Atom - QM

http://www.tcm.phy.cam.ac.uk/~bds10/aqp.html

http://www.tcm.phy.cam.ac.uk/~bds10/aqp/handout_atomic.pdf

He_3 electron structure

The lowest excitated state of helium is represented by the electron configuration $1s 2s$

the triplet state of the configuration $1s 2s$ lower in energy than the singlet state