Energies of Singlet and Triplet States

Triplet spatial wavefunction is zero when the two electrons are at the same position, wheras,
the singlet wavefunction is nonzero. Because the electrons repel each other more when they are close to one another, we therefore expect the singlet to have more electronelectron repulsion and a higher energy. This rule turns out to hold quite generally and is called Hund’s rule : for degenerate noninteracting states, the configuration with highest spin multiplicity lies lowest in energy. Hence, triplet are expected with lower energies. more can be found on this site which may be helpful to understand soem more topics discussed in class.
http://ocw.mit.edu/courses/chemistry/5-61-physical-chemistry-fall-2007/lecture-notes/lecture26.pdf