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F block Elements


The elements constituting the f-block are those in which 4f and 5f orbitals are progressively filled. These elements are formal members of group 3 but are shown separately as f-block of the periodic table.The elements of f-block are also called inner transition elements. 

All the f-block elements are metals. All the elements of actinide series are radioactive. The two series of inner transition elements 4f and 5f series is known as lanthanoids and actinoids respectively.

Inner Transition Elements 

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The elements constituting the f-block are those in which 4f and 5f orbitals are progressively filled. These elements are formal members of group 3 but are shown separately as f-block of the periodic table. The inner transition elements such as U, Th and Pa are proving good sources of nuclear energy.

Lanthanides constitute a group of elements that occupy one sixth of all naturally occurring elements on earth. With exception of promethium the lanthanides are nonradioactive elements that are often found mixed with early actinides thorium and uranium. Lanthanides are also proposed as nuclear fission products in nuclear reactors.

Actinides from americium to lawrencium display solution phase chemical features that resemble those of the trivalent lanthanides. Conversely in certain oxidation states the early actinides often mimic transition metals.
Lanthanides also called rare earth elements comprise of a group of 14 elements of which only one promethium does not occur naturally in the Earth's crust, while the other fourteen are relatively abundant in rocks and soils.

The lanthanides include lanthanum plus the 14 elements in which the 4f electron shell is filled. The most characteristic fact about the lanthanides is the sequence of close packed phases which can be seen either by changing the atomic number or by isothermal compression of a single element.
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The actinide from actinium to lawrencium. The actinide series derives its name from the group 3 element actinium means actinium-like and therefore should exclude actinium, it is usually included in the series for the purpose of comparison.

The actinides form a series at the end of the periodic table that consist of a gradual filling of the 5f electron shell. The elements that concern us here are Th, Pa, U, Np, Pu, Am, Cm and Bk.

Electronic Configurations

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It is not possible to know the electronic configurations of lanthanides with certainty. The probable reason for this is that the 5d and 4f electrons are so close in energy that is not always possible to ascertain whether the electron is entering 5d-orbital or 4f-orbital.

The electronic configuration of lanthanides are listed below.

S.No Elements Electronic configuration
1 La57 [Xe]4f05d16s2
2 Ce58 [Xe]4f25d06s2
3 Pr59 [Xe]4f35d06s2
4 Nd60 [Xe]4f45d06s2
5 Pm61 [Xe]4f55d06s2
6 Sm62 [Xe]4f65d06s2
7 Eu63 [Xe]4f75d06s2
8 Gd64 [Xe]4f85d06s2

The electronic configuration of actinides are listed below.

S.No Elements Electronic configuration
1 Ac89 [Rn]5f06d17s2
2 Th90 [Rn]5f16d17s2
3 Pa91 [Rn]5f26d17s2
4 U92 [Rn]5f36d17s2

Atomic and Ionic Radii

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Atomic radii and Ionic radii of lanthanides decreases from La through Lu. However the decrease is very small.

In lanthanides electrons are being filled in 4f orbitals. It is deep seated orbital and the screening of one 4f-electron by another form the effects of the nuclear charge are very weak due to the shapes of the f-orbitals.

Hence with increasing atomic number and nuclear charge the effective nuclear charge felt by each 4f electron increases. This causes a decrease in the radii of the atoms or ions from La to Lu. This is known as lanthanide contraction.

Atomic radii do not vary regularly. However, Ionic radii decrease with increasing atomic number that is actinite contraction similar to lanthanide contraction.
The +3 oxidation state of lanthanides is common and most stable. It is because the sum of the three ionization energies are not very high. Oxidation states of +2 and +4 are also found when it corresponds with the stable electron configuration.

Actinides exist in a greater range of oxidation states +3 to +7. Highest oxidation states are found with Np and Pu +3 to +7. Oxidation states decreases with increase in atomic number. High range of oxidation states is due to the fact that the 5f, 6d and 7s orbitals are of comparable energies. Such large range of oxidation states makes chemistry of these element complexs.
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Chemical Reactivity

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Lanthanides are highly reactive which has been in agreement with the values of their ionization energy and electronegativity. For practical purposes the size of these ions is almost identical which gives rise to similar chemical properties of the elements.

Actinides particularly the lighter ones, display multiple oxidation states and complex chemical behavior. Fundamental chemical properties of the lighter actinides including oxidation states, hydrolysis and complexation characteristics form the basis of their separation.


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Lanthaides and their compounds find a large number of applications which are given below.
  1. Metallurgical applications
  2. Ceramic applications
  3. Catalytic applications
  4. Electronic applications
  5. Nuclear applications
  6. Medical applicatons - used in the treatment of diseases like eczema, leprosy etc.
Actinides are used as a source of energy and neutrons arising from their radioactive decay. It is used as an oxide framework with a fine mash for generating a bright gas light. It is used for constructing nuclear reactors also used as alloys for heating coils in electric ovens.