Actinides are a group of 15 chemical elements with atomic numbers 89 through 103, beginning with actinium, Ac, and ending with lawrencium, Lr. They form the second of the two principal f block series in the periodic table, corresponding broadly to the progressive filling of the 5f subshell. The group includes actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium and lawrencium.
The name actinides is derived from actinium, just as lanthanides are named after lanthanum. The elements are commonly displayed in a separate row beneath the main periodic table to make the table more compact, although their electronic positions belong to periods 7 and the f block. All actinides are radioactive, which is one of their defining practical characteristics. Several occur naturally, especially thorium and uranium, while many of the heavier members are predominantly or entirely produced artificially in nuclear reactors, particle accelerators or through radioactive decay chains.
Actinide chemistry is strongly influenced by the availability of multiple oxidation states. Unlike many lanthanides, whose plus 3 oxidation state dominates much of their chemistry, actinides can commonly exhibit several oxidation states because the energies of their 5f, 6d and 7s electrons are sufficiently close that different numbers of electrons can participate in bonding. Uranium can form important plus 3, plus 4, plus 5 and plus 6 compounds, while plutonium can display several oxidation states in solution and solid compounds.
This variable oxidation chemistry makes actinides chemically diverse and has major consequences for nuclear fuel processing, environmental transport, separation chemistry and waste management. Actinide ions can form complexes with inorganic and organic ligands, and their chemical behaviour depends strongly on oxidation state, acidity, ligand identity and surrounding materials. The greater radial extension of 5f orbitals compared with 4f orbitals also means that actinide electrons can participate more directly in chemical bonding than the highly shielded 4f electrons of lanthanides.
Like the lanthanides, actinides exhibit a progressive contraction across their series, commonly called the actinide contraction. As nuclear charge increases across the series, the added 5f electrons do not completely compensate for the additional positive charge of the nucleus. The resulting increase in effective nuclear attraction causes a general decrease in atomic and ionic size. The actinide contraction influences bonding, coordination chemistry, oxidation state stability and comparisons between actinides and later elements.
The trend is not perfectly smooth because relativistic effects, electron configurations, oxidation states and other electronic factors complicate simple periodic patterns. Actinide chemistry is also strongly affected by relativistic physics because the nuclei are highly charged. Relativistic effects alter orbital energies and radial distributions, contributing to unusual electronic structures and chemical properties in heavy elements. Consequently, the actinides provide important examples of where simple periodic trends remain useful but must be supplemented by detailed quantum mechanical and relativistic descriptions.
Radioactivity is fundamental to the behaviour and applications of the actinides. Every actinide nucleus is unstable and undergoes radioactive decay with a characteristic half life. Different isotopes can decay through alpha emission, beta processes, spontaneous fission or combinations of decay pathways. Uranium 238, uranium 235, thorium 232 and plutonium 239 are particularly important isotopes in nuclear science. Alpha decay is especially common among heavy actinide nuclei and involves emission of an alpha particle consisting of two protons and two neutrons.
Some heavy actinide isotopes can also undergo spontaneous fission, in which a nucleus splits into smaller nuclei and releases neutrons and energy. The radioactive properties of actinides create both useful applications and significant hazards, requiring specialised containment, monitoring, radiation protection and waste management. Their behaviour is governed by nuclear structure rather than ordinary chemical instability, so chemical reactions do not cause an actinide nucleus to become nonradioactive.
Uranium and plutonium illustrate the central importance of actinides in nuclear energy and nuclear technology. Uranium 235 can undergo induced nuclear fission after absorbing a suitable neutron, and this process can sustain a chain reaction under appropriate conditions. Uranium 238 is abundant in natural uranium and can absorb neutrons to form uranium 239, which through radioactive decay ultimately produces plutonium 239. Plutonium isotopes have important roles in nuclear energy and other nuclear technologies.
Thorium 232 is not itself a fissile isotope in the same sense as uranium 235, but it can participate in nuclear fuel cycles by absorbing neutrons and ultimately producing uranium 233, which is fissile. These nuclear transformations demonstrate that actinides are important not only because of their chemical properties but also because their nuclei can undergo transformations involving enormous energy changes. Nuclear applications therefore require distinguishing chemical reactions, which involve electrons, from nuclear reactions, which change atomic nuclei.
Actinides have applications beyond conventional nuclear power. Americium 241 is used in some smoke detectors because its alpha radiation can ionise air in an ionisation chamber, allowing changes caused by smoke to be detected. Californium 252 is a strong neutron source and has applications in specialised neutron activation, materials analysis and other scientific or industrial contexts. Certain actinides have been used in scientific instruments, research reactors and specialised radioisotope applications.
Their usefulness depends on specific isotope properties such as half life, radiation type, decay energy and availability. At the same time, their radioactivity means that applications require carefully controlled handling and disposal. Actinides also serve as important subjects for fundamental nuclear physics because measurements of their decay, fission and electronic properties help researchers understand the behaviour of matter at extreme nuclear charge and mass.
The history of the actinides is closely connected with the development of modern nuclear science. Uranium was known as a chemical element long before the discovery of nuclear fission, but the discovery of radioactivity and subsequent nuclear research transformed understanding of heavy elements. The discovery of elements beyond uranium during the twentieth century expanded the known periodic table and led to the recognition that these elements formed a coherent series.
Neptunium was identified in 1940, followed by plutonium and several additional transuranium elements. The development of nuclear reactors and particle accelerators made it possible to create and study elements that occur only in extremely small natural quantities or do not persist naturally. The term transuranium refers specifically to elements with atomic numbers greater than uranium, so most actinides after uranium are transuranium elements. However, actinide and transuranium are not synonyms because actinium and several early actinides have atomic numbers below uranium.
Actinides are sometimes misunderstood as being chemically identical because they are displayed together, but their chemistry can vary considerably more than that of the lanthanides. Their multiple oxidation states, complex formation, redox behaviour and radioactive transformations produce substantial diversity. Another important distinction is between an element and an isotope. Uranium is an element defined by having 92 protons, whereas uranium 235 and uranium 238 are different isotopes with different neutron numbers and nuclear properties.
The chemical identity of the element remains uranium even when the isotope changes. Actinides therefore provide a particularly clear demonstration that atomic structure has two interconnected levels. Electron configuration largely determines chemical behaviour, while nuclear composition determines isotope mass and radioactive behaviour. Understanding actinides consequently requires chemistry, quantum mechanics, nuclear physics, materials science, radiation physics and environmental science.
