and why does it occur?

The lanthanide contraction is the gradual decrease in atomic and ionic radii across the lanthanide series. It occurs because the added f electrons poorly shield the outer electrons from the increasing nuclear charge. As a result, the effective nuclear charge experienced by the outer electrons increases, pulling them closer to the nucleus and causing a contraction in size.

6. How does the lanthanide contraction affect the properties of elements following the lanthanide series?

The lanthanide contraction has a significant impact on elements following the lanthanide series. It results in very similar atomic radii for elements in the same group but different periods (e.g., Zr and Hf, Nb and Ta). This similarity in size leads to comparable chemical properties, making separation of these elements challenging and influencing their occurrence in nature.

7. What are the common oxidation states of lanthanides?

The most common oxidation state for lanthanides is +3. This is because removing three electrons (two from the 6s orbital and one from the 4f or 5d orbital) results in a stable electronic configuration. However, some lanthanides can exhibit other oxidation states, such as +2 (e.g., Eu2+, Yb2+) or +4 (e.g., Ce4+), depending on their electronic configuration and chemical environment.

8. Why do f-block elements generally show variable oxidation states?

F-block elements often exhibit variable oxidation states due to the small energy differences between their f, d, and s orbitals. This allows for relatively easy promotion or demotion of electrons between these orbitals. Additionally, the f electrons are less effective at shielding the outer electrons from the nuclear charge, which can lead to unexpected stabilities of certain oxidation states.

9. What is the actinide series, and how does it differ from the lanthanide series?

The actinide series consists of elements from atomic number 89 (actinium) to 103 (lawrencium). While similar to lanthanides in many ways, actinides differ in that they show a wider range of oxidation states, are generally more reactive, and many are radioactive. Actinides also tend to have larger atomic and ionic radii compared to lanthanides due to the presence of 5f orbitals instead of 4f.

10. Why are most f-block elements silvery-white in appearance?

Most f-block elements appear silvery-white because of how their electrons interact with light. The free electrons in the metal can easily move and oscillate in response to the electric field of incoming light. This causes the light to be reflected, giving the characteristic silvery-white appearance. This is a common property of many metals, not just f-block elements.

11. How does the reactivity of f-block elements compare to other groups in the periodic table?

F-block elements are generally more reactive than d-block elements but less reactive than s-block elements. Their reactivity is influenced by their large atomic size and low ionization energies. Lanthanides react readily with water and acids, forming hydroxides and hydrogen gas. Actinides are even more reactive, with some reacting vigorously with air and water.

12. What is the "gadolinium break" in the lanthanide series?

The "gadolinium break" refers to a discontinuity in the trends of certain properties in the lanthanide series at gadolinium (Gd). This occurs because gadolinium has a half-filled 4f subshell (4f7), which provides extra stability. Properties such as melting point, boiling point, and magnetic behavior show a break in their trends at this point in the series.

13. How do the magnetic properties of f-block elements differ from other elements?

F-block elements often exhibit strong magnetic properties due to their unpaired f electrons. Many lanthanides and actinides are paramagnetic, meaning they are attracted to magnetic fields. Some, like gadolinium, show ferromagnetism at low temperatures. These unique magnetic properties make f-block elements valuable in various technological applications, such as in strong magnets and MRI contrast agents.

14. Why are many f-block elements, especially actinides, radioactive?

Many f-block elements, particularly actinides, are radioactive due to their large, unstable nuclei. These nuclei have a high proton-to-neutron ratio, making them energetically unfavorable. To achieve stability, they undergo radioactive decay, emitting particles or energy. This instability increases as we move across the actinide series, with all elements beyond uranium (92) being entirely synthetic and highly radioactive.

15. How does the color of f-block element compounds differ from those of d-block elements?

F-block element compounds often display vibrant and distinct colors, similar to d-block elements. However, the origin of these colors is different. In f-block elements, the colors arise from f-f electronic transitions, which are typically sharp and well-defined. This is in contrast to d-block elements, where colors result from d-d transitions that are often broader. As a result, f-block element compounds can show more subtle variations in color under different conditions.

16. What is the "actinide hypothesis" and how does it relate to the periodic table?

The "actinide hypothesis," proposed by Glenn Seaborg, suggests that the actinide elements form a series similar to the lanthanides, with the 5f orbitals being filled progressively. This hypothesis led to a reorganization of the periodic table, placing the actinides as a separate series below the main body, similar to the lanthanides. This arrangement better reflected the chemical and physical properties of these elements and helped predict the existence of transuranic elements.

17. How do the ionization energies of f-block elements compare to other groups?

F-block elements generally have lower ionization energies compared to d-block elements but higher than s-block elements. This is due to their large atomic radii and the effective shielding of the outer electrons by the inner f electrons. However, the ionization energies increase across the series as the nuclear charge increases, following the lanthanide contraction.

18. Why do f-block elements often form complexes with high coordination numbers?

F-block elements tend to form complexes with high coordination numbers (often 8 or 9) due to their large ionic radii and the availability of f orbitals for bonding. The f orbitals can accommodate more ligands around the central metal ion compared to d-block elements. Additionally, the ionic nature of f-block element bonding allows for more flexible geometries, further enabling high coordination numbers.

19. How does the basicity of lanthanide hydroxides change across the series?

The basicity of lanthanide hydroxides generally decreases across the lanthanide series. This trend is primarily due to the lanthanide contraction, which causes a decrease in ionic radius and an increase in the charge density of the Ln3+ ions. As a result, the Ln-OH bond becomes stronger and less ionic, making it harder for the hydroxide to dissociate and act as a base.

20. What is the "tetrad effect" in f-block elements?

The "tetrad effect" is a phenomenon observed in f-block elements where certain properties show a pattern of variation in groups of four elements. This effect is attributed to the increased stability associated with quarter-filled, half-filled, three-quarter-filled, and fully filled f-subshells. It can be observed in properties such as formation constants of complexes, extraction behavior, and some thermodynamic properties.

21. How do the melting and boiling points of f-block elements compare to other elements?

F-block elements generally have high melting and boiling points compared to s-block and p-block elements, but they are often lower than those of d-block elements. This is due to the weak metallic bonding in f-block elements, caused by the limited involvement of f electrons in bonding. The melting and boiling points tend to increase across the series due to the lanthanide contraction, with some exceptions near half-filled and fully filled f-subshells.

22. Why are many f-block elements classified as rare earth elements?

Many f-block elements, particularly the lanthanides, are classified as rare earth elements. Despite the name, most are not particularly rare in the Earth's crust. They are called "rare" because they are rarely found in concentrated mineral deposits and are difficult to separate from each other due to their similar chemical properties. The term "earth" is an old chemical term for oxides, as these elements were first isolated as oxides.

23. How does the electronegativity of f-block elements compare to other groups?

F-block elements generally have low electronegativity values compared to other groups in the periodic table. This is due to their large atomic radii and the fact that their valence electrons are in inner orbitals (f orbitals), making them less attracted to the nucleus of other atoms. The electronegativity values slightly increase across the series due to the lanthanide contraction, but they remain relatively low compared to p-block elements.

24. What is the "actinide concept" and how did it change our understanding of the periodic table?

The "actinide concept," proposed by Glenn Seaborg, suggests that the actinide elements form a series analogous to the lanthanides, with the 5f orbitals being filled progressively. This concept revolutionized our understanding of the periodic table by recognizing that elements beyond actinium should be placed in a separate series, similar to the lanthanides. This rearrangement better explained the chemical properties of these elements and helped predict the existence and properties of transuranic elements.

25. How do the atomic volumes of f-block elements change across the series?

The atomic volumes of f-block elements generally decrease across the series, a trend known as the lanthanide contraction. This contraction occurs because the added f electrons poorly shield the outer electrons from the increasing nuclear charge. As a result, the effective nuclear charge experienced by the outer electrons increases, pulling them closer to the nucleus and causing a decrease in atomic volume. This trend is more pronounced in lanthanides than in actinides.

26. Why do f-block elements often have similar chemical properties?

F-block elements often have similar chemical properties because their 4f (for lanthanides) or 5f (for actinides) orbitals are shielded by outer s and p orbitals. This means that the f electrons do not significantly participate in bonding or chemical reactions. Instead, the chemistry of these elements is largely determined by their +3 oxidation state, which results from losing two s electrons and one d or f electron. This similarity makes separating these elements challenging.

27. How does the stability of the +3 oxidation state in lanthanides compare to that in actinides?

The +3 oxidation state is generally more stable for lanthanides than for actinides. In lanthanides, the 4f orbitals are more compact and closer to the nucleus, making them less available for bonding. This results in a strong preference for the +3 state. In actinides, the 5f orbitals are more diffuse and can participate more readily in bonding, allowing for a wider range of oxidation states, including +3 but also higher states like +4, +5, and +6, especially in the early actinides.

28. What is the "lanthanide paradox" and how does it affect the properties of these elements?

The "lanthanide paradox" refers to the unexpected trend in some properties of lanthanide compounds, particularly their solubility and complex stability. Despite the decreasing ionic radius across the series (due to the lanthanide contraction), the solubility of some lanthanide salts and the stability of some lanthanide complexes actually increase. This paradox is attributed to changes in the hydration of the ions and the increasing charge density, which can lead to stronger interactions with solvents and ligands.

29. How do the spectroscopic properties of f-block elements differ from those of other elements?

F-block elements exhibit unique spectroscopic properties due to their partially filled f orbitals. Unlike d-block elements, where d-d transitions result in broad absorption bands, f-block elements show sharp, well-defined absorption and emission lines. These arise from f-f transitions, which are typically forbidden by selection rules but become weakly allowed due to mixing with other states. This results in characteristic, fingerprint-like spectra that are useful for identifying and studying f-block elements.

30. Why are some f-block elements, particularly certain lanthanides, used in lighting technologies?

Some f-block elements, especially certain lanthanides, are used in lighting technologies due to their unique luminescent properties. These elements can absorb energy and then emit it as light of specific wavelengths, a process known as photoluminescence. For example, europium can emit red light, terbium green light, and cerium blue light. This property is utilized in fluorescent lamps, LED screens, and other lighting applications to produce specific colors or enhance overall light quality.

31. How does the reactivity of f-block elements with water compare to that of alkali metals?

While f-block elements are generally reactive with water, their reactivity is typically less vigorous than that of alkali metals. Lanthanides react with cold water to form hydroxides and hydrogen gas, but the reaction is slower than with alkali metals. The reaction rate increases across the lanthanide series due to the lanthanide contraction. Actinides can be even more reactive, with some early actinides reacting vigorously with water. However, the overall trend is still less reactive than alkali metals due to the lower reduction potentials of f-block elements.

32. What is the "lanthanide break" and how does it affect the properties of elements in the sixth and seventh periods?

The "lanthanide break" refers to the significant difference in properties between the elements before and after the lanthanide series in the periodic table. This break is caused by the lanthanide contraction, which results in the post-lanthanide elements having smaller atomic radii than expected. As a result, elements in the same group but different periods (e.g., Zr and Hf, Nb and Ta) have very similar atomic radii and chemical properties. This similarity affects their chemical behavior, occurrence in nature, and methods of separation.

33. How do the redox properties of f-block elements compare to those of transition metals?

The redox properties of f-block elements are generally less diverse than those of transition metals. While transition metals can exhibit a wide range of oxidation states due to their partially filled d orbitals, f-block elements, especially lanthanides, tend to prefer the +3 oxidation state. This is because the f electrons are less involved in bonding and are more difficult to remove. However, some f-block elements, particularly actinides, can show a wider range of oxidation states, especially in the early part of the series where the 5f orbitals are more accessible for bonding.

34. Why do many f-block elements form colored compounds despite having colorless ions?

Many f-block elements form colored compounds despite having colorless ions due to charge transfer processes. While the f-f transitions responsible for the color of the ions are often forbidden and weak, charge transfer transitions between the metal and ligands can be much stronger. These metal-to-ligand or ligand-to-metal charge transfer transitions often occur in the visible region of the spectrum, resulting in intensely colored compounds. This is particularly noticeable in complexes with certain ligands or in solid-state compounds.

35. How does the coordination chemistry of f-block elements differ from that of d-block elements?

The coordination chemistry of f-block elements differs from d-block elements in several ways: