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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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.
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).
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.
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.
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.
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 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 :
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