Diproton

Helium-2 (diproton)

Helium-2, He, is unbound. The only bound atom with a mass number of 2 is deuterium. The nucleus of He, a diproton, consists of two protons with no neutrons. Its instability is due to spin–spin interactions in the nuclear force and the Pauli exclusion principle, which states that within a given quantum system two or more identical particles with the same half-integer spins (fermions) cannot simultaneously occupy the same quantum state; so He's two protons have opposite-aligned spins and the diproton itself has negative binding energy.