Characteristics of Actinides

Contents

1 Characteristics of Actinides
2 Chemical properties of Actinides
3 Similarities between lanthanoids and actinides
4 Differences between lanthanoids and actinides
5 Uses of Actinides

The fourteen elements which follow actinium from thorium (Z = 90) to lawrencium (Z = 103) in the periodic table are called actinides. They involve the filling of 5f orbitals.

1) Electronic Configurations

The actinides involve the filling of 5f-subshell. Actinium has the electronic configuration 6d¹7s². From thorium ( Z= 90) onwards, 5f orbitals get progressively filled. Because of almost equal energy of 5f and 6d subshell, there are some uncertainties regarding the filling of 5f and 6d subshells. The electron may enter either of these two subshells. Whether thorium contains 5f electrons or not and two electronic configurations are suggested for this:

Th= 5f⁰6d²7s² or 5f¹6d¹7s^²

The general electronic configurations of actinides may be written as [Rn] 5f^1-146d^0-1 7s²

Element	Atomic number	Outer electronic configuration
Actinium (Ac)	89	[Rn]6d¹ 7s²
Thorium (Th)	90	[Rn]6d² 7s²
Protactinium (Pa)	91	[Rn] 5f²6d¹ 7s²
Uranium (U)	92	[Rn] 5f³6d¹ 7s²
Neptunium (Np)	93	[Rn] 5f⁴6d¹ 7s²
Plutonium (Pu)	94	[Rn] 5f⁶6d⁰ 7s²
Americium (Am)	95	[Rn] 5f⁷6d⁰ 7s²
Curium (Cm)	96	[Rn] 5f⁷6d¹ 7s²
Berkelium (Bk)	97	[Rn] 5f⁹6d⁰ 7s²
Californium (Cf)	98	[Rn] 5f¹⁰6d⁰ 7s²
Einsteinium (Es)	99	[Rn] 5f¹¹6d⁰ 7s²
Fermium (Fm)	100	[Rn] 5f¹²6d⁰ 7s²
Mendelevium (Md)	101	[Rn] 5f¹³6d⁰ 7s²
Nobelium (No)	102	[Rn] 5f¹⁴6d⁰ 7s²
Lawrencium (Lr)	103	[Rn] 5f¹⁴6d¹ 7s²

2) Ionisation Enthalpies

The ionisation enthalpies of the early actinides are lower than those of early lanthanoids. This is due to the fact that when the 5f orbitals are beginning to be occupied, they penetrate less into the inner core of electrons and therefore, they are more strongly shielded from the nuclear charge than are the 4f electrons of the corresponding lanthanoids. As a result, the outer electrons of actinides are less firmly held by the nucleus and hence their ionisation enthalpies are lower.

3) Physical Properties

(a) Melting and boiling points: The melting and boiling points of actinides are moderately high but they are considerably lower than those of transition elements. However, there is no regular trend in the melting and boiling point values.(b) Density: The densities of actinides first increase and then decrease.(c) Enthalpies of fusion and enthalpies of vaporisation: The enthalpies of vaporisation, in general decrease from Th to Am.

4) Oxidation States

Actinides show different oxidation states such as +2, +3, +4, +5, +6 and +7.However +3 oxidation state is most common among all the actinides. The wide range of oxidation states of actinides is attributed to the fact that the 5f, 6d and 7s energy levels are of comparable energies. Therefore, all these three subshells can participate.All the actinides exhibit in general +3 oxidation state. The elements in the first half of the series exhibit higher oxidation states.

For example: The maximum oxidation state increases from +4 in Th to +5, +6 and +7 in Pa, U and Np respectively. After Np, the maximum oxidation state decreases in succeeding elements. Therefore, the actinides resemble the lanthanoids in having more compounds in +3 state than in +4 state. However, +3 and +4 ions tend to hydrolyse .

The actinides form ionic species. The principle ionic species are M³⁺ , M⁴⁺ , MO²⁺ , MO₂²⁺.The lower oxidation states tend to be ionic while the higher ones are covalent. The dioxo ions, MO₂²⁺ are formed due to high charge density.

Example : UO₂²⁺are formed both in solid and solution state. Oxo cations are stable in acid and in aqueous solutions,.

5) Ionic Radii and Actinide Contraction

Like lanthanoids, actinides also show actinides contraction. The size of the atoms or ions of actinides decrease regularly along the series with the increase in atomic number from actinium to lawrencium. This steady decrease in the ionic radii with the increase in atomic number is called actinide contraction.

The actinide contraction is due to the imperfect shielding of one 5f electron by another in the same subshell. Therefore, as we move along the series, the nuclear charge and the number of 5f electrons increases by one unit at each step. However due to imperfect shielding of 5f orbitals, the effective nuclear charge increases which results in contraction of the size. In actinide contraction, there are bigger jumps between the consecutive members as compared to lanthanoids. This is due to lesser shielding of 5f electrons which results in more increase in effective nuclear charge and therefore, larger attraction.

6) Magnetic Properties

Most of the actinide ions are paramagnetic because they contain unpaired electrons. The ions (Ac³+, Th⁴⁺, Lr³⁺ etc.) which have the electronic configurations f⁰ or f¹⁴ are diamagnetic.

7) Colours of ions

Actinide ions are generally coloured. The colour of actinide ions depends upon the number of 5f-electrons. The ions containing no 5f-electrons or seven electrons (exactly half filled f-subshel) are colourless. The ions containing 2 to 6 electrons in the 5f-subshell are coloured . The colours arise due to electronic transition between various energy states within the 5f-subsell (f-transitions).

Ion	Number of electrons	Colour
Ac³⁺	f⁰	Colourless
U³⁺	f³	red
Np³⁺	f⁴	blue or purple
Pu³⁺	f⁵	violet
Am³⁺	f⁶	pink
Cm³⁺	f⁷	colourless
Th⁴⁺	f⁰	colourless
Pa⁴⁺	f¹	colourless
U⁴⁺	f²	green
Np⁴⁺	f³	yellow-green
Pu⁴⁺	f⁴	orange
Am⁴⁺	f⁵	pink
Cm⁴⁺	f⁶	pale-yellow

8) Formation of Complexes

Actinides have greater tendency to form complexes in comparison to lanthanides because of their higher charge and smaller size of their ions.

Chemical properties of Actinides

1) The actinide metals are all silvery in appearance and display a variety of structures.

2) These are highly reactive metals especially in finely divided form.

3) They react with boiling water and form a mixture of oxide and hydride.

4) All metals are attacked by hydrochloric acid but most of these are slightly affected by nitric acid due to the formation of protective oxide layer.

5) They react with non-metals at moderate temperatures.

6) Alkalies have no action with these metals.

Similarities between lanthanoids and actinides

1. Both exhibit oxidation state of +3 predominantly.

2. In both the series, f-orbitals are being progressively filled.

3. Actinides and lanthanoids having same number of unpaired electrons have quite similar spectra. In the absorption spectra of the elements of both the series, sharp line like bands appear due to f-f transitions.

4. Both are electropositive and have high reactivity.

5. Like lanthanoid contraction, there is actinide contraction. These contractions are due to poor shielding of 5f and 4f electrons in actinides and lanthanoids respectively.

6. The nitrates, perchlorates and sulphates of trivalent actinoids and lanthanoids are soluble whereas the hydroxides, fluorides and carbonates of tho elements of both the series are insoluble.

7. Actinides like lanthanoids show ion exchange behaviour.

Differences between lanthanoids and actinides

Property	Lanthanoids	Actinoids
Oxidation state	Lanthanoids show mainly +3 oxidation state except in a few cases where it is +2 and +4	In addition to +3 oxidation state, actinoids also show higher oxidation states such as +4, +5, +6 and +7.
Binding energies	Binding energies of 4f are higher.	Binding energies of 5f are lower.
Shielding effect	4f electrons have greater shielding effect. Therefore the contraction in their ionic radii is less.	5f electrons have poor shielding effect. Therefore, the contraction in their sizes is more.
Tendency to form complexes	The tendency to form complexes is less.	They have greater tendency to form complexes.
Radioactivity	Except promethium, these are non-radioactive.	All the actinides are radioactive.
Colours	Most of their ions are colourless.	Most of the actinide ions are coloured.
Paramagnetic character	They are paramagnetic and their magnetic properties can be easily explained.	They are also paramagnetic but their magnetic properties cannot be easily explained.

Uses of Actinides

1) Thorium is used in the form of oxide, ThO₂ for making incandescent gas mantles. The mantle made from silk fibres is dipped into a mixed solution of thorium and cerium nitrates in the ratio of 99 and 1 respectively. When fixed in the lamp and ignited, the silk fibres burn away leaving a network of thoria (ThO₂) and ceria (CeO₂). The small amount of cerium oxide is essential because otherwise ThO₂, itself gives only a poor light.

2) Thorium is used in the manufacture of fine rods for atomic reactors.

3) Thorium salts are also nowadays used in medicines in the treatment of cancer.

4) Uranium salts impart green colour to glass.

5) Uranium is used in the production of nuclear energy by the process of nuclear fission.

6) Uranium salts are used in textile industry, ceramic industry as well as in medicines.

7) Plutonium is fissionable material and is used for fuelling atomic reactors. It is Used as an ingredient of atomic explosive weapons. It can also be used to make atomic bombs.