In its natural state, uranium primarily exists as a mixture of isotopes, with uranium-238 being the most abundant isotope, accounting for over 99% of natural uranium. Uranium-235, a less abundant but still significant isotope, makes up about 0.72% of natural uranium. These isotopes are referred to as "Uranium" without a specific oxidation state because they can exist in various oxidation states depending on the chemical environment.
On the other hand, uranium in its +3 oxidation state, referred to as uranium(III), indicates that the uranium atom has lost three electrons, resulting in a +3 charge. Uranium can form different oxidation states, including +3, +4, +5, and +6, depending on the specific conditions and compounds it is involved with.
Uranium(III) compounds are generally less common compared to other oxidation states of uranium. They are typically encountered in specialized chemical reactions or in the presence of reducing agents that can facilitate the reduction of uranium from higher oxidation states to uranium(III).
It's important to note that the term "uranium" by itself usually refers to the element in its natural state, which consists of a mixture of isotopes and can exhibit various oxidation states, unless a specific oxidation state is mentioned.