The two structures NH₂ and H-NH₂ are constitutional isomers.
Constitutional isomers have the same molecular formula but different connectivity of atoms. In this case, both structures have the same atoms (one nitrogen and two hydrogens), but their connectivity is different. In NH₂, the nitrogen atom is directly bonded to two hydrogen atoms, while in H-NH₂, there is an additional hydrogen atom bonded to the nitrogen atom, leading to a different structural arrangement.