The atomic nucleus of helium-4 is composed of how many protons and how many neutrons ?
This symmetry reflects similar underlying physics : the pair of neutrons and the pair of protons in helium 's nucleus obey the same quantum mechanical rules as do helium 's pair of electrons ( although the nuclear particles are subject to a different nuclear binding potential ) , so that all these fermions fully occupy 1s orbitals in pairs , none of them possessing orbital angular momentum , and each cancelling the other 's intrinsic spin .