Can you name as many distinct elements as possible? Do you only have a basic understanding of nitrogen, oxygen, and carbon? Is that the end of it? What about the Actinoids? Do you know what these things are? Don’t worry if you don’t; we’re here to answer any doubts you might have about this new set of elements. Let’s have a look at some of the lesser-known elements in this article.

An actinoid element is also called an actinide element. Any series of 15 consecutive chemical elements in the periodic table from actinium to lawrencium (atomic numbers 89–103). As a group, they are significant largely because of their radioactivity. Although several members of the group, including uranium (the most familiar), occur naturally, most are man-made.

In this article, we will discuss in detail Actinoids, Electronic Configuration, Oxidation states, Characteristics, Uses, etc. Continue reading to know more.

Define Actinoids

Actinides are elements with atomic numbers ranging from \(90\) to \(103,\) which come after the element Actinium. Actinoids are also referred to as Actinides, actinones, and Actinoid series. The actinoid series is also known as the second rare earth series or the second inner transition series.

This series, which consists of fourteen elements ranging from thorium (Th: At No. \( = 90\)) to lawrencium, involves the filling of \(5{\text{f}}\)-orbitals in the atoms (Lr: At. No. \( = 103.\)) These elements are found in the periodic table after actinium (Ac, At. No. \(89\)) and have similar physical and chemical properties.

Learn the Concept of Oxidation Numbers

Define Lanthanoids

The elements in which the last electron enters one of the \(4{\text{f}}\)-orbitals are called \(4{\text{f}}\)- block elements or the first inner transition series. These are also called lanthanides or lanthanons or lanthanoids because they come immediately after lanthanum.

Earlier, these \(14\) elements (\({\text{Z}} = 58\) to \(71\)) were called rare earth elements. Lanthanum, though a \({\text{d}}\)-block element, is included in the lanthanoid series because it closely resembles lanthanoids. Lanthanoids are easier to study because they only have one stable oxidation state.

Electronic Configuration of Actinoids

The electronic configuration of actinoids is uncertain. This is because the energies of the \(5{\text{f}}\)- and \(6{\text{d}}\)-subshells are nearly equal, making it difficult to determine whether the differentiating electron enters the \(5{\text{f}}\)- or \(6{\text{d}}\)-subshell.

According to Seaborg, the if-orbitals begin to fill at thorium, whereas according to Dawson: the orbitals start filling only at neptunium. As a result, the electronic configuration of actinoids is somewhat uncertain. The most widely accepted electronic configurations of actinoids, however, are shown in the table.

Electronic configuration of actinium and actinoids are given below:

The general electronic configuration of Actinoids is \(\left[{{\text{Rn}}} \right]5{{\text{f}}^{{\text{l}} – 14}}6~{{\text{d}}^{0 – 1}}{\text{7}}{{\text{s}}^2}.\)

Name of the element	Symbol(M)	At,No(Z)	Electronic configuration
Actinium	\({\text{AC}}\)	\(89\)	\(\left[{{\text{Rn}}} \right]6{{\text{d}}^1}7{{\text{s}}^2}\)
Thorium	\({\text{Th}}\)	\(90\)	\(\left[{{\text{Rn}}} \right]6{{\text{d}}^2}7{{\text{s}}^2}\)
Protactinium	\({\text{Pa}}\)	\(91\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^2}6{{\text{d}}^1}7{{\text{s}}^2}\)
Uranium	\({\text{U}}\)	\(92\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^3}6{{\text{d}}^1}7{{\text{s}}^2}\)
Neptunium	\({\text{Np}}\)	\(93\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^4}6{{\text{d}}^1}7{{\text{s}}^2}\)
Plutonium	\({\text{Pu}}\)	\(94\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^6}6{{\text{d}}^0}7{{\text{s}}^2}\)
Americium	\({\text{Am}}\)	\(95\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^7}6{{\text{d}}^0}7{{\text{s}}^2}\)
Curium	\({\text{Cm}}\)	\(96\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^7}6{{\text{d}}^1}7{{\text{s}}^2}\)
Berklium	\({\text{Bk}}\)	\(97\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^9}6{{\text{d}}^0}7{{\text{s}}^2}\)
Californium	\({\text{Cf}}\)	\(98\)	\(\left[{{\text{Rn}}} \right)5{{\text{f}}^{10}}6{{\text{d}}^0}7{{\text{s}}^2}\)
Einsteinium	\({\text{Es}}\)	\(99\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^{11}}6{{\text{d}}^0}7{{\text{s}}^2}\)2
Fermium	\({\text{Fm}}\)	\(100\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^{12}}6{{\text{d}}^0}7{{\text{s}}^2}\)
Mendelevium	\({\text{Md}}\)	\(101\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^{13}}6{{\text{d}}^0}7{{\text{s}}^2}\)
Nobelium	\({\text{No}}\)	\(102\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^{14}}6{{\text{d}}^0}7{{\text{s}}^2}\)
Lawrencium	\({\text{Lr}}\)	\(103\)	\(\left[{{\text{Rn}}} \right]5{{\text{f}}^{14}}6{{\text{d}}^0}7{{\text{s}}^2}\)

Oxidation States of Actinoids

These elements have a dominant oxidation state of \( + 3.\) Actinoids, in addition to the \( + 3\) state, have a \( + 4\) oxidation state. Some actinoids exhibit even higher oxidation states. The maximum oxidation state increases up to the middle of the series and then decreases; for example, it increases from \( + 4\) for Th to \( + 5. + 6,\) and \( + 7\) for \({\text{Pa}},{\text{U}},\) and \({\text{NP}}\) but decreases in the succeeding elements.

The actinoids resemble lanthanoids in having more compounds in the \( + 3\) State than in the \( + 4\) state. Compounds in the \( + 3\) and \( + 4\) states, on the other hand, tend to hydrolyze. Furthermore, in the case of actinoids, the distribution of oxidation states is so uneven that it is of no use to discuss their chemistry in terms of their oxidation States.

Name of the element	Oxidation states
Actinium	\( + 3\)
Thorium	\(\left({ + 3} \right), + 4\)
Protactinium	\( + 3, + 4, + 5\)
Uranium	\( + 3, + 4, + 5, + 6\)
Neptunium	\( + 3, + 4, + 5, + 6, + 7\)
Plutonium	\( + 3, + 4, + 5, + 6, + 7\)
Americium	\( + 3,\left({ + 4} \right), + 5, + 6\)
Curium	\( + 3,\left({ + 4}\right)\)
Berklium	\( + 3,\left({ + 4}\right)\)
Californium	\( + 3\)
Einsteinium	\( + 3\)
Fermium	\( + 3\)
Mendelevium	\( + 3\)
Nobelium	\( + 3\)
Lawrencium	\( + 3\)

Ionic Radii and Actinoid Contraction

Because of the inadequate shielding effect of the \(5{\text{f}}\)-electrons, the actinoids constrict. As a result, the radii of these metals’ atoms or ions gradually shrink over the series. Due to inferior shielding by \(5{\text{f}}\) electrons, the shrinkage is stronger from element to element in this sequence.

This is due to the fact that \(5{\text{f}}\) orbitals extend beyond \(6{\text{s}}\) and \(6{\text{p}}\) orbitals in space, but \(4{\text{f}}\) orbitals are buried deep within the atom.

Ionic radii of actinium and actinoids in \( + 3\) and \( + 4\) oxidation state

Element

\({\text{Ac}}\)

\({\text{Th}}\)

\({\text{Pa}}\)

\({\text{U}}\)

\({\text{Np}}\)

\({\text{Pu}}\)

\({\text{Am}}\)

\({\text{Cm}}\)

\({\text{Bk}}\)

\({\text{Cf}}\)

\({\text{Es}}\)

\({\text{Fm}}\)

\({\text{Md}}\)

\({\text{No}}\)

\({\text{Lr}}\)

Ionic radii in \({\text{pm}}\left({{{\text{M}}^{3 + }}} \right)\)

\(112\)

–

–

\(103\)

\(101\)

\(100\)

\(99\)

\(99\)

\(98\)

\(98\)

–

–

–

–

–

Ionic Radii in \({\text{pm}}\left({{{\text{M}}^{4 + }}} \right)\)

–

\(99\)

\(96\)

\(93\)

\(92\)

\(90\)

\(89\)

\(88\)

\(87\)

\(86\)

–

–

–

–

–

General Characteristics of Actinoids

1. Silvery appearance: Actinoids, like lanthanoids, are metals with a silvery appearance.

2. Structural variability: Because they have significantly more irregularities in their metallic radii than lanthanoids, they have a high degree of structural variability.

3. Colour: These metals are silvery-white in colour. Actinoid cations, on the other hand, are usually coloured. The colour of the cation is determined by the number of \(5{\text{f}}\) -electrons. Colorless cations are those that have no \(5{\text{f}}\)-electron or have seven \(5{\text{f}}\)-electrons.

4. Boiling and melting points: Actinoids, like lanthanoids, have extremely high melting and boiling points. They do not, however, show a consistent trend with increasing atomic numbers.

5. Density: Except for thorium and americium, all actinoids have high densities.

6. Ionization enthalpies: actinoids have lower ionization enthalpies than lanthanoids. Because \(5{\text{f}}\) is less penetrating than \(4{\text{f}},\) it shields the nuclear charge more effectively.

7. Electropositive character: All actinoid metals that have been discovered are highly electropositive. In this regard, they are similar to the elements of the lanthanoid series.

8. Magnetic behaviour: Actinoid elements, like lanthanoids, are strongly paramagnetic.

9. Reducing agent: All actinoids are powerful reducing agents.

10. Radioactivity: Every actinoid element is radioactive. The first few members have fairly long half-lives. The remaining members, on the other hand, have half-lives ranging from a few days to a few minutes.

11. Chemical behaviour: They are extremely reactive metals, especially when finely divided.

12. Complex Formation: Actinoids have a higher tendency to form complexes than lanthanoids. This is due to the higher charge and smaller size of their ions.

Difference between Lanthanoids and Actinoids

The main difference between Lanthanoids and Actinoids are as follows:

Lanthanoids	Actinoids
Lanthanoids can have a maximum oxidation state of \( + 4,\) with the other oxidation states being \( + 2\) and \( + 3.\)	Actinoids can have a maximum oxidation state of \( + 7,\) with other oxidation states of \( + 2, + 3, + 4, + 5,\) and \( + 6.\)
Their paramagnetic properties are easily explained.	Paramagnetic properties are difficult to be explained.
Lanthanolds, except promethium, are not radioactive.	All actinoids are radioactive.
Lanthanoids do not form complexes easily, particularly with \(\pi \)-bonding ligands. They form complexes only with the same chelating ligands.	Actinoids have a much higher proclivity tendency to form complexes. They form complexes with \(\pi \)-bonding ligands as well.
They do not form oxocations.	Several oxocations of actinoids such as \({\text{U}}{{\text{O}}^ + },{\text{U}}{{\text{O}}_2}^{2 + },\) etc., are known to exist.
Their compounds are less basic.	Their compound is more basic.

Applications of Lanthanoids and Actinoids

The important uses of lanthanoids and actinoids are given below:

Uses of Lanthanoids

In the pure state, lanthanoids do not find any significant use. However, they are quite useful in the form of their alloys and compounds. Some examples are as follows:

1. Pyrophoric alloys containing rare earth metals are used to make ignition devices such as tracer bullets, shells, and lighter flints.
2. Cerium salts are used in qualitative and quantitative analysis, as well as in cotton dyeing and medicine.
3. In the petroleum industry, cerium phosphate is used as a catalyst.
4. \({\text{N}}{{\text{d}}_2}{{\text{O}}_4}\) and \({\text{P}}{{\text{r}}_2}{{\text{O}}_3}\) are used for colouring glass and tor making optical filters.
5. \({\text{Mg}}\)-alloys containing about \(30\% \) Misch metal and \(1\% \,{\text{Zr}}\) are used for making parts of the jet engines.

Uses of Actinoids

Actinoids in the pure state as well as in the form of their compounds find several applications. Some important applications are as follows:

1. Nuclear chemistry makes use of thorium and its compounds.
2. Uranium and plutonium are used as atomic reactor fuels.
3. \({\text{Th}}{{\text{O}}_2}\) is used in the production of incandescent gas mantles.
4. Thorium salts are used in the treatment of cancer in medicine.
5. Uranium salts are used in the glass, ceramic, textile, and pharmaceutical industries.

Summary

Actinides are elements with atomic numbers ranging from \(90\) to \(103,\) which come after the element Actinium. Actinoids are also referred to as Actinides, actinones, and Actinoid series. The general electronic configuration of Actinoids is \(\left[{{\text{Rn}}} \right]5{{\text{f}}^{1 – 14}}\,{\text{6}}{{\text{d}}^{0 – 1}}\,7{{\text{s}}^2}.\) Actinoids are metals with a silvery appearance. Actinoids are used in Nuclear chemistry, atomic reactor fuels, production of gas mantles, and in the treatment of cancer in medicine.

FAQs

We have provided some frequently asked questions on Actinoids here:

Q.1. Distinguish between lanthanoids and actinoids.
Ans: The difference between lanthanoids and actinoids are as follows:

Lanthanoids	Actinoids
Lanthanoids have a maximum oxidation state of \( + 4,\) with the other oxidation states being \( + 2\) and \( + 3.\)	Actinoids have a maximum oxidation state of \( + 7\) with other oxidation states of \( + 2, + 3, + 4, + 5,\) and \( + 6.\)
Their paramagnetic properties are easily explained.	Paramagnetic properties are difficult to be explained.
Lanthanolds, except promethium, are not radioactive.	All actinoids are radioactive.
Lanthanoids do not form complexes easily, particularly with \(\pi \)-bonding ligands. They form complexes only with the same chelating ligands.	Actinoids have a much higher proclivity tendency to form complexes. They form complexes with \(\pi \)-bonding ligands as well.
They do not form oxocations.	Several oxocations of actinoids such as \({\text{U}}{{\text{O}}^ + },{\text{U}}{{\text{O}}_2}^{2 + },\) etc., are known to exist.
Their compounds are less basic.	Their compounds are more basic.

Q.2. What are Actinoids used for?
Ans: The uses of Actinoids are as follows:
1. Nuclear chemistry makes use of thorium and its compounds.
2. Uranium and plutonium are used as atomic reactor fuels.
3. \({\text{Th}}{{\text{O}}_2}\) is used in the production of incandescent gas mantles.
4.Thorium salts are used in the treatment of cancer in medicine.
5. Uranium salts are used in the glass, ceramic, textile, and pharmaceutical industries.

Q.3. Which actinoids are used as nuclear fuel?
Ans: Uranium is used as a nuclear fuel.

Q.4. Is uranium an actinoid?
Ans: Yes, uranium is an actinoid with the symbol \({\text{U}}\) and atomic number is \(92.\) The electronic configuration of Uranium is \(\left[{{\text{Rn}}} \right]5{{\text{f}}^3}{\text{6}}{{\text{d}}^1}7{{\text{s}}^2}.\)

Q.5. What is the meaning of actinoid?
Ans: Actinides are elements with atomic numbers ranging from \(90\) to \(103,\) which come after the element Actinium.

Q.6. Write two uses of actinoids.
Ans: The two uses of actinoids are as follows;
1. Nuclear chemistry makes use of thorium and its compounds.
2. Uranium and plutonium are used as atomic reactor fuels.

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