The elements from Z = 89 to 103 are actinides. They constitute the second inner transition series within the fourth transition series. They are denoted by the symbol An.
General Characteristics
These elements have the general electronic configuration 5f0-146d0-27s².
All of them are metallic in nature and readily form their cations.
They form their +III ions quickly; the +III state is the common oxidation state for these elements. The stability of the +III state of the actinides increases with increasing atomic number; some of them form other oxidation states also corresponding to stable electron structures.
Their atomic radii decrease with increasing Z, similar to the decrease with the lanthanides; this decrease in radius is called the actinide contraction.
Their ions have characteristic UV-visible absorption spectra.
Several compounds of these elements are paramagnetic.
U, Np, Pu and Am are all quite similar in properties.
Like the lanthanides, the actinides form complexes with high coordination numbers up to 12.
The various cations of U, Np, Pu and Am have very complex solution chemistry. Their chemistry is complicated by hydrolysis, polymerisation, complexation and disproportionation reactions. The actinide contractions occur with both the atoms and the corresponding ions.
The dominant feature of the actinides is the instability of their nuclei. They undergo spontaneous decay, mainly emitting alpha particles. Some of them undergo fission to smaller nuclei.
Except for U and Th, the other actinides are synthetic.
Actinide Contraction
The ionic radii of the +III actinides are given below.
| Ac3+ | Th3+ | Pa3+ | U3+ | Np3+ | Pu3+ | Am3+ | Cm3+ |
| 0.111 | - | - | 0.103 | 0.101 | 0.100 | 0.099 | 0.098 nm |
The size of an ion depends largely upon (i) the quantum number of the outermost electrons and the effective nuclear charge. In the + ions of these elements, the outermost electrons are in a completed op shell. The effective nuclear charge increases with an atomic number because the screening effect of extra electrons in the 5 level does not compensate entirely for the increased nuclear charge. The outermost electrons are held tighter and tighter with increasing atomic number; this contracts the radius (actinide contraction). Thus, there is a contraction in the size of the ions, similar to the lanthanide contraction, from actinium to mendelevium. However, this contraction is greater from one actinide to the next than from one lanthanide to the next. Similarly, the radii of An4+ ions decrease from 0.099 nm for Th4+ to 0.080 nm for Am4+.
Absorption Spectra
The actinide ions absorb in the visible region of the electromagnetic spectrum due to atomic transitions involving the 5f electrons. The absorption bands are sharp. The extinction coefficients in the actinide series are higher than in the lanthanides, commonly by a factor of ten. The spectra of the +III ions show particular resemblances to their lanthanide analogues, for example, U3+ to Nd³+; Pu³+ to Sm³+; and Am3+ to Eu3+. These spectra confirm the electronic structures of the actinides.
The spectra of the actinide compounds arise from three types of electronic transitions.
(i) f → f transitions: These occur in the UV and visible regions and produce colours of aqueous actinide ions.
(ii) 5f → 6d transitions: These produce bands which are broader but more intense compared to the f → f transitions. These transitions give rise to bands mainly in the UV region.
(iii) Metal → ligand charge transfer: These produce intense, broad absorptions in the UV region, sometimes trailing into the visible region. They produce intense colours which characterize many actinide complexes containing oxidisable ligands.
In contrast to the lanthanides, the spectra of the actinide ions are sensitive to their environments. These spectra are more complex than those of the lanthanide ions.
Magnetic Properties
The actinide ions containing spin-free electrons are paramagnetic. The paramagnetism of appropriate transuranic ions and the corresponding lanthanide ions show a remarkable similarity. However, unlike with the lanthanide ions, the experimental magnetic values for the actinide ions are lower than expected because the 5f electrons in them are less effectively screened from the crystal field that quenches the orbital contribution. The magnetic moment confirms the electronic structures of the ions.
Ion types and colour in aqueous solutions
| Element | M3+ | M4+ | MO2+ | MO22+ |
| Ac | Colourless | - | - | - |
| Th | - | Colourless | - | - |
| Pa | Colourless | Colourless | ||
| U | Red | Green | - | Yellow |
| Np | Purple-blue | Yellow-green | Green | Pink |
| Pu | Violet | Orange-brown | Red purple | Orange |
| Am | Pink | Pink | Yellow | Brown |
| Cm | Colourless | Unknown | - | - |
