For hydrogen, the notation 1 s
1 is used, where the superscript denotes a single electron in the 1 s state.
Because an electron can have a spin quantum number of 1/2 or 1/2, two electrons having opposite
spins can occupy the 1 s state. The helium atom, having two electrons, has the configuration 1 s
2 with
the electrons having spins of 1/2 and 1/2.