In the first case, the possible spin states are the three symmetric ones listed above, and this state is commonly referred to as a triplet. The triplet state is not allowed in the ground state of the helium atom, since the spatial function in this case is symmetric and the spin function needs to be anti-symmetric. We can observe that if we take r → → 1 ≈ ≈ r → → 2 , the probability amplitude tends to zero, meaning that the electrons are unlikely to be close to each other, which is referred to as Fermi hole and is responsible for the space-occupying properties of matter.