Like the lanthanides, all actinides are highly reactive with halogens and chalcogens; however, the actinides react more easily.
Actinides, especially those with a small number of 5f-electrons, are prone to hybridization.
This is explained by the similarity of the electron energies at the 5f, 7s and 6d shells.
Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium, +4 for thorium and plutonium and +3 for actinium and other actinides.

Chemically, actinium is similar to lanthanum, which is explained by their similar ionic radii and electronic structure.
Like lanthanum, actinium almost always has an oxidation state of +3 in compounds, but it is less reactive and has more pronounced basic properties.
Among other trivalent actinides Ac³⁺ is least acidic, i.e. has the weakest tendency to hydrolyze in aqueous solutions.

Thorium is rather active chemically.
Owing to lack of electrons on 6d and 5f orbitals, the tetravalent thorium compounds are colorless.
At pH < 3, the solutions of thorium salts are dominated by the cations [Th(H₂O)₈]⁴⁺. The Th⁴⁺ ion is relatively large, and depending on the coordination number can have a radius between 0.95 and 1.14 Å. As a result, thorium salts have a weak tendency to hydrolyse.
The distinctive ability of thorium salts is their high solubility, not only in water, but also in polar organic solvents.

Protactinium exhibits two valence states; the +5 is stable, and the +4 state easily oxidizes to protactinium(V).
Thus tetravalent protactinium in solutions is obtained by the action of strong reducing agents in a hydrogen atmosphere.
Tetravalent protactinium is chemically similar to uranium(IV) and thorium(IV).
Fluorides, phosphates, hypophosphate, iodate and phenylarsonates of protactinium(IV) are insoluble in water and dilute acids.
Protactinium forms soluble carbonates.
The hydrolytic properties of pentavalent protactinium are close to those of tantalum(V) and niobium(V).
The complex chemical behavior of protactinium is a consequence of the start of the filling of the 5f shell in this element.

Uranium has a valence from 3 to 6, the last being most stable.
In the hexavalent state, uranium is very similar to the group 6 elements.
Many compounds of uranium(IV) and uranium(VI) are non-stoichiometric, i.e. have variable composition.
For example, the actual chemical formula of uranium dioxide is UO₂₊ₓ, where x varies between −0.4 and 0.32.
Uranium(VI) compounds are weak oxidants.
Most of them contain the linear "uranyl" group, UO²⁺
₂. Between 4 and 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group.
The uranyl group acts as a hard acid and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands.
NpO²⁺
₂ and PuO²⁺
₂ are also the common form of Np and Pu in the +6 oxidation state.
Uranium(IV) compounds exhibit reducing properties, e.g., they are easily oxidized by atmospheric oxygen.
Uranium(III) is a very strong reducing agent.
Owing to the presence of d-shell, uranium (as well as many other actinides) forms organometallic compounds, such as Uᴵᴵᴵ(C₅H₅)₃ and Uᴵⱽ(C₅H₅)₄.
Neptunium has valence states from 3 to 7, which can be simultaneously observed in solutions.
The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds.
Neptunium metal is very reactive.
Ions of neptunium are prone to hydrolysis and formation of coordination compounds.

Plutonium also exhibits valence states between 3 and 7 inclusive, and thus is chemically similar to neptunium and uranium.
It is highly reactive, and quickly forms an oxide film in air.
Plutonium reacts with hydrogen even at temperatures as low as 25–50 °C; it also easily forms halides and intermetallic compounds.
Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse.
Plutonium(V) can enter polymerization reactions.

The largest chemical diversity among actinides is observed in americium, which can have valence between 2 and 6.
Divalent americium is obtained onl
Like the lanthanides, all actinides are highly reactive with halogens and chalcogens; however, the actinides react more easily. Actinides, especially those with a small number of 5f-electrons, are prone to hybridization. This is explained by the similarity of the electron energies at the 5f, 7s and 6d shells. Most actinides exhibit a larger variety of valence states, and the most stable are +6 for uranium, +5 for protactinium and neptunium, +4 for thorium and plutonium and +3 for actinium and other actinides.
Chemically, actinium is similar to lanthanum, which is explained by their similar ionic radii and electronic structure. Like lanthanum, actinium almost always has an oxidation state of +3 in compounds, but it is less reactive and has more pronounced basic properties. Among other trivalent actinides Ac³⁺ is least acidic, i.e. has the weakest tendency to hydrolyze in aqueous solutions.
Thorium is rather active chemically. Owing to lack of electrons on 6d and 5f orbitals, the tetravalent thorium compounds are colorless. At pH < 3, the solutions of thorium salts are dominated by the cations [Th(H₂O)₈]⁴⁺. The Th⁴⁺ ion is relatively large, and depending on the coordination number can have a radius between 0.95 and 1.14 Å. As a result, thorium salts have a weak tendency to hydrolyse. The distinctive ability of thorium salts is their high solubility, not only in water, but also in polar organic solvents.
Protactinium exhibits two valence states; the +5 is stable, and the +4 state easily oxidizes to protactinium(V). Thus tetravalent protactinium in solutions is obtained by the action of strong reducing agents in a hydrogen atmosphere. Tetravalent protactinium is chemically similar to uranium(IV) and thorium(IV). Fluorides, phosphates, hypophosphate, iodate and phenylarsonates of protactinium(IV) are insoluble in water and dilute acids. Protactinium forms soluble carbonates. The hydrolytic properties of pentavalent protactinium are close to those of tantalum(V) and niobium(V). The complex chemical behavior of protactinium is a consequence of the start of the filling of the 5f shell in this element.
Uranium has a valence from 3 to 6, the last being most stable. In the hexavalent state, uranium is very similar to the group 6 elements. Many compounds of uranium(IV) and uranium(VI) are non-stoichiometric, i.e. have variable composition. For example, the actual chemical formula of uranium dioxide is UO₂₊ₓ, where x varies between −0.4 and 0.32. Uranium(VI) compounds are weak oxidants. Most of them contain the linear "uranyl" group, UO²⁺
₂. Between 4 and 6 ligands can be accommodated in an equatorial plane perpendicular to the uranyl group. The uranyl group acts as a hard acid and forms stronger complexes with oxygen-donor ligands than with nitrogen-donor ligands. NpO²⁺
₂ and PuO²⁺
₂ are also the common form of Np and Pu in the +6 oxidation state. Uranium(IV) compounds exhibit reducing properties, e.g., they are easily oxidized by atmospheric oxygen. Uranium(III) is a very strong reducing agent. Owing to the presence of d-shell, uranium (as well as many other actinides) forms organometallic compounds, such as Uᴵᴵᴵ(C₅H₅)₃ and Uᴵⱽ(C₅H₅)₄.
Neptunium has valence states from 3 to 7, which can be simultaneously observed in solutions. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of coordination compounds.
Plutonium also exhibits valence states between 3 and 7 inclusive, and thus is chemically similar to neptunium and uranium. It is highly reactive, and quickly forms an oxide film in air. Plutonium reacts with hydrogen even at temperatures as low as 25–50 °C; it also easily forms halides and intermetallic compounds. Hydrolysis reactions of plutonium ions of different oxidation states are quite diverse. Plutonium(V) can enter polymerization reactions.
The largest chemical diversity among actinides is observed in americium, which can have valence between 2 and 6. Divalent americium is obtained onl
Actinide