Obtain an orderofmagnitude estimate for the singlettriplet s

Obtain an order-of-magnitude estimate for the singlet-triplet splitting of the energy levels of the two electrons in helium due to a direct spin-spin interaction.

Solution

If two electrons of He in 1s state,then one electron must be spin up and the other spin down by the Pauli exclusin principle.In other words the electron spins are anti parallel and the state is a singlet ( S=0)

However, if only one electron is in the 1s state and the other electron is in a higher energy level, the two electrons can have parallel or anti parallel spins.if the spins are parallel (S=1) ,it is a triplet state.