Inner Transition Elements


The elements in which the differentiating electron enters the penultimate energy level i.e. (n−2)f, are called f-block elements. Due to such electronic configuration where the last electron enters the 4f or 5f orbitals that are lower than the outermost electrons, f-block elements are also named as inner transition elements.

Depending upon the fact whether the last electron enters the 4f or 5f-orbitals, f-block elements are differentiated into lanthanoids and actinoids.


The f-block consists of the two series, lanthanoids and actinoids.

  • Lanthanum closely resembles the lanthanoids for which the general symbol Ln is often used.
  • The names, symbols and electronic configurations of lanthanum and lanthanoids are given:

Electronic Configurations of Lanthanoids

·         The general electronic configuration of the lanthanoids is

Atomic and Ionic Sizes of lanthanoids

In lanthanide series with increasing atomic number there is a progressive decrease in the atomic as well as ionic radii. This regular decrease is known as lanthanide contraction. This is due to the poor shielding of f orbitals, which are unable to counter balance the effect of increasing nuclear charge. Net result is contraction in size.

lanthanoid contraction

The lanthanoid contraction is a term used to describe the trend in the decrease of the atomic and ionic radii of the lanthanoid series (also called the lanthanide series) elements from atomic number 57 (lanthanum) to 71 (lutetium), as one moves from left to right across the series.

This phenomenon occurs because the additional electrons in each subsequent element are added to 4f orbitals, which are not very effective at shielding the increased nuclear charge experienced by the outermost electrons. As a result, the effective nuclear charge experienced by the outermost electrons increases, leading to a contraction in their orbitals and a decrease in atomic and ionic radii. This also results in an increase in the density, melting and boiling points of the elements.

Oxidation States of lanthanoids

·         The most common oxidation state of lanthanides is + 3 which is obtained by using two electrons in 6s and one electron from 5d subshell.

Exception: Some elements show +2 and +4 oxidation states. This irregularity arises mainly from the extra stability of empty, half-filled or filled f subshell.

For example:

General Characteristics of lanthanoids

Physical properties of lanthanoids:

·         All the lanthanoids are silvery white soft metals and tarnish rapidly in air, the hardness increases with increasing atomic number.

·         The melting points range between 1000 to 1200 K but samarium melts at 1623 K.

·         They are also good conductors of heat and electricity.

·         Lanthanoids are all used in the steel industry for making alloy steels. The important and well-known alloy is misch-metal and it consists of lanthanoid (90-95%), iron (4-5%) and the trace amount of S, C Ca and Al.

Chemical properties of lanthanoids:

·         Some important chemical reactions of lanthanoids are:

Some important chemical reactions of lanthanoids are:



The 14 elements immediately following actinium (89), with atomic numbers 90 (Thorium) to 103 (Lawrencium) are called actinoids. They belong to second inner transition series. In actinoids, the filling of electrons takes place in the anti-penultimate subshell.

Electronic Configurations of Actinoids

The general electronic configurations for the actinoids is [Rn]5f1−146d0−17s, where Rn is the electronic configuration of the element Radium. The fourteen electrons are formally added to  though not in Thorium  but onwards from it and the 5f subshell is complete at Lr (Z = 103). The irregularities in the electronic con-figuration of the actinoids are related to the stabilities of  of empty, half-filled or filled f subshell.

Ionic Sizes of actinoids

In actinides, the ionic radii decreases as we move down the series. This decrease in ionic radius is termed as actinide contraction. This effect is due to poor screening offered by 5f electrons.


Oxidation States of actinoids

The dominant oxidation state of actinoids is +3. However they also show variable oxidation states due to the comparable energy of 5f, 6d and 7s subshells.For example: The uranium shows oxidation states of and. The element neptunium (Z = 93) show an oxidation state upto +7.

General Characteristics of Actinoids and Comparison with Lanthanoids

  • Actinoid metals are highly reactive and have irregular metallic radii, which are much greater than in lanthanoids.
  • They are silvery in appearance and exhibit a variety of structures.
  • They react readily with most non-metals at moderate temperatures.
  • Hydrochloric acid attacks all actinoid metals, but they are slightly affected by nitric acid due to the formation of protective oxide layers, and alkalies have no action.
  • The magnetic properties of the actinoids are more complex than those of the lanthanoids.
  • The ionisation enthalpies of the early actinoids are lower than those of the early lanthanoids, as the 5f electrons are shielded from the nuclear charge by the core electrons.
  • The availability of outer electrons for bonding is higher in actinoids due to their less firm holding.
  • The early actinoids exhibit gradual variation in properties without change in oxidation state, similar to lanthanoids.
  • The lanthanoid and actinoid contractions have extended effects on the sizes and properties of the elements succeeding them in their respective periods.


Some Applications of d- and  f-Block Elements

·        Misch-metal is used in making tracer bullets, shell and lighter flint.

·        Mixed oxides of lanthanoids are used as a catalyst in petroleum cracking. Some individual oxides of lanthanoids are used as phosphors in television screens and similar fluorescing surface.

·        Iron and steels are important construction materials.

·        Their production involves the reduction of iron oxides, removal of impurities, and addition of carbon and alloying metals.

·        Compounds like TiO and MnO2 are manufactured for special purposes.

·        Group 11 elements are known as coinage metals and Ag and Au are valuable collection items.

·        Cu/Ni alloy is used for contemporary UK 'silver' coins and copper-coated steel for 'copper' coins.

·        Many metals and/or their compounds are essential catalysts in the chemical industry.

·        V2O5 catalyses the oxidation of SO2 to sulphuric acid.

·        TiCl4 with Al(CH3)3 forms the basis of the Ziegler catalysts used in polyethylene manufacturing.

·        Iron catalysts are used in the Haber process for ammonia production.

·        Nickel catalysts enable the hydrogenation of fats to proceed.

·        PdCl2 catalyses the oxidation of ethyne to ethanal in the Wacker process.

·        Nickel complexes are useful in the polymerisation of alkynes and other organic compounds.

·        AgBr is used in the photographic industry due to its light-sensitive properties.