6.D.: The aufbau principle
Wave function for a lithium atom.
Since the two possible 1s orbitals,
and
, are occupied, the third electron must occupy a 2
s
orbital. The right-hand side of the above equation is often abbreviated as 1
s
2
2
s
.
and 
,
are occupied, the third electron must occupy a 2s orbital.
The right-hand side of the above equation is often abbreviated as
1s22s.