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Actinides

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What are Actinides?

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Actinides are group 3 elements. Actinide derives its name from the first element in the series Actinium. It is represented with the chemical symbol An and the series actinoid or actinides elements comprises the 15 metallic chemical elements from atomic numbers 89 to 103 (actinium to lawrencium). All actinides are radioactive and also release energy when radioactive decay occurs. The most abundant actinides on Earth are Uranium and Thorium (which are naturally occurring) and Plutonium, which is synthetically produced. These actinides are used in nuclear weapons and nuclear reactors. Uranium and Thorium have also been important historically. Americium is used in the ionization chambers for contemporary smoke detectors.

Actinide Series

The term ‘actinide series’ is derived because of the first element of the series- Actinium. An is the symbol used to refer to any of the elements representing the Actinide series. All the actinide series are radioactive in nature and release a large amount of energy on radioactive decay. 

Electronic Configuration of Actinides

[Rn] 5f1-14 6d0-1 7s2 is the general electronic configuration of actinides, where [Rn] represents the electronic configuration of Radium (which is the nearest noble gas). The energy of 6d and 5f electrons are equally similar and therefore, the electrons enter into the 5f orbital.

F Block Elements, Actinides and Lanthanides

Actinides are the second series of the F block elements and the first series of the f block elements are called lanthanides which include elements with atomic numbers starting from 57 to 71. The former, i.e. actinides, are radioactive in nature whereas lanthanides are non-radioactive except promethium. Lanthanides are soft metals having silvery-white colour. Lanthanoid contraction takes place when there is a decrease in atomic and ionic radii from lanthanum to lutetium. Lanthanides are good conductors of electricity and heat and have melting points in the range of 1000 K-1200 K and exception is Samarium with 1623K. Properties of f block elements are such that these have electrons added to the ‘f’ sub-orbitals of n-2 level and are placed between (n-1)d and ns block elements in the periodic table. Their properties are the same as that of d-block elements. 

Talking about the similarities between Actinides and Lanthanides, both are having a prominent oxidation state of +3. Both get involved in the filling of (n-2)f orbitals. Both are highly electropositive and reactive in nature. In both cases, there is a decrease in ionic and atomic size with an increase in atomic number. Both actinides and lanthanides show magnetic properties. 

Actinide Contraction

The ionic radii or atomic size of tri positive actinide ions tend to decrease steadily from Th to Lw because of the increasing nuclear charge and electrons entering inner (n-2) f orbital. Hence, this gradual reduction in the size with an increase in atomic number is called actinide contraction and occurs similarly to lanthanide contraction. Contraction may be larger along the period because of the poor shielding by 5f electrons.

Formation of Colourful Ions

Like lanthanide ions, actinides also have electrons in f-orbital and also comprise empty orbitals such as the d-block elements. When these absorb a frequency of light, the f-f electron transition produces a visible colour leading to the formation of coloured ions.

Ionization of Actinides

As compared to lanthanides, actinides have lower ionization enthalpies and this is because 5f electrons are properly shielded from nuclear charge than 4f.

Oxidation State of Actinides

Variable oxidation states can be found in actinides due to the smaller energy gap between 5f, 6d and 7s orbitals. Though 3+ is the most stable oxidation state, other oxidation states occur due to the good shielding of f-electrons. It is such that the maximum oxidation state initially increases up to the middle of the series and then decreases. It can also be said that increase takes place from +4 for Th to +5, +6 and +7 for Pa, V and Np and then decreases in the succeeding elements.

Complexes Formation by Actinides

As compared to the lanthanides, actinides are known to be better complexing agents than lanthanides due to the smaller size, however, they have a higher nuclear charge. Actinides can also form  Pπ – complexes too.

Chemical Reactions by Actinides

Having lower ionization energy, actinides are electropositive as compared to lanthanides and are most reactive. Actinides can react with hot water as well as react with oxidizing agents to form a passive coating, halides and hydrides. Actinides are also known to be strong reducing agents.

Physical Properties of Actinides

Let’s learn about some of the physical properties of actinides as follows:

  1. The Density of Actinides: All actinides except thorium and americium are having very high densities.

  2. Boiling Point and Melting Point of Actinides: Actinides are known to have fairly high melting points similar to lanthanides, however, they do not have a definite trend in the melting and boiling points of lanthanides.

  3. Magnetic Properties: Being paramagnetic in nature, it depends on the presence of unpaired electrons in Actinides. Because of the shielding of 5f electrons, they have quenched orbital angular moment and the observed magnetic moment is lesser than the calculated one.

FAQ (Frequently Asked Questions)

Q1. What is Lanthanide Contraction?

Ans: Lanthanide contraction is the gradual decrease in the size of lanthanide ions with an increasing atomic number. The atomic size or the ionic radii of tri positive lanthanide ions tend to decrease steadily from La to Lu as there is an increase in nuclear charge and electrons entering the (n-2) f inner orbital. Lanthanide contraction affects the atomic size, difficulty in the separation of lanthanides, effect on the basic strength of hydroxides, complex formation and the ionization energy of d-block elements. The size of the atom of the third transition series is nearly similar to that of the atom in the second transition series during this time. 

Q2. Which are the Most Abundant Actinides in Nature?

Ans: Thorium and Uranium are known to be the most abundant actinides in nature with the mass concentrations of 16 ppm and 4 ppm, respectively. Uranium is found mostly in the Earth’s crust in the form of a mixture of the oxide in minerals like Uraninite. The most abundant thorium minerals are known to be thorianite and Monazite.