- 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
Characteristics 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
Th= 5f06d27s2 or 5f16d17s2
The general electronic configurations of actinides may be written as [Rn] 5f1-14 6d0-1 7s2
|Element||Atomic number||Outer electronic configuration|
|Actinium (Ac)||89||[Rn]6d1 7s2|
|Thorium (Th)||90||[Rn]6d2 7s2|
|Protactinium (Pa)||91||[Rn] 5f2 6d1 7s2|
|Uranium (U)||92||[Rn] 5f3 6d1 7s2|
|Neptunium (Np)||93||[Rn] 5f4 6d1 7s2|
|Plutonium (Pu)||94||[Rn] 5f6 6d0 7s2|
|Americium (Am)||95||[Rn] 5f7 6d0 7s2|
|Curium (Cm)||96||[Rn] 5f7 6d1 7s2|
|Berkelium (Bk)||97||[Rn] 5f9 6d0 7s2|
|Californium (Cf)||98||[Rn] 5f10 6d0 7s2|
|Einsteinium (Es)||99||[Rn] 5f11 6d0 7s2|
|Fermium (Fm)||100||[Rn] 5f12 6d0 7s2|
|Mendelevium (Md)||101||[Rn] 5f13 6d0 7s2|
|Nobelium (No)||102||[Rn] 5f14 6d0 7s2|
|Lawrencium (Lr)||103||[Rn] 5f14 6d1 7s2|
2) Ionisation Enthalpies
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
7) Colours of ions
|Ion||Number of electrons||Colour|
|Np3+||f4||blue or purple|
8) Formation of Complexes
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
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.|
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.