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F Block Elements

The long form of the periodic table is a tabular representation of all known elements. Elements are arranged in this table in the increasing order of their atomic number. We know that an atom is neutral due to the same number of electrons and protons therefore the atomic number of element provides the complete information about the electronic configuration of elements.

On the basis of the valence shell configuration and position of valence electron in the element, the long form of the periodic table can be classified in 4 blocks. The s-block elements are placed in 1st and 2nd group on the table and called as alkali metal and alkaline earth metal. The p-block elements are placed from a 13th group to 18th in the periodic table. There are metals, metalloids and non-metals in this block. The d-block elements are placed from the 3rd group to 12th group of the periodic tables. They are also known as the transition elements.

There are only metals in this block which show variable oxidation states and widely used as catalyst for several organic and inorganic reactions. There are two series of 14 elements, placed at the bottom of main table which are commonly known as f-block elements. These elements show unique properties compared to other elements therefore are placed at the bottom of the table. Let’s discuss some chemical and physical properties of f-block elements.

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F-block Elements

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F- block elements are also called Inner transition elements. The last electron of these elements enters the 'f' sub shell. They are mostly radioactive elements. They are present beneath the main periodic table, in the form of two rows, one row belonging to lanthanides and the other row belonging to actinides.

Lanthanides and Actinides

The first row of the f- block , the 4f series consists of the Lanthanides and the second row, the 5f series are the Actinides.

Lanthanides Series

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Lanthanides or Lanthanones as they are also called, belong to the 4f series, the first series of f- block elements. In these elements, the differentiating electron enters the 4f- orbitals. This series has Lanthanum( Z= 57) and the next fourteen elements (Z = 58 to 71). But, La, z = 58 has a completely empty f- sub -shell. So, the elements from Ce to Lu (58 to 71) usually consists of the lanthanide series.

As the number of electrons in the outermost and the penultimate shell remains the same, the fourteen elements resemble each other. The lanthanides are also called as rare-earths. The name lanthanides is given because of the strong resemblance of these elements to lanthanum. The name rare earths was given to these elements because they were originally extracted from oxides, which were called 'earths' earlier and which were considered rare. 

The 4f electrons of these elements are completely shielded and do not take part in the chemical bonding, unlike the d- electrons of transition series. These are silvery white metals and are good conductors of heat. Among all the elements only some elements have a fully filled and half-filled f-orbital. The elements with these f-orbitals are more stable. 

Example : Europium and gadolinium has half-filled f-orbital. Ytterbium and lutetium has fully filled f-orbital. The list of elements in this series, with atomic number and symbol are

Elements+symbol Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Atomic number 58 59 60 61 62 63 64 65 66 67 68 69 70 71


Groups of Lanthanides

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Klemm has divided lanthanides into two groups of seven elements each. These are
  1. From Cerium Ce(58) to Gadolinium - Gd ( 64)
  2. From Terbium (Tb - 65) to Lutetium (Lu - 71)
In the case of the first group, the half filling of 4f orbitals takes place and in the case of second group, the pairing of electrons in the 'f' sub-shell takes place.

Oxidation States

All the elements of this series exhibit an oxidation state of +3 (Ln3+ ). Some of these elements also exhibit +2 and +4 states, but these states are not as stable as the +3 state. Only +3 state is exhibited by La, Gd and Lu which is due to extract-ability of empty, completely half filled or completely filled 4f orbitals. Along with +3, other two oxidation states, +2 and +4 are exhibited by some members of this series. 

Example: Eu and Yb can acquire +2 state as it provides them with extra stability.
The Ce4+ ion is stale and it can convert easily to Ce3+ thereby making the 4+ ion a very strong oxidizing agent. 

Chemical Reactivity

Lanthanides are chemically reactive. They react with carbon, hydrogen, oxygen and sulfur and form
carbides, hydrides, oxides and sulphides respectively. The oxides formed are M2O3 oxides. They react with water and form insoluble hydroxides. The oxides and hydroxides react with CO2 and form carbonates, M2(CO3)3.

In moving along the lanthanide series from Ce(58) to Lu (71) a regular decrease in the size of the atom/ion with increase in atomic number is observed. This decrease in size is called the Lanthanide contraction. In this series the size of Lanthanum is maximum and that of Lu, lutetium is minimum. The decrease is size, though continuous, is not regular.

Reason for Lanthanide Contraction

The cause of lanthanide contraction can be traced to the imperfect shielding of one 4f electron by another in the same sub-shell. On moving along the lanthanide series the number of 4f electrons increases by one unit at each step and the imperfect shielding increases, causing the contraction in electron cloud of the 4f sub-shell. Ionic radii changes from 1.06 Α to 0.85A.

Important Consequences

The lanthanide contraction plays a significant role in the chemistry of lanthanides. The important consequences of it are:

  1. There is a steady decrease in ionic size.
  2. There is a slight increase in electronegativity of the trivalent ions.
  3. The Eo values for M3+ + 3e → M(g) increases regularly from Lanthanum -252V to 2.25 V for Lutetium.
  4. Since there is a very small change in the size of ions in the lanthanide series and there is no change in the outermost shell, there is a close resemblance in chemical properties.
  5. Lanthanide contraction plays a significant role in the chemistry of lanthanides and heavier transition elements.
  6. The atomic radii of 5d transition elements are very close to those of the corresponding 4d transition elements. Due to this the crystal structure and other properties of lanthanides are very similar.

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Applications of Lanthanides

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  1. Metallurgical applications: Some of the alloys of lanthanide elements find important metallurgical applications as reducing agents. Example: Misch meals (Ce- 30 to 35%)
  2. Ceramic applications: Ce(III) and Ce(IV) oxides find use in glass polishing powders whereas Nd and Pr oxides are extensively used in coloring glass and in the production of standard light filters.
  3. Catalytic applications: Some lanthanide compounds are used as catalysts. Example: Cerium phosphate is used in petroleum cracking as a catalyst.
  4. Electronic applications: The ferromagnetic garnets of 3Ln2O3.5Fe2O3 type are used in microwave devices.
  5. Nuclear applications: These elements and some of their compounds are used in nuclear control devices, shielding devices and fluxing devices. Sm - 140, Eu - 153, Gd- 155, Gd- 157 and Dy - 164 are some of the important isotopes used in nuclear technology.

Actinides Elements

The 5f block elements are also called actinoids or actinones. This series includes 15 elements, from Thorium(Z = 90) to Lawrencium (Z = 103). The name actinides is derived from actinium, the very first member of the series.

F-block Periodic Table

F-block elements are present beneath the main periodic table, in the form of two rows.
  1. First row makes up the Lanthanide series.
  2. Second row makes up the Actinide series.
More topics in F block Elements
Actinide Series
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