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Actinide Series

The periodic table can be represented by using their valence shell configuration. Whole periodic table classified in four blocks. The name of block represents the position of valence shell of an element. Like an s-block elements has its valence electron in s-orbital. d- Block elements are also known as transition element and f- block are also called and inner transition element. Let’s take an example; you have an element with atomic number 11, and electronic configuration 2, 8, 1 or 1$s^2$, 2$s^2$, 2$p^6$, 3$s^1$. Out of these 4 blocks, s-block is consisted of 1st and 2nd group whereas p-block contains group-13 to group-18. From 3rd to 12th groups, elements are commonly known as d-block elements.

There are 2 series of 14 elements at the bottom of main table which are commonly known as f-block elements. The first series is called as lanthanide and 2nd series is known as actinide series.  Glenn Seaborg’s discovered many elements especially after uranium. Starting with plutonium in 1940, he discovered elements from 94 to 102 and placed them in the periodic table as the lanthanide/actinide series at the bottom of the table. The elements of this block show unique physical and chemical properties. Almost all radioactive elements are placed in this block. Let’s discuss actinide series with the chemical and physical properties of elements.

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The elements in which the extra electron, also called the differentiating electron enters the (n-2) f orbitals are called as 'f' block elements. Since the extra electron goes to (n-2) f-orbitals, these elements have been given the name f-block elements. Since (n-2) f orbitals lie comparatively deep within the kernel, (being the inner penultimate shell), these elements are called "Inner transition elements".

The (n-2) f orbitals may either be the 4f or 5f orbitals. Thus, f- block elements are classified into two, namely.
  • 4f- series elements: These elements are called Lanthanides or lanthanones.
  • 5f- series elements: These elements are called actinides or actinones.

What are Actinides?

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The elements with atomic numbers 89-103, in which the 5f orbitals are being filled up, constitute the actinides series. Just as in the lanthanides, where the anti penultimate 4f orbital is filled successively by the addition of one electron at a time, in actinides the anti-penultimate 5f orbital is successively filled by the addition of one electron at each step. Thus, lanthanides constitute the first inner transition series and the actinides constitute the second inner transition series.

In the atoms of 5f-block elements the extra electron enters the 5f-orbital. The elements of this series are also called as actinides or actinones, since these elements follow actinium, Ac(Z= 89) in the periodic table and show close similarities with Ac.

Although, the fourteen elements from Thorium (Th- 90) to Lawrencium (Lw- 103) are called actinides, due to the similarities in properties among the 15 elements, from Ac -89 to Lw-103, all these 15 elements are considered to be members of the actinides series. Thus, Ac-89 is also studied under actinides. The actinides constitute the second inner transition series.

Actinides Properties

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1. Occurrence

Excepting Ac, Th , Pa and U, which occur in nature in Uranium minerals, all the remaining actinides are unstable and made artificially by nuclear transmutations. The actinides elements lying beyond Uranium, are called trans-uranium or trans-Uranic elements All the actinides are radioactive.

2. Electronic configuration

There is some uncertainty about the electronic configuration of these elements. There is doubt whether thorium, protactinium and uranium contains the differentiating electron in the 5f or 6d orbital. The uncertainty is due to the entry of electrons in the 5f or 6d orbital, which are of almost same energy. Thus there are two views regarding the electronic configuration of these elements. According to one(Seaborg view) the 5f orbital starts filling at thorium while according to other(Dawson view) the 5f orbital starts filling at neptunium

Example: Thorium 5f1 6d1 7s2 (Seaborg view) and 6d2 7s2 (Dawson view).

3. Oxidation state

Unlike lanthanides, actinides show a variety of oxidation states, viz. uranium shows all the oxidation states from +3 to +6,Np and Pu shows +3 to +7. However the principle oxidation states are +3 and +4.
It may be seen from these oxidation states that the +2 oxidation state is shown by two elements namely, Am and Th in a few compounds like ThBr2, ThI2, ThS etc. +3 oxidation state is shown by all actinides.+3 state becomes more stable as the atomic number increases. +4 oxidation state is shown by Th, Pa, U, Np, Pu, Am and Cm, while +5 state is shown by Th, Pu, U, Np, Pu and Am.

4. Ionic radius

The ionic radius of actinides decreases regularly along the series. The decrease in ionic radius of actinides is called actinides contraction, and is due to the poor screening effect of the nuclear charge by the f electrons.

5. Color

Actinides ions are usually colored. The color depends upon the number of 5f electrons, ions with 5f 0 electrons and 5f 7 electrons are colorless. The color is due to f-f electronic transitions. Most of the tri positive and tetra positive (3+ and 4+) ions are colored.

Example: Ac3+-colorless , U3+ - Red, Np3+ - Purple, Pu3+ - Violet, Am3+ - pink, Cm3+ - colorless, U4+ - Green, Np4+ - Yellow-green.

6. Complex formation

The actinides have a greater tendency to form complexes than lanthanides because their ions have high charge and are smaller ions.

The degree of complex formation decreases in the following order:


The complexing power of different singly charged and doubly charged anions following order.



Actinides will also form complexes with a large number of organic substances.

Actinides Series

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Actinides lie in the 7th period (Incomplete period with n=7) and III B group of the periodic table.

The following flow chart shows the Actinide series :


Actinides Periodic Table

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Prior to the discovery of trans-Uranium elements, (Trans-uranium elements are the elements lying beyond Uranium in the periodic table), the naturally occurring heaviest known elements namely, Th- 90, Pa- 91 and U - 92 were placed below Hf-72, Ta- 73 and W-74 in IV B, V B and VI B groups of the periodic table, because these elements showed +4, +5 and +6 oxidation states and resembled Hf, Ta and W respectively in many of their properties. The then undiscovered trans-uranium elements with atomic numbers 93 to 100 were thus expected to occupy the positions in the periodic table below Re- 75, Os- 76, Ir- 77 , Pt- 78, etc.

The discovery of Neptunium (Np - 93) came in 1940 and this discovery was followed shortly by the discovery of Plutonium, Pu- 94 in 1941. The tracer elements Np- 93 and Pu - 96 showed that the chemical properties of these two elements very much resemble Uranium - 92 and not at all those of Re- 75 and Os- 76.

On this basis, all the three elements, U, Np and Pu were placed under W- 74 in group VI B in a column. But later, this was also proved wrong, since many other elements discovered did not follow this pattern. In 1944, Seaborg suggested that all the elements with atomic number greater than 89 constitute a second series of inner-transition elements, similar to the lanthanide series.

Thus, they were placed below the lanthanides and named as Actinides series. So, the actinides form a second row below lanthanides and these two series are separated and placed below the main periodic table.
  • In the atoms of the elements of both series, the extra electron enters (n-2) f orbitals.
  • The elements of both the series show +3 oxidation states.
  • Like lanthanide contraction seen in lanthanides, actinides also have actinides contraction.
  • Elements of both the series have low electronegativity and are very reactive.
  • Most of the lanthanide and actinides cations are paramagnetic.
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