* Uranium salt is a compound containing uranium and a salt (a compound formed by the reaction of an acid and a base).

* Boron is an element.

* Hydrogen is also an element, but it's the simplest and most abundant element in the universe. It's very different in its properties and behavior compared to the other two.

The other two (uranium salt and boron) are both involved in nuclear chemistry or reactions related to the nucleus of atoms. Hydrogen, while involved in some nuclear reactions, has a much broader range of chemical roles.