Uranium is a heavy metal element, designated by the chemical symbol U and atomic number 92. As an actinide, it is primarily recognized for its association with nuclear power and its presence in the environment. The element naturally occurs in low concentrations within soil, rock, and water.
The Charge of a Neutral Uranium Atom
The electrical charge of any atom is determined by the balance between positively charged protons and negatively charged electrons. Uranium has an atomic number of 92, meaning it contains 92 protons. In its natural, unreacted state, the atom is electrically neutral because it also possesses 92 electrons, which perfectly counterbalance the protons. The net electrical charge on a neutral uranium atom is zero.
The Range of Ionic Charges
Uranium rarely remains neutral, instead readily forming ions by losing valence electrons to create compounds. The charge of these ions is described by the oxidation state, which indicates the number of electrons lost. Uranium exhibits ionic charges ranging from positive three to positive six.
Common Oxidation States
The +3 state (U\(^{3+}\)) is highly unstable and quickly oxidizes in the presence of water or air. The +4 state (U\(^{4+}\)) is reasonably stable and is present in the compound uranium dioxide (UO\(_{2}\)). The +5 state (UO\(_{2}^{+}\)) is less common and unstable in water. The highest and most stable oxidation state is +6, which plays the largest role in environmental and industrial chemistry.
The Stability and Structure of the Uranyl Ion
The most prevalent and stable charged form of uranium in aerobic environments is the uranyl ion, which represents the +6 oxidation state. This ion is a complex ion with the chemical formula UO\(_{2}^{2+}\), consisting of a uranium atom centrally bonded to two oxygen atoms, carrying an overall charge of positive two. The uranyl ion adopts a distinct linear structure, often represented as [O=U=O]\(^{2+}\). The bonds between uranium and oxygen are exceptionally short and strong, contributing significantly to its stability.
Because of its stability and structure, the UO\(_{2}^{2+}\) ion is highly soluble in water. This solubility means that the uranyl ion is the dominant form of uranium found in natural waters, soils, and biological systems. The ion’s positive two charge allows it to readily form complexes with other negatively charged molecules, influencing its movement and behavior in the environment.