Hydrogen ions (H+) are very reactive and will readily combine with other molecules. They don't form stable molecules on their own. However, they can combine with other species to form:

1. Hydronium ion (H3O+): This is the most common form of hydrogen ions in aqueous solutions. It forms when a hydrogen ion (H+) combines with a water molecule (H2O).

2. Dihydrogen cation (H2+): This is a less common species, formed when two hydrogen ions (H+) combine. It is highly reactive and unstable, making it difficult to isolate.

It's important to remember that hydrogen ions are just a proton (H+) and don't exist independently in solution. They are always associated with other molecules or ions.