The general features of the helium atom are revised here . The important points to note are as follows. The overall antisymmetry of the wave function (Pauli exclusion principle) means that the ground state has to be a spin-singlet (S=0). All excited states of helium have one electron in the lowest orbital and one in a higher one (1s2s; 1s3d etc). For these the spin can either be 0 (singlet) or 1 (triplet); the latter states do not convert readily to the former and were originally thought to be a different form of the element; they are still called orhohelium while the singlet states including the ground state are termed parahelium. Coulomb repulsion between the electrons results in the triplet states being lower in energy than the corresponding singlets. This is explained and the level diagrams given here .