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Actinide Series

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The actinides are the 14 elements that follow actinium in the periodic table, just as the lanthanides are the 14 elements that follow lanthanum. The two groups of elements are characterized by the progressive filling of the 5f and 4f electronic sub-shells respectively.

Actinides with incomplete 5f shell have many interesting electronic and nuclear properties. Unlike lanthanides early actinides exhibit multi valence. Most actinides possess the close-packed crystal structures typical of metallic elements, but the most heavily used actinides are U and Pu have low symmetry crystal structures.

Definition

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The elements with atomic number 90 to 103 in the periodic table are called actinides. They are placed after actinium. Most of the actinides are artificially made. Actinides occupy the place below lanthanides at the bottom of the periodic table.

The name actinides are derived from actinium the very first member of the series. The 5f orbitals are being filled up are called actinide series. These elements are also known as actinons. Only the first four elements of this series namely actinium, thorium, protoactinium and uranium occur in nature, the other elements are made artificially by nuclear bombardment.

Elements

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The actinide elements are radioactive metals that begin with the element actinium and continue through the element lawrencium and there is a sub series of the actinide series known as the transuranic elements.
Actinides

It is important to remember that the transuranic elements are part of the actinide series. In other words the actinide series includes the transuranic elements which are considered a continuation of period 7 on the periodic table. Actinide series is also called the second rare earth series since they are homologous to the lanthanide rare earth elements just above them in the periodic table.

Among the actinide series, only two elements exist as long lived nuclides and occur naturally and in relatively large quantity on earth - uranium and thorium.

Properties

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1. Electronic configuration

Dawson suggests that actinium has a 5f06d17s2 electronic configuration with Th having 5f06d27s2 configuration, with the subsequent filling of the d3 and d4 in Pa and U respectively.

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
5 Np93 [Rn]5f46d17s2
6 Pu94 [Rn]5f56d17s2
7 Am95 [Rn]5f77s2
8 Cm96 [Rn]5f76d17s2
9 Bk97 [Rn]5f86d17s2
10 Cf98 [Rn]5f96d17s2
11 Es99 [Rn]5f106d17s2
12 Fm100 [Rn]5f116d17s2
13 Md101 [Rn]5f126d17s2
14 No102 [Rn]5f147s2
15 Lw103 [Rn]5f146d17s2

2. Oxidation states


Actinides has only +3 oxidation state with increasing atomic number, while less heavier elements like thorium, protoactinium, uranium, neptunium, plutonium and americium along with +3 oxidation state, the stable oxidation states of +4, +5, +6 are also seen.

The oxidation states of actinides are listed below.

S.No Elements Oxidation states
1 Ac89 3
2 Th90 3, 4
3 Pa91 3, 4, 5
4 U92 3, 4, 5, 6
5 Np93 3, 4, 5, 6
6 Pu94 3, 4, 5, 6
7 Am95 3, 4, 5, 6
8 Cm96 3, 4
9 Bk97 3, 4
10 Cf98 3
11 Es99 3
12 Fm100 3
13 Md101 2, 3
14 No102 2, 3
15 Lw103 3

3. Atomic and Ionic radii


The Ionic radii of actinides systematically decrease with increase in atomic number. The reason for the ionic radius decreases with the atomic number is the increasing nuclear charge attracting more effectively the 5f electron shell. This decrease is known as the actinide contraction.
Ionic Radii of Actinides

Applications

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The applications of actinides are listed below.
  • Thorium is the major component in gas mantles. It finds application in ceramics, lighting and aerospace alloys. Current world production of thorium is 1500 tonnes per annum.
  • Uranium oxide has been used for many years as a yellow pigment for glass and ceramics but apart from this relatively small scale application, the main use of uranium is as a nuclear fuel.
  • Plutonium is used in long lasting portable power sources and in the miniature batteries used in heart pacemakers. These radioisotope thermoelectric generators rely on the heat produced during the $\alpha$-decay of $^{238}\textrm{Pu}$.
  • Americium is an $\alpha$ and $\gamma$ emitter with a half life of 433 years and is found in most homes as a component of domestic smoke detectors. When smoke enters the ionization chambers of the smoke detector, they absorb the $\alpha$-particles, resulting in a fall in electric current which sets off the alarm.