Editing Neptunium (section)

===Hydroxides===
The oxides and hydroxides of neptunium are closely related to its ions. In general, Np hydroxides at various oxidation levels are less stable than the actinides before it on the periodic table such as [[thorium]] and uranium and more stable than those after it such as plutonium and americium. This phenomenon is because the stability of an ion increases as the ratio of atomic number to the radius of the ion increases. Thus actinides higher on the periodic table will more readily undergo [[hydrolysis]].<ref name="Yoshida753" /><ref name="Yoshida768" />

Neptunium(III) hydroxide is quite stable in acidic solutions and in environments that lack oxygen, but it will rapidly oxidize to the IV state in the presence of air. It is not soluble in water.<ref name="migration" /> Np(IV) hydroxides exist mainly as the electrically neutral Np(OH)<sub>4</sub> and its mild solubility in water is not affected at all by the pH of the solution. This suggests that the other Np(IV) hydroxide, {{chem|Np|(OH)|5|-}}, does not have a significant presence.<ref name="Yoshida768" /><ref>{{cite journal |author=Trygve E Eriksen |display-authors=4 |author2=Pierre Ndalamba |author3=Daqing Cui |author4=Jordi Bruno |author5=Marco Caceci |author6=Kastriot Spahiu |date=1993 |title=Solubility of the redox-sensitive radionuclides <sup>99</sup>Tc and <sup>237</sup>Np under reducing conditions in neutral to alkaline solutions. |journal=SKB Technical Report |volume=93-18 | pages=1–32 |url=http://www.skb.se/upload/publications/pdf/TR93-18webb.pdf}}</ref>

Because the Np(V) ion {{chem|NpO|2|+}} is very stable, it can only form a hydroxide in high acidity levels. When placed in a 0.1&nbsp;[[molar concentration|M]] [[sodium perchlorate]] solution, it does not react significantly for a period of months, although a higher molar concentration of 3.0&nbsp;M will result in it reacting to the solid hydroxide NpO<sub>2</sub>OH almost immediately. Np(VI) hydroxide is more reactive but it is still fairly stable in acidic solutions. It will form the compound NpO<sub>3</sub>· H<sub>2</sub>O in the presence of [[ozone]] under various [[carbon dioxide]] pressures. Np(VII) has not been well-studied and no neutral hydroxides have been reported. It probably exists mostly as {{chem|[NpO|4|(OH)|2|]|3-}}.<ref name="Yoshida768" /><ref>{{cite journal |author=Neck, V. |author2=Kim, J. I. |author3=Kanellakopulos, B. |date=1992 |title=Solubility and hydrolysis behaviour of neptunium(V) | journal=Radiochim. Acta |volume=56 | pages=25–30|doi=10.1524/ract.1992.56.1.25 |s2cid=99239460 }}</ref><ref>{{cite journal |author=Kato, Y. |author2=Kimura, T. |author3=Yoshida, Z. |author4=Nitani, N. |date=1996 |title=Solid-Liquid Phase-Equilibria Of Np(VI) And Of U(VI) Under Controlled CO<sub>2</sub> Partial Pressures |journal=Radiochim. Acta |volume=74 | pages=21–25|doi=10.1524/ract.1996.74.special-issue.21 |s2cid=100097624 }}</ref><ref>{{cite journal |author=Nikonov, M. V. |author2=Bessonov, A. A. |author3=Krot, N. N. |author4=Perminov, V. P. |date=1994 |title=Synthesis and characterization of Np(VI, VII) mixed-valence compound |journal=Radiochemistry |volume=36 | pages=237–8}}</ref>