We now can return to the shell
models discussed in the first six chapters, and reinterpret them
in terms of quantum levels of the atom. The first ten elements are
diagramed across the top of this page (and pages 10
and 11). The hydrogen atom has only a single
electron, and in its lowest-energy state, or ground state,
the electron occupies the 1s quantum level.
The helium atom has two electrons, and in its ground state both
will occupy the 1s orbital as shown.
Electrons have one more property
that has not been mentioned so far, spin. Electrons behave
magnetically as if they were tiny spinning spheres of negative charge,
with the north pole pointing either up or down. Two electrons can
occupy the same orbital if, and only if, they have opposite spin
directions. Hence we say that the 1s orbital in helium is
occupied completely by a pair of electrons of opposite spins.